Chapter 8: Respiration and Circulation

In the early 1600s, an English physician named William Harvey did something unusual for his time. Instead of accepting what the old textbooks said, he decided to count.

The teaching of that era, inherited from the ancient physician Galen, was that blood was manufactured in the liver, carried outward to the organs, and there used up — consumed like fuel, never to return. Harvey looked at the heart and asked a simple question: how much blood does it push out with each beat? He multiplied that amount by the number of beats in an hour and arrived at a figure so large that no liver on earth could have produced it. The blood, he realised, could not be made fresh each time. It had to be the same blood, going round and round.

He tested the idea on the human arm. Tying a band around it, he watched the veins swell, then pressed along a vein with his finger and showed that the blood inside would move only in one direction — towards the heart. Small valves inside the veins made sure of it. In 1628 he published his conclusion: the heart is a pump, and blood travels in a circle.

Diagram

Fig. 8.0 – William Harvey’s ligature demonstration on the forearm: a band tied above the elbow, swollen surface veins, a finger pressing along a vein, with arrows showing blood moving only towards the heart past the internal valves.

Curiosity Corner

Q1. Why does blood in our heart and veins flow in only one direction and not slide backwards? Q2. Why is the wall of the left ventricle so much thicker than the wall of the right ventricle? Q3. Oxygen is barely soluble in water, so how does enough of it travel in our blood to keep every cell alive? Q4. A heart removed from the body can keep beating for a while. If the brain is not sending the command, who is setting the beat? Q5. Why does a person working near a running car engine in a closed garage suddenly feel dizzy and faint, even though there is plenty of air in the room?

Each of these questions will be answered as you work through this chapter — watch for them as you read.

8.1 Organs of Respiratory Exchange

Why Every Cell Must Respire

Every living organism — a bacterium in pond water, a mango tree, a human being — needs a constant supply of energy to run its life processes. Growth, movement, transport across membranes, repair of tissues and even thinking all draw on this energy.

The body does store energy, but it stores it in a locked-up form: complex organic compounds such as carbohydrates, fats and proteins. This is potential energy — present, but not directly usable. A cell cannot spend a molecule of glucose the way it spends a molecule of ATP.

So the stored energy must first be converted into a usable currency. That conversion is exactly what respiration does. It unlocks the chemical energy of food, step by step, and repackages it as ATP, the form every cell can actually spend.

Definition

(Respiration is a biochemical process in which organic compounds are oxidised in an orderly, step-by-step manner so that the chemical energy locked in them is liberated in the form of ATP.)

Notice the words “in an orderly manner.” Oxidation of glucose can be summarised as a single overall reaction:

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 38\ \text{ATP}\]

Here \(C_6H_{12}O_6\) is glucose, \(O_2\) is oxygen taken in from the environment, \(CO_2\) is the carbon dioxide released, \(H_2O\) is metabolic water, and 38 ATP is the useful energy trapped by the cell from one molecule of glucose. But the cell never carries this out in one leap — it releases the energy gradually, in many small steps, so that it can be captured rather than wasted.

Try Yourself

What would happen if respiration took place in one single step, instead of many small ones?

The Respiratory Surface

For respiration to continue, oxygen must keep entering the body and carbon dioxide must keep leaving it. This two-way traffic of gases between the organism and its environment is called gaseous exchange.

Gaseous exchange does not happen anywhere on the body — it happens at a particular site. The part of the body where gases actually pass between the organism and its surroundings is called the respiratory surface.

Definition

(The respiratory surface is the site on or in the body of an organism where gaseous exchange takes place between the organism and its environment.)

Respiratory exchange is a simple physical process — it is driven by diffusion, not by any pumping or active carrier. Gases simply move from where they are in higher concentration to where they are in lower concentration. Because it depends on diffusion, the surface must be built in a way that makes diffusion fast and easy.

For efficient gaseous exchange, the respiratory surface should have the following features:

It should have a large surface area.
It should be thin, highly vascular and permeable to allow exchange of gases.
It should be moist.

Each feature has a clear reason behind it. A large area means many gas molecules can cross at the same time. A thin, permeable wall means the distance a gas molecule has to travel is short. Being highly vascular — richly supplied with blood vessels — means the gas that crosses is carried away at once, so the difference in concentration across the surface is maintained. And moisture matters because gases must first dissolve in a film of water before they can diffuse across a living membrane.

Structure and Function

The respiratory surface (thin, moist, permeable wall → gases dissolve and cross a very short distance quickly → rapid diffusion; large surface area → many molecules cross at once → high rate of exchange; rich blood supply → gas is carried away immediately → the concentration gradient is never lost)

Memory Aid

The three features of a respiratory surface Think L-T-M: Large (surface area), Thin (and vascular/permeable), Moist.

Gaseous Exchange in Plants

Plants have no lungs, no gills and no blood to carry gases — and yet every living cell of a plant respires. Their shape and structure themselves make diffusion possible.

A terrestrial flowering plant has many air spaces between the cells of stem, leaf and root, and these air spaces are continuous with one another. This means the interior of the plant is not a solid block of tissue; it is a connected network of air channels reaching almost every cell. Exchange of gases takes place between the air in these spaces and the surrounding tissue.

Oxygen diffuses into the air space through stomata — the pores present on leaves and young stems — while carbon dioxide and water vapour diffuse out through the same pores. The pathway is shown in FC 8.1.

Flowchart

FC 8.1 – Path of oxygen into a leaf cell: atmospheric air → stoma (stomatal pore) → continuous intercellular air spaces → moist cell wall → cell membrane → cytoplasm of the living cell. (Carbon dioxide and water vapour travel the same route in reverse.)

Roots respire too. In aerated soil — that is, soil with air spaces between its particles — oxygen dissolves in the film of moisture or water around the root tissue and enters the root by diffusion. Notice that even here the rule holds: the gas dissolves first, then crosses.

Woody flowering plants such as trees and shrubs face a special problem. Their stems are covered by an external impervious bark, and gases cannot pass through it. In these plants, gaseous exchange occurs through small pores in the stem surface called lenticels, as shown in Fig. 8.1.

Diagram

Fig. 8.1 – Organs of gaseous exchange in plants: (a) leaf surface with a magnified stomatal pore showing guard cells and stoma; (b) a woody stem in section showing bark with lenticels.

Real-World Application

Farmers know that a waterlogged field can kill a crop even though the plants are surrounded by water. When soil is flooded, water fills the air spaces between the soil particles and pushes the air out. The roots can no longer obtain enough oxygen by diffusion, respiration in the root cells slows down, and the roots begin to rot. This is why fields are drained, and why an over-watered potted plant wilts instead of thriving.

Try Yourself

What are the main features of a respiratory surface?
Which are the parts of a plant that help in the process of gaseous exchange?

Respiration in Animals

Compared with plants, animals show a wide variety of respiratory surfaces and respiratory organs. The reason lies in the way animals live: they are usually more active, they use energy faster, and their bodies are more thickly built, so simple diffusion across the body surface is not always enough.

In animals, the respiratory organ that develops depends on two things — the complexity of the body’s organisation, and the medium in which the animal lives (water, air, or soil). Certain parts of the body have become specialised into different types of respiratory organs to suit these conditions.

In the higher animals, these respiratory organs are also associated with a transport system. This is an important link: the respiratory organ only brings oxygen into the body: something else must then carry it to the deeply placed cells. That “something” is the circulatory system, which we take up later in this chapter.

Table 8.2 sets out the respiratory surface or organ found in different groups of organisms, along with the habitat in which each lives.

Table 8.2 : Respiratory surface / organ in organisms

Organism	Habitat	Respiratory surface / organ
Protists, Sponges and Coelenterates	Aquatic	Plasma membrane
Flatworms like Planaria, Annelids (earthworm, nereis, leech), amphibians (frog)	Aquatic or semiaquatic	Plasma membrane, general body surface (moist skin)
Insects	Terrestrial	Tracheal tubes and spiracles
Arachnids like spiders and scorpions	Terrestrial	Book lungs
Limulus (Arthropod)	Aquatic	Book gills
Amphibian tadpoles of frog, salamanders and newts	Aquatic	External gills
Fish	Aquatic	Internal gills
Reptiles, Birds and Mammals	Terrestrial	Lungs
Turtles	Underwater	Cloaca

Read the table as a progression rather than a list. In the simplest, smallest organisms the plasma membrane itself is the respiratory surface, because no cell is far from the water outside. As bodies grow larger and thicker, the surface must be folded, branched or pouched into a specialised organ — gills in water, tracheal tubes or lungs on land — so that a large area is packed into a small space.

Zoom In / Out

The respiratory surface (Zoom in: at the finest level, every respiratory surface is just a thin, moist plasma membrane across which gases diffuse. Zoom out: in larger animals that same membrane is folded and multiplied into a tissue, then built into a whole organ — a gill, a tracheal tube, a lung — and finally linked to an organ system, the circulatory system, that distributes the gas onwards.)

Do Not Mix Up

Respiratory surface vs respiratory organ (The respiratory surface is the actual membrane across which gases diffuse; the respiratory organ is the whole structure that houses and protects that surface. In us, the alveolar membrane is the surface, while the lung is the organ.)

Exam Corner

Q. Why can a large, active animal not carry out respiration without the help of a circulatory system?

A. Diffusion is fast only over very short distances. In a small, thin-bodied animal, oxygen can diffuse from the surface to every cell in time. In a large animal, the innermost cells are centimetres away from the respiratory surface, and diffusion alone would take far too long to supply them. A circulatory system solves this by carrying the gas in a moving fluid, so no cell is ever far from a supply line.

8.2 Human Respiratory System

The human respiratory system does three jobs: it brings air in (inspiration), it pushes air out (expiration), and it allows the exchange of gases in the lungs. Once the exchange has happened, the gases are transported by the blood — from the lungs to the different tissues and parts of the body, and back again.

For convenience, the system is divided into two parts. The upper respiratory system consists of the external nares, the nasal cavities, the internal nares, the nasopharynx, the nose, the throat and associated structures. The lower respiratory system refers to the larynx, trachea, bronchi, bronchioles and lungs.

There is a logic to this division. The upper part is mainly a conditioning and conducting zone — it cleans, warms and moistens the air. The lower part carries that prepared air deeper, and ends in the alveoli where the actual exchange takes place. The whole route is shown in Fig. 8.3 and summarised in FC 8.2.

Diagram

Fig. 8.3 – Human respiratory system: label nasal cavity, nostril, mouth, pharynx, larynx, trachea, right main bronchus, right lung, left lung, ribs, diaphragm; with an inset showing an alveolus and an air sac wrapped in blood capillaries.

Flowchart

FC 8.2 – Path of inhaled air: external nares (nostrils) → nasal cavity → internal nares → nasopharynx → oropharynx → laryngopharynx → larynx (through the glottis) → trachea → right and left primary bronchi → secondary bronchi → tertiary bronchi → terminal bronchioles → alveoli (site of gaseous exchange).

Nose

The nose has a pair of slit-like openings called the external nares or nostrils, through which air enters the nasal cavity. The nasal cavity is divided into a right and a left nasal chamber by a partition of cartilage called the mesethmoid cartilage.

Each nasal chamber is further divided into three regions, and each region has a different job to do:

Vestibule: This is the proximal part, just inside the nostrils. Its skin bears hair, which filters the air by trapping dust and suspended particles present in the inhaled air.
Respiratory part (conditioner): This is the middle part. It is thin-walled and highly vascular — rich in blood vessels — and it warms and moistens the inhaled air.
Olfactory or sensory chamber: This uppermost part is lined by olfactory epithelium, which detects smell.

So the nose is not merely a hole for air. It filters, warms, moistens and smells the air before it goes any further — and each of these functions protects the delicate lung tissue lying below.

Structure and Function

The nose (hairs in the vestibule → trap dust and particles → the air reaching the lungs is filtered; thin, highly vascular middle region → blood close to the surface gives up heat and moisture → the air is warmed to body temperature and made humid, so the delicate alveolar membrane is never dried or chilled; olfactory epithelium above → detects odour molecules → warns us of smoke, gas leaks and spoiled food)

Try Yourself

Kavya underwent a surgical procedure called rhinoplasty. What could have been the reason for such a surgery? On which part of the body is it carried out?

Pharynx

The pharynx is the common chamber lying behind the nose and mouth. It is divisible into three parts.

The nasopharynx is the uppermost part. Air coming from the nasal chamber enters it, and from there it leads into the oropharynx, which is the common passage for food and air. Below the oropharynx the passage continues as the laryngopharynx.

Between the nasopharynx and the oropharynx lies the palate bone, which separates the nasal route above from the mouth below. The pharynx also carries a set of lymphoid organs called tonsils, which guard this crossroads against germs entering with food and with air.

Try Yourself

What is the role of tonsils in our body? How many pairs of tonsils do we have?

Larynx

The larynx is called the voice box. It is the part of the respiratory tract that contains the vocal cords, which produce sound. It extends from the laryngopharynx and the hyoid bone down to the trachea, as shown in Fig. 8.4.

Diagram

Fig. 8.4 – Larynx: label hyoid bone, epiglottis, thyrohyoid membrane, thyroid cartilage, laryngeal prominence (Adam’s apple), tracheal cartilage.

The larynx is a hollow, tubular structure. Its wall is made up of cartilage plates held together by membranes and muscles. Internally, it is lined by a pair of folds of elastic vocal cords, called the true vocal cords.

Voice is produced when air passes between the vocal cords, setting them vibrating. The sound thus produced is then modulated — shaped into recognisable speech — by the tongue, teeth, lips and nasal cavity. This is why a blocked nose or a swollen throat changes the way a person sounds.

The larynx opens into the laryngopharynx through a slit-like opening called the glottis. This opening of the trachea, or wind pipe, is guarded by a leaf-like flap called the epiglottis, which prevents the entry of food into the trachea.

Exam Corner

Q. The oropharynx is a common passage for food and air. Why, then, does food not normally slip into the trachea?

A. Because of the epiglottis. During swallowing, this leaf-like flap folds down over the glottis and seals the entrance to the trachea, so the food bolus is directed into the oesophagus behind it. When we talk or laugh while eating, the flap may not close in time — and the food that slips towards the trachea sets off a violent cough, the body’s way of throwing it back out.

Real-World Application

Choking happens when a piece of food does get past the glottis and lodges in the airway. The person cannot speak or cough, and the airway must be cleared at once — usually by firm back blows between the shoulder blades or by abdominal thrusts (the Heimlich manoeuvre), followed by immediate medical help. The urgency is easy to understand once you know that the trachea is the only route air has to the lungs.

Trachea (Wind Pipe)

The trachea is a long tube, 10 to 12 cm in length. It runs down through the neck in front of the oesophagus and extends into the thoracic cavity, reaching up to its middle.

The trachea is supported by 16 to 20 ‘C’-shaped rings of cartilage. These rings are the reason the trachea does not collapse. Air is drawn in by lowering the pressure inside the airway; without a stiff support, the soft tube would simply cave in each time we breathed in — much as a thin straw collapses when you suck too hard.

The reason the rings are ‘C’-shaped rather than complete circles is equally practical. The gap in the ‘C’ faces backwards, towards the oesophagus, so that when a bolus of food passes down the oesophagus it can bulge into that soft gap without being blocked by hard cartilage.

Internally, the trachea is lined with ciliated, pseudostratified epithelium and mucous glands. The mucus traps unwanted particles that have escaped the nose, and the cilia sweep this mucus upwards, preventing the particles from entering the lungs.

Structure and Function

Trachea (C-shaped cartilage rings → keep the tube permanently open against falling pressure → uninterrupted airflow; the open gap of the C faces the oesophagus → allows food to bulge past → swallowing is not obstructed; mucus + cilia lining → trap dust and sweep it upward → the lungs stay clean)

Bronchi

As the trachea reaches the middle of the thoracic cavity, it divides into the right and left primary bronchi — one for each lung. Like the trachea, the bronchi are supported internally by ‘C’-shaped incomplete rings of cartilage.

The primary bronchi divide to form secondary and then tertiary bronchi, which lead into terminal bronchioles, and these finally end in alveoli. Notice the pattern: the tubes get narrower but far more numerous at every division, like the branches of an inverted tree. This repeated branching is exactly what allows a huge exchange surface to be packed into a chest cavity of ordinary size.

Lungs

The lungs are the main respiratory organs of humans. One pair of spongy and elastic lungs is present in the thoracic cavity.

Each lung is enclosed and protected by a double pleural membrane — an outer parietal membrane and an inner visceral membrane. Between the two pleura lies a pleural cavity filled with a lubricating fluid called pleural fluid, which is secreted by the membranes themselves. This fluid lets the two layers glide over each other smoothly, so the lungs are not damaged by friction as they expand and shrink thousands of times a day.

The two lungs are not identical. The right lung is larger and is divided into 3 lobes, while the left lung is smaller and is divided into 2 lobes — the left one gives up space to the heart, which lies tilted towards that side.

Each lobe of the lung has terminal bronchioles ending in a bunch of air sacs, and each air sac carries 10 to 12 alveoli.

Try Yourself

What is the importance of pleural fluid?
Find out the total surface area covered by the alveoli.

Alveoli

The alveoli are thin-walled, lobulated structures, arranged like a bunch of grapes at the end of each bronchiole. Each alveolus is surrounded by a network of capillaries of the pulmonary arteries and veins.

The wall of an alveolus is highly elastic and is made up of a single layer of squamous epithelium resting on a basement membrane of connective tissue. A single flattened layer is the thinnest wall a living structure can have — and thinness is precisely what gases need in order to diffuse quickly.

There are about 700 million alveoli in the lungs, and it is they that provide the surface area for the exchange of gases. Individually each alveolus is microscopic; together, they add up to an enormous area folded into the chest.

Structure and Function

Alveolus (about 700 million alveoli → an enormous total surface area in a small chest → many gas molecules cross at once; wall of a single squamous epithelial layer → a very short diffusion distance → rapid exchange; wrapped in a dense capillary network → oxygen is carried away the instant it crosses → the gradient is maintained; elastic wall → the alveolus recoils after stretching → helps push air out during expiration)

Zoom In / Out

Alveolus (Zoom out: bunches of alveoli make up an air sac, air sacs make up a lobe, lobes make up the lung — the organ — which with the airways forms the respiratory system. Zoom in: the alveolar wall is a single layer of squamous epithelial cells, and each cell’s plasma membrane is the actual barrier the gas molecules cross.)

Do Not Mix Up

Bronchiole vs alveolus (A terminal bronchiole is a conducting tube — air merely passes through it and no exchange occurs there. An alveolus is the exchange chamber itself — the only place in the lung where gases actually cross into the blood.)

Diaphragm

The diaphragm is a muscular septum that separates the thoracic cavity from the abdominal cavity. At rest it is dome-shaped, curving upwards into the chest, and on contraction it becomes flattened.

That single change of shape is the engine of breathing. When the dome flattens, it drops downward and the thoracic cavity becomes larger — and, as we shall see in the next section, a larger cavity means a lower pressure, and lower pressure draws air in.

8.3 Mechanism of Respiration

Respiration, taken as a whole, is a biological process involving the exchange of gases between the atmosphere and the lungs, and it ends in the formation of ATP. It is not a single event but a chain of four linked processes:

A. Breathing B. External respiration C. Internal respiration D. Cellular respiration

Read that list as a journey. Breathing moves air in and out of the lungs; external respiration moves gases between the alveolar air and the blood; internal respiration is the transport of those gases in the blood and their exchange at the tissues; and cellular respiration is the final oxidation inside the cell, where the ATP is actually made. If any one link fails, the cells at the end of the chain starve.

A. Breathing

Breathing is a physical process by which gaseous exchange takes place between the atmosphere and the lungs. It involves two steps — inspiration and expiration — shown in Fig. 8.5.

Diagram

Fig. 8.5 – Breathing: (a) Inspiration — rib cage moves up and forward, diaphragm contracts and moves down, pressure in lungs decreases and air rushes in; (b) Expiration — rib cage moves down and inward, diaphragm relaxes and moves up, air is pushed out.

Both steps involve the same set of structures: parts of the thoracic cage — the ribs and the sternum — together with the intercostal muscles and the muscles of the diaphragm. Note that the lungs themselves have no muscle of their own. They cannot pull air in; they are pulled open by the cage around them.

Inspiration

During inspiration, atmospheric air is taken into the lungs. It occurs because of a pressure gradient formed between the lungs and the atmosphere.

Inspiration is an active process — it costs energy, because muscles must contract. The diaphragm contracts, becomes flat and moves downward. At the same time the external intercostal muscles contract, so the ribs and the sternum move upward and outward.

Both movements enlarge the chest. This leads to an increase in the thoracic volume and therefore a decrease in the pressure of the thorax and the lungs. To equalise this low pressure inside the lungs, air from the atmosphere rushes into the lungs. This is inspiration.

Flowchart

FC 8.3 – Inspiration: diaphragm contracts and flattens + external intercostal muscles contract → ribs and sternum move up and out → thoracic volume increases → pressure in thorax and lungs decreases below atmospheric pressure → air rushes in from the atmosphere → lungs inflate.

Expiration

During expiration, the thorax contracts, causing air to be exhaled.

The diaphragm relaxes and is pushed upwards, becoming dome-shaped again. The intercostal muscles also relax, pulling the rib cage inward and downward. This causes a decrease in thoracic volume and leads to an increase in pressure in the thorax and the lungs, as compared to the atmospheric pressure. So air from the lungs rushes out. This is expiration.

Because it is chiefly a relaxation — helped by the elastic recoil of the stretched lung tissue — quiet expiration is a passive process, unlike inspiration.

Flowchart

FC 8.4 – Expiration: diaphragm relaxes and returns to its dome shape + intercostal muscles relax → rib cage moves down and in → thoracic volume decreases → pressure in thorax and lungs rises above atmospheric pressure → air rushes out of the lungs.

One inspiration and one expiration together make one breath.

Compare Table

Exam Corner

Q. Air moves in and out of the lungs — but what actually makes it move?

A. Only a pressure difference. Air, like any gas, flows from higher pressure to lower pressure. The muscles never touch the air; they only change the size of the chest, which changes the pressure inside it, and the air follows that gradient. This is why a hole in the chest wall (which lets outside air into the pleural cavity) can stop a lung from inflating even though the muscles are working perfectly.

Try Yourself

Count the number of breaths you take (a) after a good night’s sleep, (b) during a vigorous activity such as running or climbing stairs, and (c) after the vigorous activity. Do you find any difference in the count?

Pulmonary Volumes and Capacities (Normal Values)

Not all the air in the lungs is exchanged with every breath. To describe how much air moves and how much stays behind, physiologists use a set of standard volumes and capacities. A volume is a single quantity of air; a capacity is the sum of two or more volumes.

Lung Volumes

Tidal volume (T.V.): It is the volume of air inspired or expired during normal breathing. It is 500 ml.

Inspiratory reserve volume (IRV): The maximum volume of air, or the extra volume of air, that is inspired during forced breathing in addition to T.V. Its value is 2000 to 3000 ml.

Expiratory reserve volume (ERV): The maximum volume of air that is expired during forced breathing after normal expiration. Its value is 1000 to 1100 ml.

Dead space (DS): The volume of air that is present in the respiratory tract (from the nose to the terminal bronchioles), but not involved in gaseous exchange. It is 150 ml. This air is sitting in the conducting tubes, where there are no alveoli — so it never meets a blood capillary.

Residual volume (RV): The volume of air that remains in the lungs and the dead space even after maximum expiration. It is 1100 to 1200 ml. Because of this residual air, the alveoli never fully collapse, and gas exchange can continue even between two breaths.

Lung Capacities

Total Lung capacity: The maximum amount of air that the lungs can hold after a maximum forceful inspiration (5200 to 5800 ml).

Vital capacity (VC): The maximum amount of air that can be breathed out after a maximum inspiration. It is the sum total of TV, IRV and ERV, and is 4100 to 4600 ml.

\[\text{VC} = \text{TV} + \text{IRV} + \text{ERV}\]

Do Not Mix Up

Vital capacity vs Total lung capacity (Vital capacity is the air a person can actually move out with the deepest possible breath. Total lung capacity is vital capacity plus the residual volume — the air that can never be expelled. So total lung capacity is always the larger of the two, and vital capacity can be measured, while residual volume cannot be blown out and measured directly.)

Numerical 8.1

For a healthy adult, tidal volume is 500 ml, inspiratory reserve volume is 3000 ml, expiratory reserve volume is 1100 ml and residual volume is 1200 ml. Calculate the person’s vital capacity and total lung capacity.

Real-World Application

A spirometer, the instrument that measures these volumes, is used in hospitals to test lung function. In asthma and in emphysema — both discussed later in this chapter — the amount of air a patient can force out is reduced, so the readings fall well below the normal range even when the person feels reasonably well. This is why doctors follow spirometry values over time rather than relying only on how breathless a patient reports feeling.

B. External Respiration / Exchange of Gases at the Alveolar Level

An alveolus consists of a layer of simple squamous epithelium resting on a basement membrane, and it is intimately associated with a dense network of capillaries. The capillary wall is also made up of simple squamous epithelium resting on a thin basement membrane.

Both these layers have a similar structure and are thin-walled. Together they make up the respiratory membrane, through which gaseous exchange occurs — that is, between the alveolar air and the blood. Fig. 8.6 shows this arrangement.

Diagram

Fig. 8.6 – Exchange of gases between alveolus and capillary: label alveolus, alveolar membrane, respiratory membrane, surface fluid, capillary, blood from pulmonary artery, blood going to pulmonary vein; arrows showing O₂ diffusing into red blood cells and CO₂ diffusing into the alveolus.

Definition

(The respiratory membrane is the combined thin barrier — alveolar epithelium, its basement membrane, and the capillary epithelium with its basement membrane — across which gases are exchanged between the alveolar air and the blood.)

Why the Gases Move

Diffusion of gases will take place from an area of higher partial pressure (pp) to an area of lower partial pressure, and it continues until the partial pressure in the two regions reaches equilibrium.

Partial pressure is simply the pressure contributed by one particular gas in a mixture of gases. It is the “pushing power” of that one gas, and it is what decides the direction in which the gas will diffuse — not the total pressure of the mixture.

The partial pressure of carbon dioxide (\(pCO_2\)) of blood entering the pulmonary capillaries is 45 mmHg, while the partial pressure of carbon dioxide in alveolar air is 40 mmHg. Due to this difference, carbon dioxide diffuses from the capillaries into the alveolus.

Similarly, the partial pressure of oxygen (\(pO_2\)) of blood in the pulmonary capillaries is 40 mmHg, while in alveolar air it is 104 mmHg. Due to this difference, oxygen diffuses from the alveoli into the capillaries.

Look at the two gradients side by side and one thing stands out: they point in opposite directions, and they do so at the same place, at the same moment. That is why one single membrane can load oxygen and unload carbon dioxide simultaneously.

Compare Table

Exam Corner

Q. Why is gas exchange in the alveolar region so very rapid?

A. Four reasons act together. The respiratory membrane is extremely thin, so the diffusion distance is minimal. The total alveolar surface area is enormous. The blood in the capillary is spread out in a very thin film, so almost every red cell touches the wall. And the steep partial-pressure gradients are constantly renewed — fresh air keeps arriving by breathing, and fresh blood keeps arriving by circulation — so the gradient never runs down.

C. Internal Respiration

The gases have now entered the blood. They must next be carried to the tissues and released there. The two main components of blood involved in the transport of the respiratory gases — \(CO_2\) and \(O_2\) — are the RBCs and the plasma.

i. Transport of Oxygen

Of the total oxygen transported, only 3% is transported in a dissolved state by the plasma. The remaining 97% is bound to the haemoglobin (Hb) present in the RBCs.

That 3% figure tells us something important. Oxygen dissolves very poorly in water, and plasma is mostly water — so if the blood had to rely on dissolved oxygen alone, it could never supply an active human body. Haemoglobin is the solution to that problem.

Curiosity Corner

Q3. Oxygen is barely soluble in water, so how does enough of it travel in our blood to keep every cell alive?

A. Because the blood does not depend on dissolved oxygen. Only 3% of the oxygen travels dissolved in the plasma; the other 97% is picked up by haemoglobin inside the red blood cells and carried as oxyhaemoglobin. Haemoglobin acts as a chemical carrier that lets blood hold far more oxygen than plain water ever could.

Haemoglobin acts as the respiratory carrier. It has a high affinity for \(O_2\) and combines with it to form oxyhaemoglobin. Theoretically, one molecule of Hb has 4 \(Fe^{++}\), each of which can pick up a molecule of oxygen (\(O_2\)).

\[Hb + 4O_2 \rightarrow Hb(4O_2)\]

Oxyhaemoglobin is transported from the lungs to the tissues, where it readily dissociates to release \(O_2\).

\[Hb(4O_2) \rightarrow Hb + 4O_2\]

Notice that both reactions involve the same molecule. Haemoglobin’s usefulness lies not in binding oxygen tightly, but in binding it where oxygen is plentiful and letting go of it where oxygen is scarce. A carrier that never released its cargo would be of no use at all.

The Oxygen Dissociation Curve

However, the degree of saturation of Hb with \(O_2\) depends upon the \(O_2\) tension, i.e. \(ppO_2\).

100% saturation is rare.
Maximum saturation of 95 to 97% is at the \(ppO_2\) in the alveoli (100 mmHg).
The degree of saturation decreases with the drop in \(ppO_2\). This begins the dissociation of \(HbO_2\).
At 30 mmHg of \(ppO_2\), only 50% saturation can be maintained.
The relationship between \(HbO_2\) saturation and oxygen tension (\(ppO_2\)) is called the oxygen dissociation curve. This oxygen–haemoglobin dissociation curve is a sigmoid curve, and it shifts towards the right due to — an increase in \(H^+\) concentration, an increase in \(ppCO_2\), a rise in temperature, and a rise in DPG (2,3-diphosphoglycerate), formed in the RBCs during glycolysis. A right shift lowers the affinity of haemoglobin for oxygen.

The curve is shown in Fig. 8.7.

Diagram

Fig. 8.7 – Oxyhaemoglobin dissociation curve (Bohr effect curves): X-axis, oxygen tension ppO₂ of blood (mm Hg), 0 to 100; Y-axis, Hb % O₂ saturation, 0 to 100; three sigmoid curves marked high CO₂ (low pH), normal CO₂ (normal pH) and low CO₂ (high pH); mark the “body tissues” region on the left of the X-axis and the “lung alveoli” region on the right.

Interpret Data

The oxyhaemoglobin dissociation curve (The X-axis gives the oxygen tension of the blood, and the Y-axis gives the percentage of haemoglobin that is saturated with oxygen. Read the right-hand end first: at the alveolar \(ppO_2\) of 100 mmHg, saturation is 95–97% — haemoglobin loads almost fully in the lungs. Now read the left-hand end: at 30 mmHg, saturation has fallen to about 50% — so haemoglobin has released roughly half its oxygen. The curve’s S-shape means that in the middle range, a small fall in \(ppO_2\) causes a large release of oxygen. That is exactly the range found in respiring tissues, so the shape itself makes haemoglobin unload generously precisely where oxygen is needed. Now compare the three curves: the “high CO₂, low pH” curve lies to the right of the others, meaning that at any given \(ppO_2\) the saturation is lower — the haemoglobin is giving up more oxygen. An active muscle produces exactly that condition, and so it automatically receives more oxygen.)

Principle / Law

(Bohr effect — The shift of the oxyhaemoglobin dissociation curve due to a change in the partial pressure of \(CO_2\) in the blood.)

Principle / Law

(Haldane effect — Oxyhaemoglobin functions as an acid. It decreases the pH of blood. Due to the increase in the number of \(H^+\) ions, \(HCO_3^-\) changes into \(H_2O\) and \(CO_2\).)

In the alveoli, where \(ppO_2\) is high and \(ppCO_2\) is low, oxygen binds with haemoglobin. But in the tissues, where \(ppO_2\) is lower and \(ppCO_2\) is high, haemoglobin does not hold as much \(O_2\). It releases \(O_2\) for diffusion into the tissue cells.

So the two effects work as a pair: the presence of carbon dioxide in a tissue helps oxygen to be released there, and the presence of oxygen in the lung helps carbon dioxide to be released there. Each gas assists the unloading of the other, exactly where that unloading is needed.

Do Not Mix Up

Bohr effect vs Haldane effect (The Bohr effect describes how carbon dioxide (and the acidity it produces) affects oxygen carriage — it makes haemoglobin release oxygen at the tissues. The Haldane effect works the other way — oxygenation of haemoglobin affects carbon dioxide carriage, driving \(CO_2\) out of the blood at the lungs. Remember: Bohr → effect on \(O_2\); Haldane → effect on \(CO_2\).)

Carbon Monoxide Poisoning

Side Note

The affinity of haemoglobin for carbon monoxide is about 250 times more than for oxygen. In the presence of carbon monoxide, haemoglobin readily combines with it to form a stable compound, carboxyhaemoglobin. The haemoglobin is thus blocked by carbon monoxide, which prevents oxygen from binding with it. As a result, less haemoglobin is available for oxygen transport, and the cells are deprived of oxygen. This is carbon monoxide poisoning.

Curiosity Corner

Q5. Why does a person working near a running car engine in a closed garage suddenly feel dizzy and faint, even though there is plenty of air in the room?

A. A running engine gives off carbon monoxide. Haemoglobin binds carbon monoxide about 250 times more readily than oxygen, and the carboxyhaemoglobin formed is stable, so it does not let go. The blood may still be full of haemoglobin and the room still full of air, but very little of that haemoglobin is now free to carry oxygen. The brain, which is highly sensitive to oxygen shortage, is affected first — hence the dizziness and fainting.

ii. Transport of \(CO_2\)

Carbon dioxide is readily soluble in water and is transported by the RBCs and the plasma in three different forms.

a. By plasma in solution form (7%): Only 7% of \(CO_2\) is transported in a dissolved form as carbonic acid, which can break down into \(CO_2\) and \(H_2O\).

\[CO_2 + H_2O \rightleftharpoons H_2CO_3\]

b. By bicarbonate ions (70%): Nearly 70% of the carbon dioxide released by the tissue cells diffuses into the plasma and then into the RBCs. What happens next is the main route by which our body carries \(CO_2\), and it is worth following step by step.

In the RBCs, \(CO_2\) combines with water in the presence of a Zn-containing enzyme, carbonic anhydrase, to form carbonic acid.
Carbonic anhydrase enzyme is found in the RBCs and not in the plasma.
The rate of formation of carbonic acid inside the RBC is therefore very high as compared to its formation in the plasma.
Carbonic acid, being unstable, almost immediately dissociates into \(HCO_3^-\) and \(H^+\) in the presence of the enzyme carbonic anhydrase (CA), leading to a large accumulation of \(HCO_3^-\) inside the RBCs.

\[CO_2 + H_2O \xrightleftharpoons[\text{CA}]{\text{CA}} H_2CO_3 \xrightleftharpoons[\text{CA}]{\text{CA}} H^+ + HCO_3^-\]

The \(HCO_3^-\) thus moves out of the RBCs. This would bring about an imbalance of the charge inside the RBCs — negative ions are leaving, so the inside would be left too positive. To maintain the ionic balance between the RBCs and the plasma, \(Cl^-\) diffuses into the RBCs. This movement of chloride ions is known as chloride shift or Hamburger’s phenomenon.
\(HCO_3^-\) that comes into the plasma joins with \(Na^+\) / \(K^+\), forming \(NaHCO_3\) / \(KHCO_3\) (to maintain the pH of the blood).

\[HCO_3^- + Na^+ \rightarrow NaHCO_3 \quad (\text{Sodium bicarbonate})\]

\(H^+\) is taken up by protein (haemoglobin).

\[Hb + H^+ \rightarrow HHb \quad (\text{Reduced Hb})\]

These \(H^+\) ions might be expected to lower the blood pH, but they are buffered by haemoglobin through the formation of deoxyhaemoglobin (reduced haemoglobin). This is why carrying large amounts of an acidic gas does not turn our blood acidic.

At the level of the lungs, in response to the low partial pressure of carbon dioxide (\(ppCO_2\)) of the alveolar air, hydrogen ions and bicarbonate ions recombine to form carbonic acid, and under the influence of carbonic anhydrase this yields carbon dioxide and water.

\[H^+ + HCO_3^- \xrightleftharpoons[\text{CA}]{\text{CA}} H_2CO_3 \xrightleftharpoons[\text{CA}]{\text{CA}} CO_2 + H_2O\]

The whole sequence runs one way at the tissue and exactly in reverse at the lung, as Fig. 8.8 and Fig. 8.9 show. This reversibility is the key idea: the same enzyme, the same reaction, the same ions — only the local partial pressures decide which direction the chemistry will run.

Diagram

Fig. 8.8 – Oxygen release and carbon dioxide pickup at the tissue: tissue cell, interstitial fluid, red blood cell and blood plasma; show CO₂ entering, the slow reaction in plasma and the fast carbonic-anhydrase reaction in the RBC, HCO₃⁻ moving out, Cl⁻ shifting in, HHb formation, and O₂ being released from HbO₂.

Diagram

Fig. 8.9 – Oxygen pickup and carbon dioxide release in the lungs: alveolus with fused basement membranes, red blood cell and blood plasma; show HCO₃⁻ re-entering the RBC, Cl⁻ shifting out, H₂CO₃ re-forming, CO₂ leaving into the alveolus, and O₂ binding HHb to form HbO₂.

Flowchart

FC 8.5 – Chloride shift at the tissue: CO₂ from tissue cell → plasma → into RBC → CO₂ + H₂O (carbonic anhydrase) → H₂CO₃ → H⁺ + HCO₃⁻ → HCO₃⁻ diffuses out into plasma → inside of RBC becomes electrically unbalanced → Cl⁻ diffuses into RBC (chloride shift / Hamburger’s phenomenon) → balance restored; meanwhile H⁺ is buffered by Hb → HHb, and HCO₃⁻ in plasma joins Na⁺/K⁺ → NaHCO₃/KHCO₃.

Flowchart

FC 8.6 – Reverse chloride shift at the lungs: low ppCO₂ in alveolar air → HCO₃⁻ moves from plasma back into the RBC and Cl⁻ moves out → HCO₃⁻ + H⁺ → H₂CO₃ (carbonic anhydrase) → CO₂ + H₂O → CO₂ diffuses out of the RBC → into the alveolus → exhaled.

c. By red blood cells (23%): Carbon dioxide binds with the amino group of the haemoglobin and forms a loosely bound compound, carbaminohaemoglobin. This molecule readily decomposes in the region where the partial pressure of carbon dioxide (\(ppCO_2\)) is low — that is, the alveolar region — releasing the carbon dioxide.

\[Hb + CO_2 \rightleftharpoons HbCO_2\]

The word “loosely” is doing real work in that sentence. A tight bond would trap the \(CO_2\) permanently; a loose one lets it be picked up in the tissue and dropped in the lung.

Compare Table

Trace It

One molecule of CO₂ from a leg muscle to the outside air 1. The muscle cell oxidises glucose and releases \(CO_2\), so \(ppCO_2\) in the cell rises above that in the blood. 2. The \(CO_2\) diffuses out of the cell, through the interstitial fluid, into the blood plasma of a nearby capillary. 3. Most of it (70%) diffuses on into a red blood cell, where carbonic anhydrase rapidly combines it with water to form carbonic acid. 4. The carbonic acid at once splits into \(H^+\) and \(HCO_3^-\). The \(HCO_3^-\) moves out into the plasma; \(Cl^-\) moves in to keep the charge balanced (chloride shift). The \(H^+\) is mopped up by haemoglobin as HHb. 5. Some of the \(CO_2\) (23%) instead binds to the amino groups of haemoglobin as carbaminohaemoglobin, and a small share (7%) simply stays dissolved in the plasma. 6. The blood travels through the veins to the right side of the heart, and from there to the lungs. 7. In the pulmonary capillary, the alveolar air has a low \(ppCO_2\). The whole chemistry now runs backwards: \(HCO_3^-\) re-enters the RBC (with \(Cl^-\) moving out), rejoins \(H^+\), and carbonic anhydrase converts it back to \(CO_2\) and water. 8. The \(CO_2\) diffuses across the respiratory membrane into the alveolus and leaves the body with the next expiration.

Exam Corner

Q. What is the role of haemoglobin in the transport of oxygen — and does it do anything else?

A. Haemoglobin carries 97% of the oxygen in the blood, binding it as oxyhaemoglobin where \(ppO_2\) is high and releasing it where \(ppO_2\) is low. But it is not an oxygen carrier alone: it also transports about 23% of the carbon dioxide as carbaminohaemoglobin, and it buffers the \(H^+\) ions produced during bicarbonate formation, keeping the pH of the blood steady. It is a three-in-one molecule.

D. Cellular Respiration

This is the last step, taking place inside the cell, where food is oxidised and ATP is generated. Everything discussed so far — breathing, alveolar exchange, blood transport — exists only to make this step possible. It can be shown by two steps.

a. Oxidation: The breaking down of complex organic molecules into simple inorganic molecules with the release of heat energy.

\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 686\ \text{Kcal}\]

b. Phosphorylation: This involves trapping the heat energy in the form of the high-energy bond of an ATP molecule. ATP is used to carry out vital life processes, and is therefore called the energy currency of the cell.

\[ADP + iP + 7.3\ \text{Kcal} \rightarrow ATP\]

Here ADP is adenosine diphosphate, iP is inorganic phosphate, and 7.3 Kcal is the energy stored in the new high-energy bond that is formed. Compare the two numbers: one molecule of glucose liberates 686 Kcal, while each ATP bond stores only 7.3 Kcal. This is precisely why oxidation must proceed in many small steps — a single explosive release of 686 Kcal could not be captured in these small packets, and most of it would be lost as heat.

Zoom In / Out

Respiration (Zoom out: breathing is an organ-system event, driven by the diaphragm, ribs and lungs working together with the circulatory system. Zoom in: the process ends inside a single cell, in fact inside a single mitochondrion, where oxygen finally accepts the electrons stripped from glucose and ATP is made. All the machinery of the chest exists to serve this molecular event.)

8.4 Regulation of Breathing

Breathing goes on whether we attend to it or not — while we sleep, while we read, while we run. Yet it is not fixed at one rate: it speeds up when we climb stairs and slows when we rest. Something must be measuring the body’s needs and adjusting the pump accordingly.

Respiration is under dual control: nervous and chemical. The nervous side sets and times the rhythm; the chemical side reports on the state of the blood and modifies that rhythm.

Human adults breathe about 12 times per minute, while a new-born breathes about 44 times per minute. A baby’s higher rate is not a defect — a small body loses heat and uses oxygen quickly relative to its size, so it must ventilate faster.

Normal breathing is an involuntary process. We do not have to remember to do it, which is exactly why we survive sleep and unconsciousness.

The Respiratory Centres

The steady rate of respiration is controlled by neurons located in the pons and the medulla. These are known as the respiratory centres. They regulate both the rate of breathing (how many breaths per minute) and its depth (how much air per breath).

Definition

(The respiratory centres are the groups of neurons in the pons and medulla oblongata that automatically regulate the rate and depth of breathing.)

The respiratory centre is divided into three groups:

The dorsal group of neurons in the medulla, which is the inspiratory centre.
The ventrolateral group of neurons in the medulla, which is the inspiratory and expiratory centre.
The pneumotaxic centre, located in the pons, which primarily limits inspiration, and is also involved in slow wave sleep and rapid eye movement sleep.

In addition, the apneustic centre in the medulla is antagonistic to the pneumotaxic centre — that is, the two work against each other, one prolonging inspiration and the other cutting it short. Between them they fix how long each breath lasts. The apneustic centre controls non-rapid eye movement sleep and wakefulness.

The arrangement of these centres and their connections to the breathing muscles is shown in Fig. 8.10.

Diagram

Fig. 8.10 – Regulation of breathing: label brain, breathing control centres, pons, medulla; nerve impulses travelling through the intercostal nerves and phrenic nerves to trigger contraction of the intercostal muscles and diaphragm; on the right, show the breathing control centres being stimulated by (a) CO₂ increase in the blood, acting directly, and (b) nerve impulses from O₂ sensors in the artery (aortic arch) indicating an O₂ decrease.

Exam Corner

Q. The centres sit in the medulla and pons — but the muscles that move air are in the chest. How does the command travel?

A. Through nerves. The phrenic nerves carry impulses from the medulla down to the diaphragm, and the intercostal nerves carry them to the intercostal muscles. Damage to the spinal cord high in the neck can cut these pathways, which is why such an injury may leave a person unable to breathe on their own even though the brain and the lungs are both intact.

The Hering–Breuer Reflex

The lungs also report back to the brain, and this feedback prevents them from being over-inflated.

During inspiration, when the lungs expand to a critical point, stretch receptors in the lung tissue are stimulated. Impulses are sent along the vagus nerves to the expiratory centre. The expiratory centre then sends out inhibitory impulses to the inspiratory centre — in effect, telling it to stop.

The inspiratory muscles then relax and expiration follows. As air leaves the lungs during expiration, the lungs are deflated and the stretch receptors are no longer stimulated. Thus the inspiratory centre is no longer inhibited, and a new respiration begins. These events are called the Hering–Breuer reflex.

Definition

(The Hering–Breuer reflex is the reflex, set off by stretch receptors in the inflating lungs, that inhibits the inspiratory centre and brings inspiration to an end. It controls the depth and rhythm of respiration and prevents the lungs from inflating to the point of bursting.)

Flowchart

FC 8.7 – Hering–Breuer reflex: inspiration begins → lungs expand to a critical point → stretch receptors stimulated → impulses along the vagus nerves to the expiratory centre → inhibitory impulses sent to the inspiratory centre → inspiratory muscles relax → expiration → lungs deflate → stretch receptors no longer stimulated → inhibition of the inspiratory centre is lifted → a new inspiration begins (cycle repeats).

This is a self-limiting loop, and it is worth recognising it as one. Each inspiration carries within it the signal that will end it. The result is a rhythm that needs no conscious supervision and cannot run away with itself.

Voluntary Control — and Its Limit

The respiratory centre has connections with the cerebral cortex, which means we can voluntarily change our pattern of breathing. We can hold a breath, breathe deliberately fast, or take one deep breath before diving.

This voluntary control is protective: it enables us to prevent water or irritating gases from entering the lungs. A swimmer holding her breath underwater, and a person shutting off a breath when passing through smoke, are both using it.

But the ability to stop breathing is limited by the build-up of carbon dioxide in the blood. Beyond a point, the chemical signal overrides the conscious one, and breathing resumes whether we wish it or not.

Exam Corner

Q. Why is it difficult to hold one’s breath beyond a limit?

A. Because holding the breath does not stop the cells from producing carbon dioxide. As the \(CO_2\) accumulates in the blood, it acts directly on the respiratory centres, which are highly sensitive to it, and the drive to breathe becomes stronger and stronger until it overpowers the voluntary command from the cortex. This safety mechanism is why a healthy person cannot hold their breath until they die — the reflex takes over first. Note also that the urgent signal is the rise in \(CO_2\), not the fall in \(O_2\): it is the waste gas, not the shortage of the useful one, that forces the next breath.

Real-World Application

This is the reason a patient with severe lung disease, whose blood carbon dioxide is chronically high, is given oxygen carefully and under supervision rather than in unlimited amounts. Doctors monitor blood gases precisely because breathing is driven by chemical feedback, not by willpower.

Do Not Mix Up

Nervous control vs chemical control of breathing (Nervous control means the respiratory centres in the pons and medulla generating and timing the basic rhythm, aided by the Hering–Breuer reflex from the stretch receptors. Chemical control means that rhythm being modified by the composition of the blood — chiefly a rise in \(CO_2\) acting directly on the centres, and a fall in \(O_2\) detected by sensors in the aortic arch. The nervous system sets the beat; the chemistry adjusts its speed.)

8.5 Modified Respiratory Movements

Not every movement of the chest is an ordinary breath. Some respiratory movements are different from the normal movements: they help to express emotion, or they clear the air passage of something that does not belong there.

Of these movements, some are reflexes — they happen automatically, without our permission, the moment a receptor in the airway is irritated. Others can be initiated voluntarily as well, for example coughing and yawning. This is why you can cough deliberately to clear your throat, and yet also cough helplessly when a crumb goes the wrong way.

Both kinds share a common design. In each case the ordinary rhythm of inspiration and expiration is briefly overridden, and air is either driven out with unusual force (to expel an irritant) or drawn in unusually deeply. The muscles are the same familiar ones — the diaphragm, the intercostals and the muscles of the abdominal wall — but the timing and the force are altered to suit the purpose.

Compare Table

Do Not Mix Up

Coughing vs Sneezing (Both are protective reflexes that blast air out, but they clear different regions. Coughing clears the lower respiratory tract — the trachea and bronchi — and the air is driven mainly through the mouth. Sneezing clears the upper tract — the nasal passage — and the air is driven mainly through the nose.)

Try Yourself

Find out information about the various modified respiratory movements and write it in a tabular form.
What is the significance of such movements?

8.6 Common Disorders of Respiratory System

The respiratory tract is one of the few places where the outside world is allowed deep inside the body. With every breath, air carrying dust, smoke, pollen, bacteria and viruses is drawn towards a membrane only one cell thick. The nose, the mucus and the cilia defend that membrane, but the defences can be overwhelmed — and when they are, disease follows.

Most respiratory disorders can be traced to one of three causes: infection by a pathogen, irritation by something inhaled, or an allergic overreaction of the body’s own defences. The table below shows a list of some common respiratory disorders, along with their symptoms, cause and treatment.

Table 8.11 : Common disorders of respiratory system

Disorder	Symptoms	Cause	Treatment
Emphysema	Breakdown of alveoli, shortness of breath	Smoking, air pollution	Quit smoking, avoid polluted air, administer oxygen to relieve symptoms
Chronic bronchitis	Coughing, shortness of breath	Smoking, air pollution	Quit smoking, avoid polluted air, if possible move to warmer, drier climate
Acute bronchitis	Inflammation of bronchi, shortness of breath, yellow mucous coughed up	Viruses and bacteria	If bacterial, take antibiotics, cough medicine, use vaporizer
Sinusitis	Inflammation of the sinuses, mucous discharge	Viruses and bacteria	If bacterial, take antibiotics and decongestants, use vaporizer
Laryngitis	Inflammation of larynx, vocal cords, sore throat, hoarseness of voice, mucous build up and cough	Viruses and bacteria	If bacterial, take antibiotics, cough medicines, voice rest, avoid irritants like smoke
Pneumonia	Inflammation of lungs ranging from mild to severe, cough and fever, shortness of breath, chills, sweating, chest pain, blood in mucous	Bacteria, viruses	Consult physician immediately, antibiotics, cough medicines, stay warm
Asthma	Constriction of bronchioles, mucus build up in bronchioles, periodic wheezing, difficulty in breathing	Allergy to pollen, some foods, food additives, pet hair, etc.	Use of inhalants to open passage ways, avoid irritants
Occupational Respiratory Disorders – silicosis, asbestosis	Inflammation, fibrosis, lung damage	Long term exposure to dust particles, silica and asbestos, particles during occupation	Protective mask and gear during work

Reading the Table with Understanding

The table is easier to remember once you connect each disorder back to the structure it damages.

In emphysema, the walls between the alveoli break down, so many small alveoli merge into fewer large ones. Recall from Section 8.2 why that matters: the exchange surface depends on having a very large total area. Merging the alveoli destroys that area, and the patient becomes breathless — not because there is too little air in the chest, but because there is too little surface to exchange it across. Fig. 8.11 makes the difference clear.

Diagram

Fig. 8.11 – Normal alveoli compared with emphysema: (a) a cluster of many small, thin-walled alveoli with a large total surface area; (b) the same region in emphysema, with the walls broken down into a few large, baggy sacs and a much smaller exchange surface.

In asthma, the problem is not the alveoli but the tubes leading to them. The bronchioles constrict and fill with mucus, so the airway narrows. Air can still be drawn in, but it is forced out through a narrowed passage — which is what produces the characteristic wheeze and the difficulty in breathing.

In bronchitis, sinusitis, laryngitis and pneumonia, the common thread is inflammation of a particular part of the tract — the bronchi, the sinuses, the larynx and the lung tissue respectively. Notice that the name in each case simply announces the site: the ending -itis means inflammation.

In the occupational disorders, fine particles of silica or asbestos are inhaled over years. They are too small to be trapped by the nose and too tough for the body to break down, so they lodge in the lung tissue and provoke a slow, permanent scarring called fibrosis. Scarred lung is stiff lung, and stiff lung cannot expand properly.

Memory Aid

The -itis disorders The ending -itis always means inflammation of the named part: bronchitis = bronchi, sinusitis = sinuses, laryngitis = larynx, tonsillitis = tonsils.

Do Not Mix Up

Asthma vs Emphysema (Asthma is a narrowing of the airways — the bronchioles constrict and fill with mucus — and it is usually episodic and reversible, triggered by an allergen. Emphysema is a destruction of the alveoli themselves, caused chiefly by smoking, and the lost surface area cannot be restored. One blocks the road; the other demolishes the destination.)

Real-World Application

Silicosis is a familiar tragedy in India’s stone-cutting, quarrying and mining regions, including the sandstone belts of Rajasthan. Workers who cut and grind stone for years inhale fine silica dust; the damage builds silently, and by the time breathlessness appears, the fibrosis is permanent. There is no cure — which is exactly why the “treatment” column in the table lists protective masks and gear, not medicines. Prevention is the whole of the cure here.

Artificial Ventilation

When breathing itself fails, none of the treatments above matters — air must be moved into the lungs at once.

Artificial ventilation, also called artificial respiration, is the method of inducing breathing in a person when natural respiration has ceased or is faltering. If used properly and quickly, it can prevent death due to drowning, choking, suffocation, electric shock and similar emergencies.

The process involves two main steps: establishing and maintaining an open air passage from the upper respiratory tract to the lungs, and forcing inspiration and expiration — as in mouth-to-mouth respiration, or by mechanical means such as a ventilator.

The order of those two steps is not arbitrary. There is no point forcing air in if the passage is blocked, so the airway must be cleared and kept open first.

Ventilator

A ventilator is a machine that supports breathing. It is used during surgery, in the treatment of serious lung diseases, and in other conditions when normal breathing fails. It is mainly used in hospitals as part of a life support system.

Ventilators do the following:

Get oxygen into the lungs.
Remove carbon dioxide from the lungs.
Help the patient breathe.

Read that list against what you learned in Section 8.3, and the machine stops being mysterious. A ventilator does not do the patient’s respiration for them — the alveoli still exchange the gases, the blood still carries them, and the cells still make the ATP. What it replaces is only the first link in the chain: the mechanical work of moving air in and out, normally done by the diaphragm and intercostal muscles.

8.7 Transportation in Living Organisms

All living organisms, whether unicellular or multicellular, show one important property: they exchange material with their surroundings, and also between various parts of their own cell or body.

Organisms take up oxygen and nutrients from the surrounding, and these are then circulated within the body for various metabolic activities. The wastes generated within are given out into the surrounding.

Notice that this is a continuous two-way traffic, and that it has two distinct stages. First the material must be brought in from outside; then it must be moved to the exact place inside where it is needed. A body that could absorb food and oxygen but not distribute them would be no better off than one that could not absorb them at all.

Flowchart

FC 8.8 – The traffic of materials in an organism: environment → intake of oxygen and nutrients → distribution to every cell of the body → use in metabolic activities → wastes produced → collection of wastes → released back into the environment.

The organ systems of the body are all links in this same traffic, as Fig. 8.11a shows: the digestive system supplies nutrients, the respiratory system supplies oxygen and removes carbon dioxide, the urinary system removes metabolic wastes — and the circulatory system is the network that connects all of them to the body cells.

Diagram

Fig. 8.11a – The body’s transport network: label mouth, digestive system, intestine, anus (unabsorbed matter/faeces); respiratory system taking in O₂ and giving out CO₂; heart and circulatory system carrying blood and nutrients; interstitial fluid and body cells; urinary system removing metabolic waste products (urine).

Transport Without a Circulatory System

Transportation in organisms and animals occurs by diffusion and by active transport between the cells. This mechanism is suitable where the surface area of the body is large and the distance between parts of the body in the organism is extremely small.

Both conditions matter. Diffusion is quick over micrometres and hopelessly slow over centimetres, so it can serve as a transport system only for small or thin-bodied organisms. The larger and thicker an animal grows, the less it can rely on diffusion alone — which is why bigger animals need a pump and a network of vessels.

Intracellular Transport: Cyclosis

Even inside a single cell, materials must be moved about. Cyclosis is the streaming movement of the cytoplasm, shown by almost all living organisms — for example, Paramoecium, Amoeba, the root hair cells of many plants, and WBCs in animals.

Cyclosis is for transportation within the cell, that is, intracellular transport. The flowing cytoplasm carries dissolved substances and organelles along with it, so that a molecule does not have to make its slow way across the cell by diffusion alone — it is, in effect, given a ride.

Definition

(Cyclosis is the streaming movement of the cytoplasm within a cell, which brings about transport of materials inside that cell — that is, intracellular transport.)

Extracellular Transport in Simple Animals

In animals without blood vessels, the material still has to reach the deeper parts of the body — and different groups have found different solutions.

In sponges and coelenterates, the surrounding water is circulated through the body cavities. The animal does not build a fluid of its own; it simply lets the sea water in, and that water carries oxygen and food to the cells lining the cavity.

In flatworms there is parenchymal circulation — the fluid moves through the parenchyma, the loose packing tissue that fills the spaces between the organs.

In roundworms there are no blood vessels, and the body fluid is moved around the viscera by contraction of the body wall and muscles. The animal’s own movements, in other words, double up as its circulation.

All of these are examples of extracellular transport — transport of materials outside the cells, through the body of the organism.

Compare Table

Exam Corner

Q. Why can an Amoeba manage without any circulatory system at all, while a rabbit cannot?

A. Because of size and shape. An Amoeba is a single cell: no part of it lies more than a few micrometres from the outside, so diffusion, helped by cyclosis, reaches everywhere in time. A rabbit is thousands of cells thick, and a molecule of oxygen would take days to diffuse from its skin to its liver. Diffusion has not become weaker in the rabbit; the distances have simply become too great for it. A circulating fluid solves this by bringing the supply line close to every cell.

Zoom In / Out

Transport (Zoom in: transport begins inside one cell, as cytoplasmic streaming shifting materials from one part of the cell to another. Zoom out: the same necessity, scaled up to a large multicellular body, becomes an entire organ system — the circulatory system, with a pump, a fluid and a network of vessels, which we take up next.)

Try Yourself

Which type of circulation is present in a cockroach? How is it different from that of humans?
Which type of circulation is present in amphibians and reptiles?
Make a list of organisms that have no proper transport system.

8.8 Circulation in Animals

In higher animals, the circulation is carried out by special fluids — blood and lymph. Instead of depending on the water outside or on the sluggish movement of the body fluid, these animals maintain their own fluid, drive it with a pump, and route it through vessels.

Blood Vascular System

Higher animals from Annelida to Chordata have a special circulating fluid, the blood, which is pumped to the tissues by the heart through the blood vessels.

Three components appear together in that sentence, and all three are needed: a fluid to carry the load, a pump to keep it moving, and vessels to direct where it goes. Take away any one of them and the system fails.

Types of Blood Vascular System

1. Open circulation: In animals having an open circulation, blood is circulated through the body cavities (haemocoels). The visceral organs lie in the blood-filled body cavity, and so exchange of material takes place directly between the blood and the cells or tissues of the body.

Because the blood is not confined to narrow tubes, it is not under much pressure — it flows with low pressure. It also usually does not contain any respiratory pigment like haemoglobin, so it does not transport respiratory gases. Examples are arthropods (the cockroach, studied in Std. XI) and molluscs.

If the blood carries no oxygen, how do these animals respire? Recall Table 8.2: insects have tracheal tubes that carry air directly to the tissues. Their respiration and their circulation are two separate systems, and their blood is left free to carry food and wastes.

2. Closed circulation: In all the vertebrates, higher molluscs and annelids, blood is circulated all over the body through a network of blood vessels. In this type of circulation, blood flows within the blood vessels and does not come into direct contact with the cells and body tissues.

Then how does material reach the cells? Exchange of material between the blood and the body tissues is through an intermediate fluid called lymph, which bathes the tissues. The blood flows with high pressure — it is confined to vessels, so the pressure is not lost — and it contains respiratory pigments like haemoglobin for the transportation of respiratory gases.

Compare Table

Exam Corner

Q. What is the relationship between how active an animal is and how complex its transport system is?

A. The two rise together. An active animal burns fuel and oxygen quickly, so its cells must be resupplied quickly — and speed of delivery depends on pressure and on directed flow. An open system, with blood sloshing slowly through a cavity, cannot keep pace with a flying bird or a running mammal. A closed system, with a strong pump, narrow vessels and an oxygen-carrying pigment, can. This is why the most active animals are found among those with closed, double circulation.

Scenario

Think of an open circulation as flooding a whole field to water it: the water reaches everything, but slowly, and without much force. A closed circulation is like a network of pipes with a pump at one end — the water is under pressure, arrives quickly, and can be sent exactly where it is needed. The pipes cost more to build and maintain, which is why only animals that need speed have evolved them.

Try Yourself

Observe a labelled diagram of the cockroach circulatory system and identify the parts: the dorsal tubular heart, the ostia, and the haemocoel in which the organs lie.

Single and Double Circulation

The closed circulation can be divided into two main types: single and double circulation. The difference lies in one simple question — during one complete round of the body, how many times does the blood pass through the heart?

Single circulation: In single circulation, the blood passes through the heart only once during each cycle, as in fishes.

Deoxygenated blood is pumped from the heart towards the gills, where it undergoes oxygenation. This oxygenated blood then moves towards various body parts, gets deoxygenated, and returns back to the heart for the next cycle. The route is shown in Fig. 8.12.

Diagram

Fig. 8.12 – Single circulation: heart (atrium and ventricle) → artery → gill capillaries (gill circulation) → onwards to the systemic capillaries (systemic circulation) → vein → back to the heart.

Flowchart

FC 8.9 – Single circulation in a fish: heart (deoxygenated blood) → ventral aorta → gill capillaries → blood is oxygenated → dorsal aorta → systemic capillaries of the body → blood becomes deoxygenated → veins → back to the heart (one pass through the heart per cycle).

Since the heart of a fish carries only deoxygenated blood, it is also called a ‘venous heart’.

Notice the limitation built into this design. After leaving the gills, the blood must pass through a second set of narrow capillaries — the gill capillaries — before it reaches the body. Passing through capillaries costs pressure, so the blood that finally reaches the body tissues is travelling slowly. A fish, supported by water, can afford this; a bird in flight cannot.

Double circulation: In double circulation, blood passes through the heart twice during each cycle. It occurs in birds and mammals.

In these animals, the heart pumps deoxygenated blood to the lungs for oxygenation, and it returns to the heart as oxygenated blood. This is ‘pulmonary circulation’.

The oxygenated blood is then pumped from the heart towards various body parts (except the lungs) and returns back to the heart as deoxygenated blood. This is ‘systemic circulation’. The human heart shows double circulation, as seen in Fig. 8.13.

Diagram

Fig. 8.13 – Double circulation (diagrammatic): heart with right atrium, right ventricle, left atrium, left ventricle; pulmonary artery → lungs → pulmonary veins → heart (pulmonary circulation); dorsal aorta → body organs → superior and inferior vena cava → heart (systemic circulation).

Definition

(Pulmonary circulation is the circuit in which deoxygenated blood is pumped from the heart to the lungs and returns to the heart as oxygenated blood.)

Definition

(Systemic circulation is the circuit in which oxygenated blood is pumped from the heart to all the body parts except the lungs, and returns to the heart as deoxygenated blood.)

Flowchart

FC 8.10 – Double circulation: right side of heart (deoxygenated blood) → pulmonary artery → lungs (oxygenation) → pulmonary veins → left side of heart → aorta → body organs (oxygen delivered) → venae cavae → back to the right side of heart (two passes through the heart per cycle).

The great advantage of the double circuit is that blood is repressurised between the two capillary beds. Blood that has slowed down in the lung capillaries returns to the heart and is given a fresh, powerful push before it is sent to the body. This is why birds and mammals — the two groups that keep their bodies warm and stay active — are precisely the two groups with double circulation.

Compare Table

Do Not Mix Up

Pulmonary circulation vs Systemic circulation (Pulmonary circulation is the short loop between the heart and the lungs — it carries deoxygenated blood away from the heart (in the pulmonary artery) and brings oxygenated blood back (in the pulmonary veins). Systemic circulation is the long loop between the heart and the rest of the body — it carries oxygenated blood away (in the aorta) and brings deoxygenated blood back (in the venae cavae). This is also the one place where an artery carries deoxygenated blood and a vein carries oxygenated blood, so read the direction, not the colour.)

8.9 Circulatory System in Human

The human circulatory system is composed of two parts: the blood vascular system and the lymphatic system. The first is the main highway; the second, as we shall see at the end of the chapter, is the drainage system that runs alongside it.

Blood Vascular System

In human beings, the blood vascular system consists of blood, the heart and the blood vessels. It is responsible for various functions like transport, homeostasis and protection.

These three functions are worth separating, because students often remember only the first.

Transport is the obvious one: oxygen and nutrients to the cells, carbon dioxide and wastes away from them, hormones from the glands to their target organs.

Homeostasis means keeping the internal environment steady — the blood distributes heat around the body, and it holds the pH, the salt content and the water content of the tissues within narrow limits.

Protection means defence: the white blood corpuscles fight pathogens, and the clotting mechanism seals a wound before the body bleeds out.

Blood Composition and Coagulation

The study of blood is called haematology. An average adult has about 4 to 6 litres of blood.

Blood is a red-coloured fluid connective tissue, derived from embryonic mesoderm. That phrase, fluid connective tissue, may sound like a contradiction — but a connective tissue is defined by having cells scattered in a matrix, and in blood the matrix simply happens to be a liquid (the plasma) instead of a solid one.

Blood is slightly alkaline (pH 7.4), salty and viscous, and it is heavier than water.

It has two main components — the fluid plasma (55%) and the formed elements, i.e. the blood cells (44%). These can be separated by centrifugation: when a sample is spun rapidly, the heavier cells are thrown to the bottom of the tube and the lighter plasma stays above.

Definition

(Blood is a red-coloured, slightly alkaline (pH 7.4), salty, viscous fluid connective tissue of mesodermal origin, consisting of a fluid matrix called plasma and formed elements — the RBCs, WBCs and platelets.)

Exam Corner

Q. Why does the pH of blood matter so much?

A. Because enzymes work only within a narrow pH range, and even a small drift in the blood’s pH would slow or stop the reactions that keep cells alive. Recall from Section 8.3 what happens when carbon dioxide dissolves in blood: it forms carbonic acid, which releases \(H^+\) ions. Those ions could easily push the blood towards acidity — which is why they are at once buffered by haemoglobin and by bicarbonate. Holding the pH at 7.4 is not a detail; it is a constant, active job.

Plasma

Plasma constitutes 55% of the blood. It is a straw-coloured, slightly alkaline, viscous fluid, and it consists of the following:

Table 8.14 : Composition of plasma

Contents	Percentage
1. Water	90 %
2. Proteins (albumen, globulin, properdin, prothrombin, fibrinogen)	7 to 8 %
3. Inorganic salts (Na, K, Mg, Ca, Fe, Mn and \(Cl^-\), \(HCO_3^-\) and \(PO_4^{3-}\))	1 %
4. Others: a. Food (glucose, amino acids, fatty acids, triglycerides) b. Wastes (urea, uric acid and creatinine) c. Regulators (hormones, enzymes, vitamins) d. Anticoagulants (heparin) e. Cholesterol and antibodies f. Dissolved gases (\(O_2\), \(CO_2\), \(N_2\))	1 to 2 %

Read the table as a story about function rather than a list to be memorised.

The 90% water is what makes plasma a solvent — everything else in the table is dissolved or suspended in it.

The proteins are each doing a job you have already met or will soon meet: prothrombin and fibrinogen are needed for blood clotting, globulins include the antibodies of defence, and albumen holds water within the vessels.

The inorganic salts maintain the osmotic balance and the pH — and note \(HCO_3^-\) in that list, the bicarbonate that carries 70% of our carbon dioxide.

The “others” group shows plasma acting as the body’s delivery van and rubbish cart at the same time: it carries food to the cells and wastes away from them, hormones from glands to targets, and the dissolved gases in transit.

Do Not Mix Up

Plasma vs Serum (Plasma is the fluid part of blood as it exists in the vessel, and it still contains fibrinogen and the other clotting factors. Serum is what is left after the blood has clotted — it is plasma minus fibrinogen and the clotting factors used up in the clot. In short: serum = plasma − clotting factors.)

Formed Elements

The blood cells that are produced in the body are collectively called formed elements. Human blood contains three types of formed elements: red blood corpuscles (RBCs), white blood corpuscles (WBCs) and platelets.

Each of the three matches one of the functions listed at the start of this section — the RBCs for transport, the WBCs for protection, and the platelets for sealing wounds. The next three sections take them up one by one.

Zoom In / Out

Blood (Zoom out: blood is a tissue, and with the heart and vessels it forms the circulatory system, an organ system serving the whole organism. Zoom in: that tissue is made of individual cells — erythrocytes, leucocytes and platelets — suspended in a fluid matrix, and inside a single erythrocyte lies the molecule that does the real work, haemoglobin.)

Try Yourself

Find out the percentage and functions of the different blood proteins.
What is the clinical importance of the haematocrit value?
Differentiate between plasma and serum.

8.10 Red Blood Corpuscles / Erythrocytes

Erythrocytes are the most abundant cells in the human body. They are circular, biconcave and enucleated — that is, they carry no nucleus. (In the camel and the llama they are nucleated.)

The biconcave shape — a disc dented inwards on both faces — is not an accident. It gives the cell a larger surface area for its volume than a sphere would, and it keeps every part of the cytoplasm close to the surface, so oxygen can diffuse in and out quickly. It also makes the cell flexible enough to bend and squeeze through a capillary narrower than itself. Fig. 8.14 shows the shape.

Diagram

Fig. 8.14 – Erythrocyte: (a) surface view showing the circular disc with a paler centre; (b) side view showing the biconcave shape; (c) a red blood cell bending as it squeezes through a narrow capillary.

Structure and Function

The erythrocyte (biconcave disc → large surface area relative to volume and a short internal diffusion distance → gases load and unload quickly; flexible, enucleated body → the cell can deform and pass through capillaries narrower than itself → it reaches every tissue; no nucleus and no mitochondria → the whole interior is free to be packed with haemoglobin, and the cell does not consume the oxygen it is carrying → maximum oxygen delivered to the tissues)

Number, Colour and Life Span

The red colour of RBCs is due to an oxygen-carrying pigment, the haemoglobin, present in their cytoplasm.

In males, the average number of RBCs is about 5.1–5.8 million/mm³ (per μL), and in females about 4.3–5.2 million/mm³. This is called the total RBC count. The average life span of RBCs is 120 days.

Think about what that life span implies. An RBC with no nucleus cannot repair itself or make new proteins, so it wears out — and the body must therefore replace millions of them every second, without pause, for a lifetime.

The process of formation of RBCs is called erythropoiesis.

Definition

(Erythropoiesis is the process of formation of red blood corpuscles.)

Numerical 8.2

An adult male has a total blood volume of 5 litres and a total RBC count of 5.5 million/mm³. Calculate the total number of red blood corpuscles present in his body. (1 litre = 10⁶ mm³)

Where RBCs Are Made and Destroyed

RBCs are produced from haemocytoblasts / reticulocytes. The erythropoietic organ of the foetus is the liver and spleen, while in the adult it is mainly the red bone marrow.

Vitamin B₁₂, folic acid, iron and heme protein are required for the production of RBCs. Each of these is a raw material for a specific part of the job: iron and heme protein go into building haemoglobin itself, while vitamin B₁₂ and folic acid are needed for the rapid cell division that produces new cells in the marrow. A shortage of any one of them shows up as a shortage of red cells.

The old and worn-out RBCs are destroyed in the liver and spleen — which is why these organs are sometimes called the graveyard of RBCs.

The number of red cells is not left to chance; it is regulated. The hormone erythropoietin, produced by the kidney cells, stimulates the bone marrow for the production of RBCs.

This makes the kidney an unlikely but logical sensor. The kidney receives a large share of the blood, so it is well placed to notice when the blood is delivering too little oxygen — and when it does, it releases erythropoietin and orders more carriers to be built.

A condition with an increase in the number of RBCs is called polycythemia, and with a decrease in the number of RBCs is called erythrocytopenia.

Flowchart

FC 8.11 – Life cycle of a red blood corpuscle: haemocytoblast in red bone marrow → (stimulated by erythropoietin from the kidney; needs vitamin B₁₂, folic acid, iron, heme protein) → reticulocyte → mature enucleated erythrocyte → circulates for about 120 days → worn out → destroyed in the liver and spleen → globin broken down to amino acids (recycled) and iron stored as ferritin (recycled) → porphyrin → biliverdin → bilirubin → added to bile → removed from the body with the faeces.

Real-World Application

When the kidneys fail, they stop producing enough erythropoietin — and patients on long-term dialysis therefore become anaemic, not because their marrow is damaged, but because the signal to the marrow has stopped arriving. Such patients are often given erythropoietin as a medicine. The same hormone has been misused as a doping agent by endurance athletes, because more red cells mean more oxygen delivered to working muscles; it is banned in competitive sport, and it is dangerous, since thickened blood clots easily.

Inside a Mature Erythrocyte

A mature erythrocyte is devoid of a nucleus, mitochondria and other membrane-bound cell organelles. Its cytoplasm, called the stroma, is rich in haemoglobin — the \(O_2\)-carrying proteinaceous pigment that gives the red colour to the RBCs and to the blood. The stroma also contains an enzyme, carbonic anhydrase.

You have already met that enzyme in Section 8.3, and its presence here explains one of the facts you learned there: carbonic anhydrase is found in the RBCs and not in the plasma, which is why 70% of our carbon dioxide is converted to bicarbonate inside the red cell rather than outside it.

Exam Corner

Q. An RBC has no mitochondria. Isn’t that a strange thing for a cell that spends its whole life handling oxygen?

A. It is exactly the right arrangement. A mitochondrion would consume oxygen, and an RBC that used up its own cargo would deliver less of it to the tissues. Having no mitochondria, the red cell meets its own modest energy needs by glycolysis — which, incidentally, is what produces the DPG (2,3-diphosphoglycerate) mentioned in Section 8.3 that shifts the dissociation curve to the right. Losing the nucleus and the organelles also clears out space, so that more of the cell can be packed with haemoglobin.

Functions of Erythrocytes

Erythrocytes are responsible for the transport of the respiratory gases \(O_2\) and \(CO_2\). They also help in maintaining the pH and the viscosity of the blood, and they contribute to the process of blood clotting.

The hematocrit is the ratio of the volume of RBCs to the total blood volume. It is different for men and women — which follows directly from the different RBC counts noted earlier.

Definition

(Hematocrit is the ratio of the volume occupied by red blood corpuscles to the total volume of blood.)

Side Note

Each erythrocyte approximately contains 270 million molecules of haemoglobin. The normal content of haemoglobin in the blood of men is about 14–17 gm%, and in women it is about 13–15 gm%. A condition with a lesser number of RBCs, or a lesser amount of haemoglobin, or both, is called anaemia.

Each molecule of haemoglobin is a protein–iron complex. It consists of four polypeptide (globin) chains — 2 alpha and 2 beta chains. An iron–porphyrin (haem) group is attached to each chain, and all four chains are bound together. Each haem group can carry one \(O_2\) molecule, and thus one haemoglobin molecule can carry four \(O_2\), forming oxyhaemoglobin. \(CO_2\) interacts with the amino acid residues of the globin chains and forms carbaminohaemoglobin.

After haemolysis, haemoglobin is broken down. Its globin part is broken down to recycle the amino acids. The iron of the heme group is stored as ferritin in the liver, and the porphyrin group of the heme is converted into the green pigment biliverdin and then into the red-orange coloured bilirubin. These pigments (mainly bilirubin) are added to the bile and are finally removed out of the body along with the faeces.

Real-World Application

Jaundice is the visible sign of this last pathway going wrong. If the liver is damaged or the bile duct is blocked, bilirubin cannot be cleared into the bile, so it accumulates in the blood and stains the skin and the whites of the eyes yellow. The colour of a bruise tells the same story on a small scale: as the trapped red cells break down under the skin, the mark passes from red through greenish (biliverdin) to yellow (bilirubin) before it fades.

Do Not Mix Up

Haemoglobin content vs RBC count (The RBC count tells you how many red cells are present per mm³ of blood. The haemoglobin content tells you how much pigment those cells carry, in gm%. A person may have a nearly normal number of cells and still be anaemic if each cell is poorly filled with haemoglobin — which is exactly what happens in iron deficiency.)

Try Yourself

Compare myoglobin and haemoglobin.
In erythrocytes, the nucleus and mitochondria are absent. Explain why.

8.11 White Blood Corpuscles / Leucocytes

Leucocytes are colourless, nucleated and amoeboid cells, larger than RBCs. Every one of those words marks a contrast with the red cells you have just studied — and each contrast points to a different job.

They are colourless because they carry no haemoglobin: they are not gas carriers. They are nucleated and keep their organelles because, unlike the RBC, a WBC must actively make proteins, respond to signals and, in some types, divide. And they are amoeboid, which means they can crawl.

That last property is decisive. Due to their amoeboid movement, leucocytes can move out of the capillary walls by a process called diapedesis. The RBC must stay inside the blood vessel; the WBC does not. It squeezes out between the cells of the capillary wall and goes to where the trouble is.

Definition

(Diapedesis is the process by which leucocytes squeeze out through the walls of the capillaries into the surrounding tissue, using their amoeboid movement.)

Exam Corner

Q. Why and when do leucocytes perform diapedesis?

A. Because the enemy is usually outside the blood vessel, not inside it. Bacteria enter through a cut in the skin and multiply in the tissue, where the blood cannot reach them. When an infection or injury occurs, chemical signals released at the site attract the leucocytes, which then crawl out of the capillary through the endothelial pores and into the tissue to attack the invaders. A blood cell that could not leave the blood would be useless in defending a wound.

Numbers, and What Changes Them

A normal adult has, on average, 5000–11000 WBCs per mm³ of blood.

Because the WBC count reflects what the body is fighting, doctors read changes in it as clues:

A decrease in the number of WBCs (below 4000) is called leucopenia. It is common in HIV, AIDS and TB patients, and in those exposed to radiation, shock and similar conditions.

A temporary increase in the number of WBCs is called leucocytosis. It is due to infection. It also occurs during pregnancy and in newborn babies.

An uncontrolled increase in the number of WBCs is a type of blood cancer called leukemia. Note the difference from leucocytosis: leucocytosis is a controlled, purposeful, temporary rise in response to a real threat, while in leukemia the marrow keeps producing abnormal white cells regardless of need — and those cells crowd out the normal ones.

WBCs are mainly concerned with the defense mechanism, i.e. protection.

Types of WBCs

These are colourless, irregular, nucleated cells, and they show polymorphism — they exist in variable forms. They can be classified into two main types: granulocytes and agranulocytes. The classification, as the names say, rests on whether the cytoplasm carries granules. All five types are shown in Fig. 8.15.

Diagram

Fig. 8.15 – Granulocytes and agranulocytes: (a) neutrophil with fine granules and a multi-lobed nucleus, (b) basophil with a few large granules and a twisted nucleus, (c) eosinophil with coarse granules and a bilobed nucleus, (d) lymphocyte with a large spherical nucleus and little cytoplasm, (e) monocyte with a large bean- or kidney-shaped nucleus.

Flowchart

FC 8.12 – Classification of WBCs: Leucocytes → (i) Granulocytes (from myeloid stem cells, ~72% of total WBCs) → Neutrophils, Basophils, Eosinophils/Acidophils; and (ii) Agranulocytes (from lymphoid stem cells, ~28% of total WBCs) → Lymphocytes (→ B-lymphocytes and T-lymphocytes) and Monocytes (→ macrophages).

A. Granulocytes

These are WBCs with a granular cytoplasm, also called polymorphonuclear leucocyte (PMN) cells. They have lobulated nuclei of different shapes — hence the name, “many-shaped nucleus.”

Granulocytes are formed from myeloid stem cells, and once formed they do not divide. Granulocytes constitute about 72% of the total WBCs.

The granules are actually secretory vesicles, which contain various secretions, enzymes and so on. This is worth pausing over: a granulocyte is essentially a walking store of chemical weapons, ready to be released when it reaches its target.

Depending upon the staining property of the granules, these granulocytes are classified into three types: neutrophils, basophils and acidophils.

a. Neutrophils: Their granules are very fine, large in number, evenly distributed, and stained with neutral stains (dyes) — which is where the name comes from.

Neutrophils are about 70% of the total WBCs, making them by far the commonest white cell. These cells are spherical and the nucleus is several-lobed (2–7).

They are able to perform amoeboid movements and phagocytosis, and they are responsible for destroying pathogens by the process of phagocytosis — engulfing the microbe and digesting it inside the cell.

‘Pus’ is a mixture of dead neutrophils, damaged tissues and dead microbes. So the pus in an infected wound is, in effect, the battlefield left behind after the neutrophils have done their work.

b. Basophils / Cyanophils: These cells have very few granules, of large size, and they stain with basic stains like methylene blue.

Basophils are non-phagocytic — they do not eat microbes at all. They are small, spherical cells and are about 0.5–1% of the total WBCs, the rarest of all. The nucleus is twisted.

They are present in infected and allergic conditions only. Basophils secrete heparin, histamine and serotonin. Their weapon is chemical, not mechanical: heparin prevents the blood from clotting inside the vessel, histamine widens the vessels and makes them leaky so that other defence cells can reach the tissue, and serotonin acts on vessel diameter.

c. Eosinophils / Acidophils: Acidophils contain lysosomal granules that are stained red by acidic stains like eosin.

Eosinophils are about 1–3% of the total WBCs. The nucleus is bilobed.

They destroy the antigen–antibody complex by phagocytosis. Their number increases in allergic conditions, and they show an antihistaminic property — that is, they counter the very histamine released by the basophils, which is how the allergic reaction is kept from running out of control. They are also responsible for detoxification, as they produce antitoxins.

Real-World Application

When a doctor orders a complete blood count (CBC), the differential count of WBCs is read exactly like this. A high neutrophil count points towards a bacterial infection. A high eosinophil count points instead towards an allergy or a parasitic worm infestation. A very low WBC count raises the suspicion of marrow suppression or of a disease such as HIV. The percentages you have just learned are what make that report readable.

Compare Table

B. Agranulocytes

Agranulocytes are about 28% of the total WBCs. The cytoplasm of these leucocytes is without granules.

They are formed from lymphoid stem cells and can divide by mitosis — unlike the granulocytes, which cannot divide once formed. This ability to multiply matters, because it lets the body rapidly expand the exact population of cells that recognises a particular invader.

The nuclei of agranulocytes are large in size, but are not lobulated like those of the granulocytes. There are two types of agranulocytes — lymphocytes and monocytes.

a. Lymphocytes: Lymphocytes are the smallest of all WBCs and have a large spherical nucleus, which fills up most of the cell and leaves only a thin rim of cytoplasm. They constitute about 25–30% of the total WBCs.

Depending upon function, two types of lymphocytes are present: B-lymphocytes and T-lymphocytes.

B-lymphocytes mature in the bone marrow and are responsible for antibody production, i.e. humoral immunity. It is a highly specific antigen–antibody immunity: each antibody fits one particular antigen, much as a key fits one lock.

T-lymphocytes mature in the thymus and are responsible for cell-mediated immunity — they deal with the enemy by direct cellular attack rather than by releasing antibodies. Helper T-cells, killer T-cells, memory T-cells and suppressor T-cells are the four main subtypes of T-lymphocytes.

Compare Table

Memory Aid

Which lymphocyte matures where B for Bone marrow, T for Thymus.

b. Monocytes: Monocytes are the largest of all the WBCs. Their nucleus is large and bean- or kidney-shaped. They form 3–5% of the WBCs.

Monocytes are actively motile and give rise to macrophages. They are mainly phagocytic, and destroy bacteria and dead or damaged tissue by phagocytosis.

So the monocyte is a clean-up specialist as well as a defender: it removes not only invaders but also the body’s own debris, which is why it is essential to the healing of a wound after the neutrophils have finished fighting in it.

Compare Table

Memory Aid

The WBCs in order of abundance Neutrophils (70%) > Lymphocytes (25–30%) > Monocytes (3–5%) > Eosinophils (1–3%) > Basophils (0.5–1%) — remember “Never Let Monkeys Eat Bananas.”

Do Not Mix Up

Phagocytosis vs Antibody production (Phagocytosis is the engulfing and digesting of an invader by a cell — done by neutrophils and by monocytes/macrophages. Antibody production is a chemical defence, in which a soluble protein specific to one antigen is released into the blood — done by B-lymphocytes. Phagocytosis is a general defence against many invaders; antibody production is highly specific to one.)

Try Yourself

What are the granules in granulocytes?
What are the reasons for changes in the number of neutrophils, and what is the importance of a complete blood count (CBC)?
Why and when are heparin, histamine and serotonin secreted? Are these biomolecules secreted by any other cell or organ in our body?

8.12 Thrombocytes / Platelets

Thrombocytes are cellular fragments formed from large cells called megakaryocytes. These are produced in the bone marrow.

Read that sentence carefully — a platelet is not a whole cell. It is a piece broken off a giant parent cell in the marrow. They are very small, oval-shaped cell fragments without a nucleus.

The normal count of thrombocytes in human blood is about 2.5–4.5 lakh/mm³ of blood. Fig. 8.16 shows how they appear alongside the other blood cells in a stained blood smear.

Diagram

Fig. 8.16 – Blood smear: label RBCs (numerous, biconcave, pale-centred discs), a WBC (larger, with a stained nucleus), and platelets (very small, irregular fragments).

If the number of thrombocytes decreases below normal, the condition is called thrombocytopenia. This condition causes internal bleeding (haemorrhage) — the small leaks that occur constantly in our capillaries are no longer sealed in time.

What Platelets Do

Platelets secrete platelet factors, which are essential in blood clotting.

They also seal the ruptured blood vessels by the formation of a platelet plug or thrombus. This is the body’s immediate first-aid: within seconds of an injury, platelets stick to the torn edges of the vessel and to one another, building a temporary plug before the slower chemical clotting has even begun.

They secrete serotonin, a local vasoconstrictor. Constricting the injured vessel narrows it, so less blood escapes and the platelets have less area to cover — a simple and effective way of buying time.

Structure and Function

The platelet (no nucleus, very small and numerous → thousands can be produced and packed into every mm³ of blood, and they can be spent freely at an injury; sticky surface → they adhere to the torn vessel wall and to one another → an instant platelet plug; secretory contents → release platelet factors and serotonin → the clotting cascade is triggered and the vessel narrows)

Real-World Application

In dengue fever, the platelet count often falls sharply, sometimes far below the normal 2.5–4.5 lakh/mm³. This is why a dengue patient’s platelet count is monitored daily, and why the danger signs are bleeding from the gums or the nose, and dark patches under the skin. The virus does not cut the blood vessels; it removes the body’s ability to seal the tiny leaks that happen anyway.

Blood Clotting / Coagulation of Blood

Clotting, or coagulation, is the process of converting the liquid blood into a solid form.

Definition

(Blood clotting or coagulation is the process by which liquid blood is converted into a solid mass, sealing a damaged blood vessel.)

This process may be initiated in two ways — by contact of blood with any foreign surface (the intrinsic process), or with damaged tissue (the extrinsic process).

Intrinsic and extrinsic processes both involve the interaction of various substances called clotting factors, by a step-wise or cascade mechanism. There are in all twelve clotting factors, numbered as I to XIII (factor VI is not in active use).

A cascade means that each activated factor activates the next one, and each step amplifies the signal — one activated molecule can activate many of the next kind. This is exactly why a clot can form in seconds from a tiny initial trigger. It also explains why the loss of even one factor in the chain, as in haemophilia, can break the whole process.

Interaction of these factors in a cascade manner leads to the formation of the enzyme thrombin.

Thromboplastin helps in the formation of the enzyme prothrombinase. This enzyme inactivates heparin — the natural anticoagulant that normally keeps blood from clotting inside the vessels — and it also converts inactive prothrombin into its active form, thrombin.

Thrombin then converts the soluble blood protein fibrinogen into insoluble fibrin. Fibrin forms a mesh in which platelets and other blood cells are trapped, to form the clot.

Follow the last two steps and the logic of the whole system appears. Fibrinogen is dissolved in the plasma of every healthy person, circulating harmlessly. It becomes a solid net only when thrombin acts on it — and thrombin exists only where the cascade has been triggered by an injury. So the clot forms at the wound, and nowhere else.

The two pathways and their meeting point are shown in FC 8.13.

Flowchart

FC 8.13 – Blood clotting: Extrinsic pathway: damage to tissue outside the vessel → tissue thromboplastin → activation of inactive factor X → activated factor X. Intrinsic pathway: damage to the blood vessel → cascade of clotting factors → activation of inactive factor X → activated factor X. Both pathways converge: activated factor X → prothrombin converted to thrombin → thrombin converts fibrinogen to fibrin → fibrin mesh traps platelets and blood cells → blood clot.

Do Not Mix Up

Intrinsic vs Extrinsic pathway (The extrinsic pathway is started by damage to the tissue outside the blood vessel, and it works through tissue thromboplastin. The intrinsic pathway is started by damage to the blood vessel itself, or by contact of the blood with a foreign surface, and it works through a cascade of clotting factors within the blood. Both end at the same place — the activation of factor X — after which the route to the clot is common.)

Do Not Mix Up

Fibrinogen vs Fibrin (Fibrinogen is the soluble plasma protein that circulates in everyone’s blood at all times and does nothing. Fibrin is the insoluble, thread-like protein formed from it by thrombin, and it is the actual mesh of the clot. Soluble → nothing happens; insoluble → the clot forms.)

Exam Corner

Q. Blood clots within seconds outside the body, yet it flows for a lifetime inside our vessels without clotting. Why?

A. Because clotting has to be actively switched on. The plasma carries heparin, a natural anticoagulant, and the lining of a healthy vessel is smooth and undamaged, so no cascade is triggered. Only when a vessel is torn, or when the blood meets a foreign surface, are the clotting factors activated — and prothrombinase then inactivates the heparin at that spot. The blood is, in effect, held permanently one step short of clotting, waiting for a signal.

Trace It

What happens when you cut your finger 1. The blade tears the wall of a small blood vessel, and blood begins to escape. 2. Platelets reach the torn edge, stick to it and to one another, and build a platelet plug that physically blocks the gap. 3. The platelets release serotonin, a local vasoconstrictor, which narrows the injured vessel and slows the loss of blood. 4. Damage to the tissue outside the vessel releases tissue thromboplastin (extrinsic pathway), while damage to the vessel itself triggers the cascade of clotting factors within the blood (intrinsic pathway). 5. Both routes activate factor X, and thromboplastin leads to the formation of prothrombinase. 6. Prothrombinase inactivates heparin and converts inactive prothrombin into active thrombin. 7. Thrombin converts the soluble fibrinogen of the plasma into insoluble fibrin threads. 8. The fibrin threads form a mesh across the wound, trapping the platelets and the other blood cells — and the clot is formed, sealing the cut.

Try Yourself

What is blood clotting? How and when does it occur?
What is immunity? Name its types.
Why does the platelet count decrease in a dengue patient?
Why does our immune system fail against pathogens like Trypanosoma and Plasmodium?
What is the relation between immunity and organ transplantation?
How do monocytes perform amoeboid movement and phagocytosis?
How do monocytes modify into macrophages?

8.13 Heart

The heart is the main pumping organ of the circulatory system. It is reddish brown in colour, and is a hollow, muscular organ, roughly the size of one’s fist. Its average weight is about 300 gm in males and 250 gm in females.

It is conical in shape and lies in the mediastinum — the space between the two lungs. It is broader at the upper end (the base) and conical at the lower end (the apex). The conical end is slightly tilted to the left side and rests above the diaphragm.

This tilt is the reason we feel our heartbeat on the left of the chest, and it is also the reason (recall Section 8.2) that the left lung has only 2 lobes while the right has 3 — the heart has taken the space.

Pericardium

The heart is enclosed in a membranous sac called the pericardium. The pericardium is formed of two main layers — the outer fibrous and the inner serous pericardium, as seen in Fig. 8.17.

Diagram

Fig. 8.17 – Heart wall and pericardium: label pericardium, fibrous pericardium, parietal layer of serous pericardium, pericardial space, visceral layer of serous pericardium (epicardium), myocardium (cardiac muscles), endocardium, trabeculae carneae.

The serous pericardium is soft, moist and elastic. It is formed of squamous epithelium, and is further divisible into two layers — the parietal and the visceral layer.

The parietal and visceral layers of the serous pericardium are separated by a pericardial space. This space is filled with pericardial fluid (about 50 ml), which acts as a shock absorber and protects the heart from mechanical injuries. It also keeps the heart moist and acts as a lubricant.

The idea should feel familiar. This is the same arrangement you met around the lungs in Section 8.2 — a double membrane with fluid between the layers, so that an organ that moves constantly can do so without friction and without damage.

Structure and Function

The pericardium (a tough outer fibrous layer → anchors the heart in the mediastinum and prevents it from over-filling → the heart keeps its position and shape; two smooth serous layers with about 50 ml of pericardial fluid between them → the layers glide over each other with no friction → the heart can beat about 72 times a minute for a lifetime without wearing itself out against the chest wall; the fluid also cushions → protection from mechanical injury)

Heart Wall

The heart is mesodermal in origin. Its wall is formed of three layers — the outer epicardium, the middle myocardium and the inner endocardium.

The epicardium is thin and is formed of a single layer of flat squamous epithelium resting on a basement membrane. (Notice from Fig. 8.17 that the epicardium is the same structure as the visceral layer of the serous pericardium — one name for the layer viewed as part of the sac, another for the same layer viewed as part of the heart wall.)

The myocardium is the middle, thick layer formed of cardiac muscles.

The endocardium is a single thin layer formed of squamous epithelium, lining the chambers from inside.

The epicardium and endocardium are protective in function, whereas the myocardium is responsible for the contraction and relaxation of the heart. This is why the myocardium is by far the thickest of the three — it is the only layer that does mechanical work.

Memory Aid

The three layers of the heart wall Read them from outside inwards: Epi- (upon) → Myo- (muscle) → Endo- (within). The prefix itself tells you the position.

External Structure of Heart

The human heart is four-chambered. The two superior chambers are called atria (auricles) and the inferior two are called ventricles.

Externally, the atria are separated from the ventricles by a transverse groove called the coronary sulcus or atrioventricular groove. The two ventricles are externally separated from each other by two grooves, the anterior and posterior inter-ventricular sulci. Coronary arteries and coronary veins run through these sulci.

That last detail is easy to skip over, but it is important: the heart, though full of blood, cannot feed itself from the blood inside its chambers. It has its own supply line — the coronary arteries — and a block in these vessels starves the myocardium itself. This is the basis of the heart attack, which we come to in Section 8.16.

The pulmonary trunk arising from the right ventricle and the aorta from the left ventricle are present on the anterior surface of the heart. The pulmonary trunk bifurcates into the right and left pulmonary arteries.

The aorta (systemic aorta) is divisible into three regions: the ascending aorta, the systemic arch / aortic arch, and the descending aorta.

The ligamentum arteriosum joins the pulmonary trunk and the aortic arch. It is the remnant of an embryonic duct called the ductus arteriosus.

Why should such a remnant exist? Before birth, the foetus does not breathe air, so there is little point sending blood to the lungs. The ductus arteriosus was a short-cut connecting the pulmonary trunk to the aorta, allowing blood to bypass the unused lungs. After birth it closes and shrinks to this fibrous cord — a small scar left behind by a circulation that no longer exists.

The aortic arch gives out three arteries, viz. the brachiocephalic (innominate) artery, the left common carotid and the left subclavian.

The right atrium receives the superior and inferior vena cava along its dorsal surface. The pulmonary veins open into the left atrium along the dorsal surface of the heart. The external features are shown from both sides in Fig. 8.18 and Fig. 8.19.

Diagram

Fig. 8.18 – Posterior (dorsal) view: external structure of human heart. Label left common carotid artery, left subclavian artery, brachiocephalic artery, aorta, superior vena cava, right and left pulmonary arteries, right and left pulmonary veins, left atrium, coronary sinus, inferior vena cava, right ventricle, left ventricle, apex of heart.

Diagram

Fig. 8.19 – Anterior (ventral) view: external structure of human heart. Label superior vena cava, aorta, ligamentum arteriosum, pulmonary trunk, right and left pulmonary arteries, right and left pulmonary veins, right atrium, left atrium, coronary sulcus, right coronary artery, branch of left coronary artery, right ventricle, left ventricle, inferior vena cava.

Internal Structure of Heart

Atria

These are the thin-walled receiving chambers of the heart. They are separated from each other by the inter-auricular septum. The inter-auricular septum has an oval depression called the fossa ovalis, which is a remnant of the embryonic aperture called the foramen ovalis.

Here too the embryo has left its mark. The foramen ovalis was an opening between the two atria in the foetus, another route for blood to bypass the lungs. After birth it closes, leaving only this shallow dimple.

The superior vena cava (precaval), inferior vena cava (postcaval) and coronary sinus open into the right atrium. The opening of the postcaval is guarded by a Eustachian valve, while the Thebesian valve guards the opening of the coronary sinus into the right atrium.

Four pulmonary veins open into the left atrium. These openings are without valves.

Both the atria open into the ventricles of their respective sides by atrioventricular apertures. These openings are guarded by cuspid valves. The tricuspid valve is present in the right AV aperture, and the bicuspid valve (mitral valve) is present in the left AV aperture.

All these heart valves help in maintaining a unidirectional flow of blood. They also avoid the back-flow of blood.

Curiosity Corner

Q1. Why does blood in our heart and veins flow in only one direction and not slide backwards?

A. Because of valves. Inside the heart, the tricuspid and bicuspid valves guard the openings between the atria and the ventricles, and the semilunar valves guard the exits into the pulmonary aorta and the aorta. Each opens in one direction only, so blood driven forward cannot be pushed back when the pressure rises behind it. The veins carry a similar arrangement of internal valves, as you will see in Section 8.15. William Harvey, in the story that opened this chapter, proved the circulation of blood by demonstrating exactly this — that blood in a vein could be pushed only towards the heart.

Ventricles

These are the inferior, thick-walled pumping chambers of the heart. The right and left ventricles are separated by an inter-ventricular septum.

The wall of the left ventricle is more muscular and about 3 times thicker than that of the right ventricle.

Curiosity Corner

Q2. Why is the wall of the left ventricle so much thicker than the wall of the right ventricle?

A. Because the two ventricles do very different amounts of work. The right ventricle pumps blood only to the lungs, which lie right beside the heart — a short, low-pressure pulmonary circuit. The left ventricle must drive blood through the entire systemic circuit, from the brain above to the toes below, against far greater resistance. A thicker, more muscular wall generates the greater force needed, which is why the left ventricular wall is about three times thicker.

The inner surface of the ventricles shows several ridges called columnae carnae or trabeculae carnae, which divide the lumen of the ventricle into small pockets or fissures.

The lumen of the ventricles also shows inelastic fibres called chordae tendinae. These attach the bicuspid and tricuspid valves to the ventricular wall (the papillary muscles) and regulate their opening and closing.

The chordae tendinae solve a real mechanical problem. When the ventricle contracts, the pressure inside it becomes enormous, and the flap-like cuspid valve would simply be blown backwards into the atrium — like an umbrella turning inside out in a gale. The inelastic cords, anchored to the papillary muscles, hold the valve flaps down and stop them from everting. So the valve shuts firmly and blood cannot escape backwards into the atrium.

The right ventricle opens into the pulmonary aorta and the left ventricle opens into the aorta. These openings are guarded by three semilunar valves each. These valves prevent the backward flow of blood into the ventricles when the ventricles relax. The internal arrangement is shown in Fig. 8.20.

Diagram

Fig. 8.20 – Internal structure of human heart: label superior vena cava, opening of superior vena cava, inferior vena cava, right atrium, tricuspid valve, right ventricle, pulmonary semilunar valve, pulmonary trunk, right and left pulmonary arteries, right and left pulmonary veins, left atrium, bicuspid (mitral) valve, left ventricle, chordae tendinae, papillary muscles, columnae carnae, interventricular septum, aorta, left common carotid artery.

Compare Table

Flowchart

FC 8.14 – Path of blood through the human heart: superior and inferior venae cavae (deoxygenated blood) → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → lungs (oxygenation) → pulmonary veins → left atrium → bicuspid (mitral) valve → left ventricle → aortic semilunar valve → aorta → body organs → back to the venae cavae.

Pumping Action of Heart

The heart acts as the main pumping organ of the circulatory system. The pumping action is brought about by a rhythmic contraction and relaxation of the cardiac muscles or heart muscles.

Contraction of heart muscles is systole, and relaxation of heart muscles is diastole.

A single systole followed by a diastole makes one heart beat. The heart beats 70 to 72 times per minute. This is called the heart rate.

During each heart beat, the ventricles pump about 70 ml of blood: this is called the stroke volume.

It means the heart pumps about:

\[72\ (\text{heart rate}) \times 70\ \text{ml}\ (\text{stroke volume}) = 5040\ \text{ml} \approx 5\ \text{litres of blood per minute}\]

This is called the cardiac output.

Definition

(Stroke volume is the volume of blood pumped out by a ventricle during one systole — about 70 ml.)

Definition

(Cardiac output is the volume of blood pumped out by a ventricle in one minute. It is the product of the stroke volume and the heart rate.)

Pause on that figure. An adult has about 4 to 6 litres of blood in the body (Section 8.9), and the heart pumps about 5 litres every minute — which means that the entire blood volume of the body passes through the heart roughly once a minute, all day, every day.

Conducting Tissue of Heart

The human heart is myogenic, i.e. the heart is capable of generating a cardiac contraction independent of nervous input. It also shows autorhythmicity, i.e. it can generate its own rhythm by specialized muscles.

Definition

(A myogenic heart is one in which the impulse for contraction originates in the cardiac muscle itself, and not in any nerve.)

Curiosity Corner

Q4. A heart removed from the body can keep beating for a while. If the brain is not sending the command, who is setting the beat?

A. The heart itself. The human heart is myogenic and autorhythmic: a specialized cardiac musculature called the nodal tissue generates the impulse for each beat without any input from nerves. The SA node sets the pace, and the impulse spreads through the conducting system to the rest of the heart. Nerves and hormones can speed the heart up or slow it down — but they do not start it, which is why an isolated heart, given oxygen and nutrients, goes on beating on its own.

A specialized cardiac musculature called the nodal tissue is distributed in the heart. A part of this nodal tissue is present in the upper right corner of the right atrium. It is called the SA node or Sino-atrial node. It lies at the base of the opening of the superior vena cava.

Another mass of nodal tissue, the modified muscular fibres — also called autorhythmic fibres (conducting tissue) — controls the beating rate of the heart.

The conducting (nodal) tissue consists of the SA node, the AV node, the bundle of His and the Purkinje fibres.

Conducting System of the Heart

The SA node (sinu-atrial node) is present in the right atrium. It acts as the pacemaker of the heart, because it has the power of generating a new wave of contraction and making the pace of contraction. The SA node passes the contraction to the left ventricle and also to the AV node.

The AV node (atrio-ventricular node) is present in the right atrial wall, near the base of the interatrial septum. It acts as the pace setter of the heart.

The bundle of His / Tawara branches start from the AV node and pass through the interventricular septum. The bundle of His forms two branches, the right and left bundles, one for each ventricle.

These branches form a network in the ventricular walls, and these are called Purkinje fibres. The bundle of His and the Purkinje fibres spread the impulses in the ventricles. As a result, both the ventricles contract simultaneously. The whole conducting system is shown in Fig. 8.21.

Diagram

Fig. 8.21 – Conducting system of human heart: label right atrium, left atrium, sinoatrial (SA) node, atrioventricular (AV) node, atrioventricular (AV) bundle (bundle of His), right and left bundle branches, Purkinje fibres, right ventricle, left ventricle; with arrows showing the spread of the impulse.

Flowchart

FC 8.15 – Spread of the cardiac impulse: SA node (pacemaker, in the right atrium) → impulse spreads through the walls of both atria → atria contract → AV node → bundle of His (in the interventricular septum) → right and left bundle branches → Purkinje fibres in the ventricular walls → both ventricles contract simultaneously.

Exam Corner

Q. Why must the impulse be delayed at the AV node before it reaches the ventricles?

A. Because the atria must finish emptying before the ventricles begin to squeeze. If both pairs of chambers contracted at the same instant, the ventricles would be pumping while they were still being filled, and much of the blood would simply be forced back against the closed AV valves. The brief pause at the AV node ensures the correct order: atria first, ventricles a moment later — which is exactly why atrial systole and ventricular systole are separate events in the cardiac cycle you will study next.

Structure and Function

Purkinje fibres (a branching network spread through the walls of both ventricles → the impulse reaches every part of the ventricular muscle almost at the same moment → both ventricles contract simultaneously and as a single unit → the blood is squeezed out from the apex upwards in one efficient stroke rather than in an uncoordinated ripple)

Side Note

Sometimes the valves of the heart are defective. Due to this, an abnormal or adventitious sound is heard, called ‘heart murmuring’ or ‘hissing’. A leaking or narrowed valve allows blood to move where it should not, and the disturbed flow can be heard through a stethoscope.

Zoom In / Out

The heart (Zoom out: the heart is one organ within the circulatory system, which serves the whole organism by connecting the respiratory, digestive and excretory systems to every cell. Zoom in: the heart wall is built of a tissue — cardiac muscle — and within that tissue lies a small population of specialized cells, the nodal tissue, whose ability to fire spontaneously is what keeps the entire system running.)

8.14 Working Mechanism of Human Heart

Cardiac Cycle

The human heart alternately contracts and relaxes. Contraction is called systole and relaxation is called diastole. The atria and the ventricles contract alternately — never together — and you already know from Section 8.13 why: the AV node delays the impulse just long enough for the atria to finish emptying before the ventricles begin to squeeze.

Consecutive systole and diastole constitute a single heartbeat, or cardiac cycle. It is completed in 0.8 sec. On an average, 72 beats are completed in one minute in an adult, at rest.

Definition

(The cardiac cycle is one complete sequence of systole and diastole of the atria and ventricles, constituting a single heartbeat. It is completed in 0.8 sec.)

Check that the two numbers agree with each other. If one cycle takes 0.8 sec, then in 60 seconds the heart completes \(60 \div 0.8 = 75\) cycles — which is close to the resting figure of about 72 beats per minute. The duration of the cycle and the heart rate are simply two ways of stating the same fact.

a. Atrial Systole (AS)

The right atrium receives deoxygenated blood and the left atrium receives oxygenated blood.

When both the atria are completely filled with blood, pressure is exerted on the wall. In response to this pressure, the SA node gets excited and generates a cardiac impulse. Due to this, the cardiac muscles in the atrial wall contract, causing atrial systole.

Notice the elegance of the arrangement: the trigger is the filling itself. The heart does not need to be told that blood has arrived — the stretch of the atrial wall is the signal.

During atrial systole, blood is pumped into the ventricles. Blood is prevented from going back into the veins and the coronary sinus by the Eustachian valve and the Thebesian valve respectively. After completing systole, the atria go into diastole.

In normal conditions, atrial systole (AS) lasts for 0.1 sec, and atrial diastole (AD) lasts for 0.7 sec.

The atria therefore rest for seven times as long as they work. They can afford to, because they are only handing blood down into the chamber directly below them — a short push, against very little resistance.

b. Ventricular Systole (VS)

The impulse that started from the SA node now reaches the AV node, and the AV node gets excited. The AV node sends impulses to the bundle of His, and from the bundle of His to the Purkinje fibres. The Purkinje fibres spread the impulses all over the wall of the ventricles.

Due to this, the ventricular wall contracts, causing ventricular systole.

During ventricular systole, the right ventricle pumps deoxygenated blood into the pulmonary trunk, and the left ventricle pumps oxygenated blood into the aorta.

During ventricular systole, the cuspid valves close both the atrioventricular apertures, preventing the flow of blood back into the atria — and the closure of these valves produces the ‘lubb’ sound.

In normal conditions, ventricular systole lasts for 0.3 sec, and ventricular diastole (VD) lasts for 0.5 sec.

During ventricular diastole, the semilunar valves are closed, preventing the backflow of blood from the pulmonary trunk and the systemic aorta into the ventricles — and this closure produces the ‘dub’ sound.

The two heart sounds you can hear through a stethoscope are therefore not the heart “beating” in any loose sense. Each is the snap of a set of valves shutting, and each marks a precise moment in the cycle.

Compare Table

Joint Diastole

For about 0.4 second, both the atria and the ventricles are in diastole. When all the chambers of the heart are in diastole, this condition is called joint diastole or complete diastole.

This is the heart’s true rest period, and it is not wasted time: with all the chambers relaxed and the AV valves open, blood flows freely from the veins into the atria and on into the ventricles. Most of the ventricular filling actually happens here, before atrial systole gives the final top-up.

Thus, the duration of one cardiac cycle is 0.8 sec. Fig. 8.22 shows how the events overlap within that time.

Diagram

Fig. 8.22 – Diagrammatic representation of the cardiac cycle: a time axis from 0 to 0.8 sec (extended to 1.0 sec to show the next cycle beginning); horizontal bars marking atrial systole (AS, 0.1 sec) followed by atrial diastole (AD, 0.7 sec); ventricular systole (VS, 0.3 sec) followed by ventricular diastole (VD, 0.5 sec); and the overlapping 0.4 sec period of joint diastole marked below.

Interpret Data

Reading the cardiac cycle chart (The horizontal axis is time in seconds, and each bar shows when a chamber is contracting or relaxing. Start at 0: the atria contract for the first 0.1 sec, then relax for the remaining 0.7 sec. The ventricles contract from 0.1 sec to 0.4 sec — that is, for 0.3 sec — and then relax for 0.5 sec, until the cycle ends at 0.8 sec. Now look for the stretch where both bars read “diastole”: it runs from 0.4 sec to 0.8 sec, giving the 0.4 sec of joint diastole. Two things stand out at once. First, the numbers close the loop: \(0.1 + 0.3 + 0.4 = 0.8\) sec. Second, out of every 0.8 sec, the ventricles work for only 0.3 sec and rest for 0.5 sec — the heart spends more of its life resting than contracting, which is exactly how it manages to keep going for a lifetime without fatigue.)

Flowchart

FC 8.16 – One cardiac cycle (0.8 sec): atria fill with blood → SA node fires → atrial systole (0.1 sec), blood pushed into ventricles → impulse reaches AV node → bundle of His → Purkinje fibres → ventricular systole (0.3 sec); cuspid valves shut (‘lubb’), blood pumped into pulmonary trunk and aorta → ventricular diastole (0.5 sec); semilunar valves shut (‘dub’) → joint diastole (0.4 sec), all chambers relaxed and refilling → back to the start.

Exam Corner

Q. How does one cardiac cycle complete in 0.8 sec, if atrial systole lasts 0.1 sec and complete diastole lasts 0.4 sec?

A. Add the three non-overlapping phases: atrial systole (0.1 sec) + ventricular systole (0.3 sec) + joint diastole (0.4 sec) = 0.8 sec. The reason this works is that atrial diastole (0.7 sec) and ventricular diastole (0.5 sec) overlap with the other phases, so they must not be added in separately. The 0.4 sec of joint diastole is precisely the part of those two diastoles that overlaps.

Stroke Volume and Cardiac Output

The right side of the heart contains deoxygenated blood and the left side contains oxygenated blood.

The total volume of blood pumped during one ventricular systole is called the stroke volume (SV), and it is approximately 70 ml.

Cardiac output (CO) is the volume of blood pumped out per minute. For a normal adult human being it is calculated as follows:

\[CO = SV \times HR\]

\[CO = 70 \times 72 = 5040\ \text{ml/min}\]

Here SV is the stroke volume in ml, HR is the heart rate in beats per minute, and CO is therefore in ml per minute.

The formula tells you something useful about how the body responds to demand. Cardiac output can be raised either by pumping more blood per beat or by beating faster. During exercise the body does both — which is how a working muscle can be supplied with several times its resting share of blood.

Numerical 8.3

A person’s heart rate rises from 72 beats per minute at rest to 150 beats per minute during exercise, while the stroke volume rises from 70 ml to 110 ml. Calculate the cardiac output in each case, and find by how many times it has increased.

Regulation of Cardiac Activity

Though the human heart is myogenic, it is also under dual control — nervous as well as hormonal. The heart sets its own beat; the body only adjusts it.

Nervous Control

The nervous control includes a part of the autonomic nervous system.

The sympathetic system (with the hormone epinephrine as neurotransmitter) increases the rate of heartbeat during an emergency. This is the “fight or flight” response — when danger appears, the muscles need more oxygen at once, and only a faster heart can deliver it.

The parasympathetic system (with acetylcholine as neurotransmitter) reduces the rate of heartbeat. It is the system that dominates in rest and calm.

The cardiovascular centre of the autonomic nervous system lies in the medulla oblongata — the same region that houses the respiratory centres of Section 8.4. It controls the rate of the heartbeat in response to inputs from various receptors:

Proprioreceptors, which monitor the position of the limbs and muscles. (This is why the heart begins to speed up as soon as you start moving, even before the blood chemistry has changed at all.)
Chemoreceptors, which monitor chemical changes in the blood.
Baroreceptors, which monitor the stretching of the main arteries and veins — in effect, sensing the blood pressure.

Chemical Control

Chemical control of the heart rate includes conditions such as hypoxia (a shortage of oxygen), acidosis and alkalosis, all of which cause decreased cardiac activity.

Hormones like epinephrine and norepinephrine enhance the cardiac activity.

Besides these, the concentration of cations like \(K^+\), \(Ca^{++}\) and \(Na^+\) has a major effect on cardiac activity. Cardiac activity decreases with an elevated blood level of \(K^+\) and \(Na^+\).

Real-World Application

This last point explains a fact that surprises many students: an abnormal potassium level in the blood can stop a heart. Cardiac muscle depends on the movement of these ions across its membrane to generate each impulse, so a badly disturbed ion balance — as may happen in kidney failure, in severe dehydration, or in a person taking certain diuretics without supervision — can disturb the rhythm of the heart or arrest it altogether. This is why blood electrolyte levels are checked so carefully in hospitalised patients.

Compare Table

Do Not Mix Up

Myogenic heart vs Nervous control (Saying the heart is myogenic means the beat originates in the nodal tissue of the heart muscle itself — no nerve is needed to start it. Saying the heart is under nervous control means the nerves can modify that beat, making it faster or slower. The nervous system is the accelerator and the brake; it is not the engine.)

Try Yourself

Why does a transplanted heart beat at a higher rate than normal?
Why does a heart recipient have to rely on a lifetime supply of immunosuppressants?

8.15 Blood Vessels

There are three main types of blood vessels in the human circulatory system, viz. arteries, veins and capillaries.

The three are not simply three sizes of pipe. Each is built for a different job — carrying blood away under pressure, bringing it back against gravity, or letting materials pass through the wall — and in each case the structure of the wall follows from that job.

Arteries

These blood vessels carry blood from the heart to various parts or organs of the body. There they branch into arterioles, and further into fine capillaries.

Arteries normally carry oxygenated blood to all parts of the body — except the pulmonary artery, which carries deoxygenated blood. Note, then, that an artery is defined by its direction, not by the kind of blood inside it: an artery is a vessel carrying blood away from the heart.

Arteries are usually situated deep in the body, except for a few like the radial, brachial and femoral, which are superficially located. Being deep is a protection: a torn artery bleeds fiercely, and the body keeps most of them well out of harm’s way.

In a T.S. of an artery, its wall shows three layers, as seen in Fig. 8.23.

Tunica externa or tunica adventitia
Tunica media
Tunica interna or intima

Diagram

Fig. 8.23 – T.S. of artery, vein and capillary: label tunica externa, tunica media, tunica interna and lumen in the artery and the vein (showing the artery’s thicker tunica media and narrower lumen, and the vein’s thinner wall and wider lumen); and the capillary as a single thin layer around a small lumen.

The outermost tunica externa is a thick, tough layer of collagen fibres. It is the protective coat, and it prevents the vessel from being over-stretched.

The tunica media is made up of smooth muscles and elastic fibres. This thick muscular and elastic layer makes the arterial wall pulsatile — able to stretch as a surge of blood arrives and to recoil once it has passed.

The innermost tunica interna is a single layer of flat, compact endothelial cells surrounding the lumen. The angular margin around the lumen shows tesselations.

The arterial lumen is devoid of valves, and blood flows through it rapidly and with high pressure.

Why are there no valves in an artery? Because they are not needed. The blood in an artery is being driven forward by the powerful ventricular systole, and the elastic recoil of the wall keeps pushing it on between beats. There is never a moment when the blood is at risk of sliding backwards.

Structure and Function

The artery (thick, muscular and elastic tunica media → the wall stretches when the ventricle ejects blood and recoils in between → the high pressure of the pulse is absorbed, and the recoil keeps the blood moving forward even during diastole; narrow lumen and thick wall → the high pressure is contained without the vessel bursting; deep placement in the body → protection from injury)

Veins

Veins are thin-walled, mostly superficial vessels, which carry blood from the organs towards the heart. The capillaries around the various organs join to form the veins.

Except for the pulmonary veins, the other veins of the body carry deoxygenated blood towards the heart. Once again, the direction — towards the heart — is what makes a vessel a vein.

Portal vein: A portal vein, e.g. the hepatic portal vein, differs from other normal veins in that it starts as capillaries from one organ and capillarises again in some intermediate organ, e.g. the liver, before taking the blood towards the heart.

So a portal vein is a vein with a detour built into it. The hepatic portal vein carries blood from the intestine straight to the liver, so that the food absorbed from the gut is processed and detoxified before it is released into the general circulation.

Definition

(A portal vein is a vein that begins in the capillaries of one organ and ends in the capillaries of another organ, instead of returning directly to the heart.)

Histologically, the veins also show the same three layers as the arteries — the tunica externa, tunica media and tunica interna. However, the tunica media is comparatively thinner, and the lumen is wide.

Internal valves at regular intervals can be seen in veins. Blood flows with low pressure, and the valves prevent the backflow of blood.

Here the valves are essential, precisely because the pressure is low. Blood in the veins of the leg has to travel upwards to the heart, against gravity, with very little force behind it. Each time the leg muscles contract, they squeeze the veins and push a column of blood along; the valves then shut behind it, so that it cannot fall back while the muscle relaxes.

Exam Corner

Q. Why do the veins have valves, when the arteries have none?

A. Because of pressure. Blood in an artery is driven forward with great force by the ventricle, so it can never flow backwards, and no valve is needed. Blood in a vein has already passed through the capillaries and has lost most of that pressure, and in the limbs it must climb upwards against gravity. Without valves it would simply slide back down between one muscular squeeze and the next. The valves make sure that every push, however weak, moves the blood one step closer to the heart.

Real-World Application

Varicose veins are the visible result of these valves failing. When the valves in the superficial veins of the legs become weak, blood pools in them instead of being carried upwards; the veins swell, twist and stand out under the skin. This is why the condition is common in people who stand for long hours, and why doctors advise such people to move, walk and flex their calf muscles regularly — the calf muscles are, in effect, the pump on which the leg veins depend.

Capillary

These are a network of minute blood vessels. They are thin-walled, having a single layer of flat squamous epithelium resting on a single basement membrane.

Capillaries are mainly involved in the exchange of materials. This is the whole purpose of the circulation: the arteries and the veins are only the transport lines, and it is here, in the capillaries, that oxygen, nutrients, carbon dioxide and wastes actually pass between the blood and the tissue.

The wall of a capillary is formed of a single layer of squamous epithelium, and it is stretchable. Blood flows through the capillaries under high pressure.

The wall of a capillary bears small endothelial pores or fenestrae, through which blood cells (WBCs) can escape by the process called diapedesis — the process you met in Section 8.11.

Structure and Function

The capillary (wall of a single squamous epithelial layer → an extremely short diffusion distance → oxygen, nutrients and wastes cross easily; an enormous branching network → almost every cell of the body lies within diffusion range of a capillary → exchange can serve the whole body; endothelial pores (fenestrae) → leucocytes can squeeze out by diapedesis → defence cells reach the tissue where infection has occurred)

Compare Table

Zoom In / Out

The vascular tree (Zoom out: the aorta, the largest artery, leaves the heart as a single wide vessel. Zoom in: it divides into arteries, then arterioles, then capillaries — each vessel narrower than the last, but far more numerous, so that the total cross-section keeps increasing. The capillaries then unite into venules and veins, and finally into the venae cavae, which return to the heart. The tree branches, exchanges, and gathers itself again.)

Pulse

The pulse is a series of pressure waves that travel through the arteries due to ventricular systole.

Every time the left ventricle contracts, it throws a volume of blood into an already full aorta. The elastic arterial wall bulges to accommodate it, and that bulge travels along the arteries as a wave. What you feel at the wrist is not the blood arriving; it is this pressure wave passing.

The pulse is strongest in the arteries closer to the heart, and gradually becomes weak in the arteries away from the heart.

It can be felt easily in the superficial arteries like the radial artery in the wrist and the carotid artery in the neck. The pulse can be felt at particular points on the body. All the locations where the pulse can be felt are shown in Fig. 8.24.

Diagram

Fig. 8.24 – Pulse points on the human body: label temporal, facial, carotid, brachial, radial, femoral, popliteal, posterior tibial and dorsalis pedis.

Exam Corner

Q. Why can you feel a pulse at the wrist or the neck, but never over a vein?

A. Because a pulse is a pressure wave produced by ventricular systole, and it can only be felt where that wave is still strong. By the time the blood has passed through the capillaries and reached the veins, the pressure surge has been completely smoothed out and lost, and the blood there flows in a steady, low-pressure stream with no wave in it. You feel a pulse only in a superficial artery — one close to the skin, like the radial or the carotid — where the wave can be pressed against a bone underneath.

Pulse rate is equal to heart rate. A pulse rate higher than normal (above 100 beats/min) is called tachycardia, and a pulse rate slower than normal (below 60 beats/min) is called bradycardia.

Do Not Mix Up

Tachycardia vs Bradycardia (Tachy- means fast: tachycardia is a heart rate above 100 beats per minute. Brady- means slow: bradycardia is a heart rate below 60 beats per minute. Note that neither is automatically a disease — a trained athlete may have a resting rate below 60 simply because a strong heart pumps a larger stroke volume and therefore needs fewer beats.)

Trace It

A drop of blood from the left ventricle to the tip of your finger and back 1. The left ventricle contracts, the aortic semilunar valves open, and the drop is ejected into the aorta under high pressure — the surge you would feel as a pulse. 2. It travels along the aortic arch, into the arteries of the arm, and reaches the radial artery at the wrist. 3. The artery branches into arterioles, and these into fine capillaries within the tissue of the fingertip. 4. In the capillary, the drop gives up oxygen and nutrients to the tissue cells and picks up carbon dioxide and wastes. It has now become deoxygenated, and most of its pressure has been spent. 5. The capillaries unite into venules and then into the veins of the arm, where valves ensure that the blood can move only towards the heart. 6. The blood is squeezed onward by the contraction of the arm muscles, passes into the superior vena cava, and returns to the right atrium of the heart — ready, as you saw in Section 8.13, to be sent to the lungs.

8.16 Blood Pressure (B.P.)

The pressure exerted by blood on the wall of the blood vessels is called blood pressure. It is measured by the sphygmomanometer, and it is usually measured from the arteries.

Definition

(Blood pressure is the pressure exerted by the blood on the walls of the blood vessels.)

Why measure it in an artery and not a vein? Because the artery is where the pressure is generated and where it matters. As you saw in Section 8.15, the pulse wave from the ventricle is strong in the arteries and is spent by the time the blood reaches the veins. A reading taken from a vein would tell us almost nothing about how forcefully the heart is pumping.

Arterial Blood Pressure

The pressure exerted by blood on the wall of an artery is the arterial blood pressure. It is not one number but two, because the pressure inside an artery rises and falls with every heartbeat.

The pressure on the arterial wall during ventricular contraction (systole) is the systolic pressure (SP). For a normal healthy adult, the average value is 120 mmHg.

The pressure on the arterial wall during the relaxation of the ventricles is the diastolic pressure (DP). For a normal healthy adult it is 80 mmHg.

\[B.P. = SP/DP = 120/80\ \text{mmHg}\]

Blood pressure is normally written as 120/80 mmHg.

Notice that the diastolic pressure never falls to zero, even though the heart has stopped pushing. Recall the elastic tunica media of Section 8.15: the artery, stretched during systole, recoils during diastole and keeps squeezing the blood forward. The 80 mmHg is the pressure maintained by that elastic recoil.

The difference between systolic and diastolic pressure is called the pulse pressure. Normally, it is 40 mmHg.

\[\text{Pulse pressure} = SP - DP = 120 - 80 = 40\ \text{mmHg}\]

Numerical 8.4

A patient’s blood pressure is recorded as 150/95 mmHg. Calculate the pulse pressure, and state whether this reading lies above or below the threshold of hypertension.

When Blood Pressure Goes Wrong

Deviations from the normal blood pressure value indicate malfunctioning of the heart. It may be due to high or low blood volume, arterial inelasticity or hardening of the arteries (arteriosclerosis), deposition of fats like cholesterol in the arteries (atherosclerosis), renal diseases, emotion-induced hormonal changes, obesity, and so on.

Blood pressure lower than normal, i.e. below 90/60 mmHg, is called hypotension, and blood pressure higher than normal, i.e. above 140/90 mmHg, is hypertension.

Do Not Mix Up

Arteriosclerosis vs Atherosclerosis (Arteriosclerosis is the general hardening and loss of elasticity of the arterial wall — a stiff pipe. Atherosclerosis is the deposition of fats such as cholesterol, along with calcium and fibrous tissue, inside the artery, which narrows the lumen — a blocked pipe. Both raise the blood pressure, but one stiffens the wall while the other obstructs the channel.)

Factors Affecting Blood Pressure

Various factors that affect the blood pressure are cardiac output, peripheral resistance, blood volume, length and diameter of blood vessels, viscosity of blood, age, gender, venous return, sleep, emotions, exercise, anxiety, and so on.

That is a long list, but every item on it acts in one of three simple ways — by changing how much blood is pushed in, how hard it is to push the blood through, or how much blood there is to push. Take them in turn.

Cardiac output. The normal cardiac output is 5 lit/min. An increase in cardiac output increases the systolic pressure. This follows directly from Section 8.14: more blood forced into the same arterial system in the same time must raise the pressure inside it.

Peripheral resistance. Peripheral resistance depends upon the diameter of the blood vessels. A decrease in the diameter of the arterioles and capillaries — under the effect of vasoconstrictors like vasopressin or ADH — causes an increase in peripheral resistance, and thereby an increase in blood pressure. Squeeze the outflow, and the pressure behind it rises, just as it does when you pinch the end of a garden hose.

Blood volume. Blood loss in accidents decreases the blood volume and thus the blood pressure. This is why a person who has lost a great deal of blood is in danger even before the oxygen shortage becomes serious — the pressure driving the circulation itself collapses.

Viscosity of blood. Blood pressure is directly proportional to the viscosity of blood. A thicker fluid is harder to drive through a tube, so a higher pressure is required to move it.

Age. Blood pressure increases with age, due to an increase in the inelasticity of the blood vessels. An artery that can no longer stretch to absorb the surge of systole allows the pressure to rise higher.

Venous return. The amount of blood brought to the heart via the veins per unit time is called the venous return, and it is directly proportional to blood pressure. The heart can only pump out what is delivered to it.

Length of the blood vessels. Blood pressure is also directly proportional to the total length of the blood vessel. The longer the pipe, the more the friction along it, and the greater the pressure needed to push blood the whole way. This is one reason obesity raises blood pressure: extra tissue means extra kilometres of capillary to be supplied.

Vasoconstriction and vasodilation. Blood pressure can also be affected by vasoconstriction (narrowing of the vessels, which raises it) or vasodilation (widening, which lowers it).

Gender. Females (before menopause) have slightly lower B.P. than males of the same age. However, the risk of high B.P. increases in females after menopause sets in.

Exam Corner

Q. Why are obese persons prone to hypertension?

A. Several of the factors above act together in obesity. Extra body tissue must be supplied with blood, so the total length of the vascular network increases — and blood pressure is directly proportional to that length. The heart must therefore raise its cardiac output to perfuse the larger body. Fat deposition in the arteries (atherosclerosis) narrows the lumen and raises the peripheral resistance at the same time. More output, more resistance and a longer network all push the pressure up together.

Measurement of Blood Pressure

Blood pressure is measured with the help of an instrument called a sphygmomanometer, shown in Fig. 8.25.

Diagram

Fig. 8.25 – (a) Sphygmomanometer: an inflatable rubber cuff covered with cloth, connected by tubes to a mercury manometer on one side and a rubber bulb on the other; (b) Stethoscope.

The instrument consists of an inflatable rubber bag cuff covered by a cotton cloth. It is connected with the help of tubes to a mercury manometer on one side and a rubber bulb on the other side.

During measurement, the person is asked to lie in a sleeping position. The instrument is placed at the level of the heart, and the cuff is tightly wrapped around the upper arm. The cuff is then inflated until the brachial artery is blocked due to the external pressure.

Then the pressure in the cuff is slowly lowered until the first pulsatile sound is heard. At this moment, the pressure indicated in the manometer is the systolic pressure.

The reasoning behind that step is worth making explicit. As long as the cuff pressure is above the systolic pressure, the artery stays completely shut and no blood — and no sound — gets through. The moment the cuff pressure falls just below the systolic pressure, the peak of each pulse is able to force the artery open for an instant, and blood spurts through the narrowed vessel with a sound. So the first sound marks the systolic pressure exactly.

The sounds heard during the measurement of blood pressure are called Korotkoff sounds.

The pressure in the cuff is further lowered until no pulsatile sound can be heard any more, because the blood flow has become smooth. At this moment, the pressure indicated in the manometer is the diastolic pressure — the artery has stopped being squeezed shut at any point in the cycle, so the turbulent, noisy flow gives way to silent, smooth flow.

An optimal blood pressure (normal) level reads 120/80 mmHg.

Trace It

Taking a blood pressure reading 1. The cuff is wrapped around the upper arm, at the level of the heart, and inflated. 2. The cuff pressure is raised above the systolic pressure — the brachial artery is completely squeezed shut, no blood flows, and the stethoscope hears nothing. 3. The cuff pressure is slowly released. When it falls just below the systolic pressure, the peak of each pulse forces the artery briefly open and blood spurts through: the first Korotkoff sound is heard. The manometer reading at this instant is the systolic pressure (normally 120 mmHg). 4. As the pressure falls further, the artery is open for longer during each cycle, and the sounds continue. 5. When the cuff pressure falls below the diastolic pressure, the artery stays open throughout the cycle, the flow becomes smooth, and the sounds disappear. The reading at this instant is the diastolic pressure (normally 80 mmHg). 6. The result is written as systolic over diastolic — 120/80 mmHg.

Hypertension

Persistently raised blood pressure, higher than the normal, is called hypertension. 140/90 mmHg is called the threshold of hypertension, and readings of 180/120 mmHg and higher are dangerous to health. It may damage the heart, the brain and the kidneys.

Notice the word persistently. A single high reading after climbing stairs or after a fright is not hypertension — emotion and exercise raise the pressure in everyone. Hypertension is a pressure that stays raised even at rest.

Under the condition of hypertension, the heart uses more energy for pumping. This causes angina pectoris — the chest pain due to a lowered blood supply to the cardiac muscles — and it may lead to myocardial infarction.

There are more chances of brain haemorrhage due to hypertension, as the arteries in the brain are less protected by the surrounding tissues as compared to those of other organs. In the kidney, hypertension may cause kidney failure.

Hypertension is often called a silent condition, and the reason should now be clear: it produces no pain of its own. The damage it does is gradual — to the walls of the arteries, and through them to the heart, the brain and the kidneys — and it is usually discovered only when one of those organs begins to fail.

Coronary Artery Disease (CAD)

Coronary Artery Disease is also known as atherosclerosis. In this disease, calcium, fat, cholesterol and fibrous tissues get deposited in the blood vessels supplying blood to the heart muscles, making the lumen narrow.

Recall from Section 8.13 that the heart cannot feed itself from the blood inside its chambers — it depends on its own coronary arteries. A narrowing of those vessels therefore starves the very muscle that is doing the pumping.

Angina Pectoris

Angina pectoris is the pain in the chest resulting from a reduction in the blood supply to the cardiac muscles, because of atherosclerosis or arteriosclerosis.

It is characterised by severe pain and heaviness in the chest. The pain may spread to the neck, the lower jaw, the left arm and the left shoulder.

The pain usually results from exertion, when there is more demand for oxygen by the heart, but the supply does not meet the requirement. This is why angina typically appears on climbing stairs and eases with rest: at rest, the narrowed artery can just about supply the heart; under exertion, it cannot.

Real-World Application

The pain of angina and of a heart attack often does not stay in the chest — it radiates to the left arm, the shoulder, the neck or the jaw. This is not because those parts are diseased, but because the nerves carrying pain from the heart enter the spinal cord at the same levels as those from the arm and shoulder, and the brain misreads the source. Knowing this is genuinely useful: a person with a crushing chest discomfort spreading to the left arm and jaw needs emergency medical help, not rest and an antacid.

Angiography

Angiography is the X-ray imaging of the cardiac blood vessels to locate the position of blockages.

Depending upon the degree of blockage, remedial procedures like angioplasty or by-pass surgery are performed.

In angioplasty, a stent is inserted at the site of the blockage to restore the blood supply — the narrowed vessel is propped open from within.

In by-pass surgery, the atherosclerotic region is by-passed with a part of a vein or artery taken from any other suitable part of the body, like the hands or the legs — a new road is built around the blocked one.

Heart Transplant

The replacement of a severely damaged heart by a normal heart from a brain-dead or recently dead donor is called a heart transplant.

A heart transplant is necessary in the case of patients with end-stage heart failure and severe coronary arterial disease — that is, when the heart muscle itself is too damaged to be repaired.

Exam Corner

Q. Why must a heart-transplant recipient take immunosuppressants for life?

A. Because the transplanted heart carries the donor’s antigens, not the recipient’s. The recipient’s T-lymphocytes (Section 8.11) recognise the graft as foreign and mount a cell-mediated attack on it — the process called rejection. Immunosuppressant drugs damp down this immune response so that the graft survives. The trade-off is real: with the immune system weakened, the patient becomes more vulnerable to infections, and must be monitored carefully for life.

Silent Heart Attack

A silent heart attack, also known as a silent myocardial infarction, is a type of heart attack that lacks the general symptoms of a classic heart attack, such as extreme chest pain, hypertension, shortness of breath, sweating and dizziness.

The symptoms of a silent heart attack are so mild that a person often confuses them for regular discomfort and thereby ignores them. It has been studied that men are more affected by silent heart attack than women.

The danger, of course, lies in exactly this mildness. The heart muscle is dying just as surely as in a classic attack, but the warning that would send the patient to a hospital never arrives.

Side Note

Akash Manoj, a teenager from Chennai, invented a non-invasive technique to predict the possibility of a silent heart attack. He developed this technique while he was in Class X, and for his innovation he was invited to Rashtrapati Bhavan as a guest of the President of India under the Innovation Scholars In-Residence Programme. His kit analyses the level of FABP3 (Fatty Acid Binding Protein-3) with the help of UV light. FABP3 is the smallest protein in the blood, and it leaks out of damaged heart muscle — so detecting it can reveal the injury that the patient never felt.

Try Yourself

Surf the internet for video clips of angiography, angioplasty and by-pass surgery, and gather more information about these medical procedures.

8.17 Electrocardiogram

You know from Section 8.13 that every heartbeat begins as an electrical impulse in the SA node and spreads through the nodal tissue to the rest of the heart. That impulse is an electrical event — and electricity, unlike a contraction, can be detected from a distance. As the wave of excitation travels through the heart wall, tiny electrical variations reach the surface of the body, where they can be picked up through the skin.

The graphical recording of the electrical variations detected at the surface of the body, during their propagation through the wall of the heart, is an electrocardiogram (ECG). This recording may be in the form of a printout or an onscreen display.

Definition

(An electrocardiogram (ECG) is the graphical record of the electrical changes that occur in the heart during a cardiac cycle, detected at the surface of the body.)

The instrument used for this recording is the ECG machine or electrocardiograph. This instrument detects and amplifies the signals — amplification is essential, because the electrical changes reaching the skin are extremely small.

Real Incident / Discovery

The first practical electrocardiograph was built in the early 1900s by Willem Einthoven, a Dutch physiologist. His machine, called the string galvanometer, was so large that it filled a room and needed several people to operate it — and yet it could record the faint electrical signals of the beating heart with remarkable accuracy. Einthoven also gave the waves the names P, Q, R, S and T that are still used today, and he was awarded the Nobel Prize for this work. What was once a room-sized apparatus is today a portable machine found in every hospital.

Recording the Signals

Various electrodes are used for the recording of the signals.

Four electrodes are positioned on the limbs — two on the arms and two on the legs. These are the limb electrodes.

Six electrodes are positioned on the chest. These are the chest electrodes.

Together they view the same electrical event from many different angles. This is what allows a physician to work out not only that something is wrong but where in the heart it is wrong.

The Waves of a Normal ECG

In a normal record, three different waves are recognised: the P-wave, the QRS complex and the T-wave, as shown in Fig. 8.26.

Diagram

Fig. 8.26 – Normal ECG: X-axis, time in seconds (0 to 1.6); Y-axis, millivolts (−1 to +2); show the P-wave, the QRS complex (with Q, R and S marked) and the T-wave; also mark the P-R interval (= 0.16 sec), the Q-T interval, the S-T segment and the R-R interval; label “atria” above the P-wave and “ventricles” above the QRS complex.

The P-wave is a small upward deflection from the baseline of the graph. It represents the atrial depolarization — the spread of the electrical impulse through the atria, which is immediately followed by atrial systole.

The QRS complex starts as a slight downward deflection from the baseline, continues as a sharp and large upright wave, and ends as a downward wave. The QRS complex represents the ventricular depolarization.

It is much larger than the P-wave, and the reason is simply mass: the ventricular walls are far thicker and more muscular than the atrial walls, so a far greater volume of muscle is being excited, and the electrical signal produced is correspondingly stronger.

The T-wave is a small, wide and upwardly elevated wave. It represents the ventricular repolarization — the recovery of the ventricular muscle, which then relaxes.

Do Not Mix Up

Depolarization vs Repolarization (Depolarization is the electrical excitation of the heart muscle — the change that triggers it to contract. Repolarization is the electrical recovery of that muscle, which allows it to relax and be ready for the next beat. The P-wave and the QRS complex are depolarizations (of the atria and the ventricles), while the T-wave is a repolarization (of the ventricles). Remember that the ECG records the electrical event, not the mechanical contraction — the contraction follows a moment later.)

Interpret Data

Reading a normal ECG (The horizontal axis is time in seconds and the vertical axis is voltage in millivolts. Read one cycle from left to right. The small P-wave comes first: the atria are being excited, and atrial systole follows. Then the tall, sharp QRS complex: the ventricles are being excited, and ventricular systole follows — this is why the QRS complex marks the loudest part of the heartbeat, when the cuspid valves snap shut with the ‘lubb’ sound. Finally the broad T-wave: the ventricles are recovering electrically and are about to relax. The gap from the start of the P-wave to the start of the QRS complex is the P-R interval, normally 0.16 sec — and it corresponds to the delay at the AV node that lets the atria finish emptying before the ventricles contract. The distance from one R peak to the next is the R-R interval, and it gives the duration of one cardiac cycle — from which the heart rate can be calculated directly.)

Numerical 8.5

In a patient’s ECG record, the R-R interval is found to be 0.75 seconds. Calculate the person’s heart rate in beats per minute, and state whether it lies within the normal range, or indicates tachycardia or bradycardia.

What an ECG Tells the Doctor

An ECG helps to diagnose:

an abnormality in the conducting pathway,
enlargement of the heart chambers,
damage to the cardiac muscles,
reduced blood supply to the cardiac muscles, and
causes of chest pain.

A physician can find out the defect in the heart by examining the wave pattern and the time interval between the waves.

Each of these follows logically from what the waves mean. If the conducting pathway is blocked, the impulse takes longer than usual to travel from the atria to the ventricles — so the P-R interval lengthens. If a chamber is enlarged, more muscle is being depolarized and the corresponding wave grows taller. And if a region of cardiac muscle is short of blood or already dead, that region no longer conducts or recovers normally, so the S-T segment and the T-wave become distorted. The ECG is, in this sense, a map of the heart’s electrical health — and it can be read within minutes, without a single cut.

Structure and Function

The ECG as a diagnostic tool (the heart’s impulse spreads through a fixed conducting pathway → each stage of that spread produces a characteristic wave at a characteristic time → any change in the size, shape or timing of a wave points to a specific fault in a specific part of the heart → the physician can localise the damage without opening the chest)

Try Yourself

What is depolarization and repolarization?
What is the correlation between depolarization and repolarization on the one hand, and contraction and relaxation of the heart on the other?
How are the signals detected and amplified by the electrocardiograph?
Who discovered the ECG?

8.18 Lymphatic System

The lymphatic system consists of lymph, lymphatic vessels, and some organs and tissues.

The word ‘lymph’ means ‘clear water’, and it is a fluid connective tissue with almost a similar composition to blood — except that it lacks RBCs, platelets and some proteins.

That single difference explains almost everything about lymph. Having no RBCs, it is colourless rather than red, and it carries no haemoglobin and therefore no significant load of oxygen. Having fewer proteins, it is thinner and more watery than plasma. What it does carry, and carry in plenty, are lymphocytes — the defence cells you met in Section 8.11.

Where Lymph Comes From, and Where It Goes

Recall from Section 8.8 that in a closed circulation the blood never touches the body cells directly: the exchange between blood and tissue takes place through an intermediate fluid. That fluid is what fills the spaces between the cells — and lymph is what happens to it next.

Fluid from the intercellular spaces of the body tissue enters into the lymphatic vessels. From here it is discharged into the blood vessels (veins) through the thoracic duct and the right lymphatic duct.

So the lymphatic system is a one-way drainage line, not a circuit. Notice the contrast with the blood vascular system: blood leaves the heart and returns to it, going round and round, whereas lymph begins blindly in the tissues and travels in one direction only — back to the veins. It has no pump of its own, and is moved chiefly by the contraction of the surrounding muscles and by valves that prevent it from slipping backwards.

The organs of the system are shown in Fig. 8.27, and the way it runs alongside the blood circulation is shown in Fig. 8.28.

Diagram

Fig. 8.27 – Lymphatic system: label tonsil, cervical lymph nodes, subclavian vein, thymus gland, thoracic duct, red bone marrow, spleen, axillary lymph nodes, appendix, inguinal lymph nodes, popliteal lymph nodes, and the network of lymphatic vessels throughout the body.

Diagram

Fig. 8.28 – Circulation and lymphatic system: (a) relationship of lymphatic capillaries to tissue cells and blood capillaries — label arteriole, blood capillary, venule, tissue cell, interstitial fluid, lymph, lymphatic capillaries; (b) details of a lymphatic capillary — label endothelium of the lymphatic capillary, opening, anchoring filament, interstitial fluid, tissue cell, lymph.

Flowchart

FC 8.17 – Path of lymph: blood capillary → fluid filters out into the intercellular (interstitial) spaces → bathes the tissue cells and exchanges materials with them → excess fluid enters the blind-ended lymphatic capillaries as lymph → lymphatic vessels → lymph nodes (lymph is filtered, lymphocytes act on any pathogens) → larger lymphatic vessels → thoracic duct and right lymphatic duct → subclavian veins → returned to the blood.

Structure and Function

The lymphatic capillary (blind-ended tube with overlapping endothelial cells that form flap-like openings → interstitial fluid can push its way in but cannot leak back out → the fluid moves in one direction only, out of the tissue; anchoring filaments tie the capillary to the surrounding tissue → when the tissue swells with fluid, the filaments pull the flaps open wider → the more fluid there is to drain, the more freely it enters)

Compare Table

Exam Corner

Q. If the blood capillaries already reach every tissue, why does the body need a second set of vessels at all?

A. Because more fluid leaves the blood capillaries into the tissue spaces than is taken back into them. If that surplus were simply left behind, it would accumulate and the tissues would swell. The lymphatic vessels collect this excess fluid and return it to the veins, keeping the tissue spaces from flooding. On the way, the fluid is passed through the lymph nodes, where lymphocytes screen it — so the drainage line doubles up as a defence checkpoint.

Real-World Application

Filariasis, spread by mosquitoes in many parts of India, is a striking demonstration of what the lymphatic system does — because it shows what happens when the system stops working. The adult worms lodge in the lymphatic vessels and block them. The tissue fluid can no longer be drained away, so it accumulates, and the affected limb swells enormously and its skin thickens — the condition known as elephantiasis. The blood circulation in the limb is intact; it is only the drainage that has failed.

Side Note

Tonsils are small lymphatic nodules in the pharyngeal region. Normally there are five tonsils, strategically positioned to fight against inhaled and ingested foreign substances — which is exactly what one would expect at the crossroads of the food and air passages you studied in Section 8.2.

Inflammation of the tonsils is called tonsillitis. It is caused by a viral or bacterial infection. The symptoms include sore throat, fever, swollen lymph nodes, nasal congestion, difficulty in swallowing, headache and so on. Viral tonsillitis cures naturally, but bacterial tonsillitis needs antibiotic treatment. Tonsillectomy — surgical removal of the tonsils — is performed in some patients who do not respond to the treatment.

Zoom In / Out

The lymphatic system (Zoom out: the lymphatic system is one half of the circulatory system of the human body, working alongside the blood vascular system — one delivers, the other drains. Zoom in: it is built of vessels, and those vessels begin as single blind-ended lymphatic capillaries, each with a wall just one cell thick, lying between the tissue cells and quietly collecting the fluid they leave behind.)

Try Yourself

Find out the location of the lymph nodes in the human body, and mark them on an outline diagram.

8.19 Summary

Respiration is the biochemical process in which organic compounds are oxidised in an orderly, step-by-step manner to liberate chemical energy as ATP: \(C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 38\ \text{ATP}\). The site where gases are exchanged with the environment is the respiratory surface.
An efficient respiratory surface must have a large surface area, must be thin, highly vascular and permeable, and must be moist. These three features together make diffusion — a purely physical process — fast enough to supply the organism.
Plants exchange gases by diffusion through stomata on leaves and young stems, through the continuous intercellular air spaces, through the moisture film around the roots in aerated soil, and through lenticels in the bark of woody plants.
Animals show a variety of respiratory organs suited to their body complexity and their medium: the plasma membrane in protists, sponges and coelenterates; the moist body surface in flatworms, annelids and amphibians; tracheal tubes and spiracles in insects; book lungs in arachnids; book gills in Limulus; external gills in tadpoles; internal gills in fish; lungs in reptiles, birds and mammals; and the cloaca in turtles. In higher animals the respiratory organ is linked to a transport system.
The human upper respiratory system consists of the external nares, nasal cavities, internal nares, nasopharynx, nose and throat; the lower respiratory system consists of the larynx, trachea, bronchi, bronchioles and lungs. The nose filters, warms and moistens the air; the larynx bears the vocal cords and its opening, the glottis, is guarded by the epiglottis; the trachea is kept open by 16 to 20 ‘C’-shaped cartilage rings.
The lungs lie in the thoracic cavity, enclosed in a double pleural membrane with pleural fluid between the layers. The right lung has 3 lobes and the left 2. About 700 million alveoli, each with a wall of a single squamous epithelial layer, provide the surface for gaseous exchange. The diaphragm separates the thoracic and abdominal cavities.
Respiration includes four processes: breathing, external respiration, internal respiration and cellular respiration.
Inspiration is an active process: the diaphragm flattens and the external intercostal muscles raise the ribs and sternum, so the thoracic volume increases, the pressure falls, and air rushes in. Expiration reverses this — the muscles relax, the volume falls, the pressure rises, and air is pushed out. One inspiration plus one expiration is one breath.
Pulmonary volumes and capacities: tidal volume 500 ml; inspiratory reserve volume 2000 to 3000 ml; expiratory reserve volume 1000 to 1100 ml; dead space 150 ml; residual volume 1100 to 1200 ml; total lung capacity 5200 to 5800 ml; and vital capacity = TV + IRV + ERV = 4100 to 4600 ml.
External respiration occurs across the respiratory membrane, by diffusion from a higher to a lower partial pressure. The \(pCO_2\) of blood entering the pulmonary capillaries is 45 mmHg against 40 mmHg in alveolar air, so \(CO_2\) leaves the blood; the \(pO_2\) of that blood is 40 mmHg against 104 mmHg in alveolar air, so \(O_2\) enters the blood.
Oxygen transport: only 3% of oxygen travels dissolved in plasma; 97% binds to haemoglobin as oxyhaemoglobin, \(Hb + 4O_2 \rightarrow Hb(4O_2)\). Saturation depends on \(ppO_2\) — about 95 to 97% at 100 mmHg in the alveoli, and only 50% at 30 mmHg. The oxygen dissociation curve is sigmoid and shifts to the right with a rise in \(H^+\), in \(ppCO_2\), in temperature and in DPG, which lowers haemoglobin’s affinity for oxygen.
The Bohr effect is the shift of the oxyhaemoglobin dissociation curve due to a change in the partial pressure of \(CO_2\). The Haldane effect is the action of oxyhaemoglobin as an acid, whereby the rise in \(H^+\) converts \(HCO_3^-\) into \(H_2O\) and \(CO_2\).
Haemoglobin’s affinity for carbon monoxide is about 250 times that for oxygen. Carbon monoxide forms the stable compound carboxyhaemoglobin, blocking oxygen transport — this is carbon monoxide poisoning.
Carbon dioxide is transported in three forms: 7% dissolved in plasma as carbonic acid; 70% as bicarbonate ions, formed inside the RBCs by the enzyme carbonic anhydrase — the outward movement of \(HCO_3^-\) is balanced by \(Cl^-\) entering the RBC, the chloride shift or Hamburger’s phenomenon; and 23% bound to haemoglobin as carbaminohaemoglobin. At the lungs the whole chemistry runs in reverse.
Cellular respiration has two steps: oxidation, \(C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 686\ \text{Kcal}\); and phosphorylation, \(ADP + iP + 7.3\ \text{Kcal} \rightarrow ATP\). ATP is the energy currency of the cell.
Breathing is under dual control, nervous and chemical. The respiratory centres in the pons and medulla — the dorsal group, the ventrolateral group, the pneumotaxic centre and the apneustic centre — set the rate and depth. Adults breathe about 12 times per minute and newborns about 44 times per minute. The Hering–Breuer reflex, triggered by stretch receptors and carried by the vagus nerves, ends each inspiration and prevents over-inflation. Voluntary control is possible through the cerebral cortex, but is limited by the build-up of \(CO_2\) in the blood.
Modified respiratory movements, such as coughing and yawning, differ from normal breathing and help express emotion or clear the air passage; some are reflexes and some can also be started voluntarily.
Common respiratory disorders include emphysema and chronic bronchitis (from smoking and air pollution), acute bronchitis, sinusitis, laryngitis and pneumonia (from viruses and bacteria), asthma (from allergy), and the occupational disorders silicosis and asbestosis (from long-term dust exposure). Artificial ventilation and the ventilator support breathing when natural respiration fails.
Transport in organisms occurs by diffusion and active transport where the body is small and thin. Cyclosis — the streaming of cytoplasm, as in Paramoecium, Amoeba, root hair cells and WBCs — is intracellular transport. Extracellular transport is seen as water circulating through the body cavities of sponges and coelenterates, parenchymal circulation in flatworms, and the muscular movement of body fluid in roundworms.
In open circulation, blood flows through haemocoels at low pressure, contacts the tissues directly, and usually carries no respiratory pigment (arthropods, molluscs). In closed circulation, blood flows within vessels at high pressure, exchanges materials through lymph, and carries respiratory pigments (vertebrates, higher molluscs, annelids).
In single circulation, as in fishes, blood passes through the heart only once per cycle, and the heart is a ‘venous heart’. In double circulation, as in birds and mammals, blood passes through the heart twice — once in the pulmonary circulation to the lungs and back, and once in the systemic circulation to the body and back.
Blood is a fluid connective tissue of mesodermal origin: red, slightly alkaline (pH 7.4), salty and viscous, with a volume of 4 to 6 litres in an adult. It consists of plasma (55%) and formed elements (44%). Plasma is 90% water, 7 to 8% proteins, 1% inorganic salts and 1 to 2% other substances. The blood vascular system carries out transport, homeostasis and protection.
Erythrocytes are circular, biconcave and enucleated, numbering 5.1–5.8 million/mm³ in males and 4.3–5.2 million/mm³ in females, with a life span of 120 days. Erythropoiesis occurs in the liver and spleen in the foetus and in the red bone marrow in the adult, and is stimulated by erythropoietin from the kidney. Worn-out RBCs are destroyed in the liver and spleen, iron being stored as ferritin and the heme converted to biliverdin and then bilirubin. Polycythemia is an increase and erythrocytopenia a decrease in RBC number; anaemia is a shortage of RBCs or of haemoglobin.
Leucocytes are colourless, nucleated, amoeboid cells, 5000–11000 per mm³, that leave the capillaries by diapedesis and defend the body. Granulocytes (about 72%) are neutrophils (70%), basophils (0.5–1%) and eosinophils (1–3%); agranulocytes (about 28%) are lymphocytes (25–30%) and monocytes (3–5%). B-lymphocytes mature in the bone marrow and give humoral immunity; T-lymphocytes mature in the thymus and give cell-mediated immunity. Leucopenia, leucocytosis and leukemia are the disorders of WBC number.
Thrombocytes are non-nucleated fragments of megakaryocytes, 2.5–4.5 lakh/mm³, which form the platelet plug and secrete platelet factors and serotonin. In clotting, the intrinsic and extrinsic pathways activate a cascade of twelve clotting factors (I to XIII, factor VI not in active use); prothrombinase converts prothrombin to thrombin, and thrombin converts soluble fibrinogen into insoluble fibrin, which meshes the cells into a clot. Thrombocytopenia causes haemorrhage.
The heart lies in the mediastinum, weighs about 300 gm in males and 250 gm in females, and is enclosed in the pericardium, whose serous layers are separated by about 50 ml of pericardial fluid. Its wall has three layers — epicardium, myocardium and endocardium.
The four-chambered heart has thin-walled receiving atria (with the fossa ovalis in the inter-auricular septum) and thick-walled pumping ventricles, the left ventricular wall being about three times thicker than the right. The tricuspid and bicuspid valves guard the AV apertures, the chordae tendinae and papillary muscles hold them shut, and three semilunar valves guard each of the two great arteries — together ensuring a unidirectional flow of blood.
The heart is myogenic and autorhythmic. Its nodal tissue consists of the SA node (pacemaker), the AV node (pace setter), the bundle of His and the Purkinje fibres, which spread the impulse so that both ventricles contract simultaneously.
One cardiac cycle takes 0.8 sec: atrial systole 0.1 sec, ventricular systole 0.3 sec, and joint diastole 0.4 sec. Closure of the cuspid valves gives the ‘lubb’ sound and closure of the semilunar valves the ‘dub’ sound. Stroke volume is about 70 ml and the heart rate 70 to 72 per minute, so \(CO = SV \times HR = 70 \times 72 = 5040\ \text{ml/min}\).
Cardiac activity is under nervous control (the sympathetic system with epinephrine speeds the heart, the parasympathetic system with acetylcholine slows it, the cardiovascular centre lying in the medulla oblongata and receiving inputs from proprioreceptors, chemoreceptors and baroreceptors) and chemical control (hypoxia, acidosis and alkalosis decrease activity; epinephrine and norepinephrine increase it; raised \(K^+\) and \(Na^+\) decrease it).
Arteries carry blood away from the heart, have a thick muscular and elastic tunica media, a narrow lumen and no valves. Veins carry blood towards the heart, have a thinner tunica media, a wide lumen and internal valves. A portal vein begins and ends in capillaries. Capillaries have a wall one cell thick and are the site of exchange, with fenestrae through which WBCs escape by diapedesis. The pulse is the pressure wave produced by ventricular systole; above 100 beats/min it is tachycardia, below 60 beats/min bradycardia.
Blood pressure is measured with a sphygmomanometer and read as \(B.P. = SP/DP = 120/80\ \text{mmHg}\), the difference of 40 mmHg being the pulse pressure. Below 90/60 mmHg is hypotension and above 140/90 mmHg is hypertension. It is affected by cardiac output, peripheral resistance, blood volume, the length and diameter of the vessels, viscosity, age, gender, venous return, sleep, emotions, exercise and anxiety.
Hypertension may damage the heart, brain and kidneys, and may lead to angina pectoris, myocardial infarction, brain haemorrhage or kidney failure. Coronary artery disease (atherosclerosis) narrows the vessels supplying the heart muscle; it is investigated by angiography and treated by angioplasty or by-pass surgery. A severely damaged heart may need a heart transplant. A silent heart attack lacks the classic symptoms and is therefore easily ignored.
An ECG is the graphical record of the electrical changes of the heart, taken with four limb electrodes and six chest electrodes. The P-wave shows atrial depolarization, the QRS complex ventricular depolarization and the T-wave ventricular repolarization. It reveals faults in the conducting pathway, enlargement of the chambers, damage to the cardiac muscle and a reduced blood supply to it.
The lymphatic system consists of lymph, lymphatic vessels, organs and tissues. Lymph is a fluid connective tissue similar to blood but without RBCs, platelets and some proteins. It collects fluid from the intercellular spaces and returns it to the veins through the thoracic duct and the right lymphatic duct. Tonsils are lymphatic nodules in the pharyngeal region; their inflammation is tonsillitis.

Flowchart

FC 8.18 – The whole chapter in one chain: atmospheric air → breathing (inspiration by the diaphragm and intercostal muscles) → alveoli → external respiration across the respiratory membrane → O₂ into the blood, CO₂ out → transport by haemoglobin in RBCs and as bicarbonate in plasma → pulmonary veins → left atrium → left ventricle → aorta → systemic capillaries → internal respiration: O₂ released to the tissue cells, CO₂ picked up → cellular respiration inside the cell → ATP → deoxygenated blood → venae cavae → right atrium → right ventricle → pulmonary artery → lungs → CO₂ exhaled (cycle repeats); excess tissue fluid → lymphatic capillaries → lymph nodes → thoracic duct → back into the veins.

8.20 Points to Ponder

Think about the following questions after you have studied the chapter. They are meant to be argued over, not simply looked up — most of them have more than one defensible answer.

Respiration releases the energy of glucose in many small steps rather than in one leap. Suppose a cell could somehow carry out the whole oxidation in a single step. What exactly would go wrong, and would the cell gain anything at all from the change?
The features of a good respiratory surface — large area, thin wall, moist, richly supplied with blood — are the same in a fish’s gill, an insect’s tracheal tube and a human alveolus. Why should such different organs, in such different media, arrive at the same set of requirements?
An insect’s blood carries no respiratory pigment, yet insects are among the most active animals on earth. What does this suggest about the relationship between an animal’s respiratory system and its circulatory system? Could a large mammal ever adopt the insect solution?
Diffusion needs no energy and no machinery, and it works perfectly in a Paramoecium. What is it about size and shape — rather than about diffusion itself — that makes this simple mechanism useless for a rabbit?
Our heart is myogenic: it starts and times its own beat, and the nerves can only speed it up or slow it down. What advantage does an organism gain from a pump that does not wait for instructions? What would be the risk if the heart were entirely under the control of the brain?
The oxygen dissociation curve shifts to the right in an actively respiring tissue, so that haemoglobin gives up more of its oxygen there. In what sense can this be called an automatic response — one that needs no sensor, no nerve and no hormone?
Only 3% of the oxygen in blood travels dissolved in plasma, while 70% of the carbon dioxide travels as bicarbonate ions. Why should the body have evolved such different arrangements for two gases that cross the very same membrane?
Blood clots within seconds at a wound, but never inside a healthy vessel. Consider what would happen if the clotting mechanism were made a little more sensitive, or a little less. Why does the body hold it so finely balanced, and what does this tell you about the cost of both extremes?
Fever, inflammation, pus and swelling all feel like the illness itself. To what extent are they instead the body’s defence at work? Where would you draw the line between a defence that helps and a defence that has become harmful?
Hypertension damages the heart, the brain and the kidneys, and yet it causes no pain of its own. What does this tell you about the value of measuring things that a person cannot feel? Should routine blood-pressure measurement begin much earlier in life than it usually does?
Emphysema destroys the alveolar walls, and the lost surface area never comes back. Compare this with a cut in the skin, which heals completely. Why are some tissues of the body able to repair themselves and others not, and what does this imply for how we should treat the lungs?
Athletes at high altitude produce more red blood cells, and the hormone erythropoietin has been misused as a doping agent. Is training at high altitude — which produces exactly the same effect — any different in principle? On what grounds would you draw the line between preparation and cheating?
A donated heart continues to beat inside a stranger’s chest, but only if that person’s immune system is deliberately weakened for life. Reflect on the trade-off: the body defends itself by rejecting anything foreign, and the very same defence must be blunted to save the person’s life.

8.21 Objective Questions

The muscular structure that separates the thoracic and abdominal cavity is ________.
1. pleura
2. diaphragm
3. trachea
4. epithelium
What is the minimum number of plasma membranes that oxygen has to diffuse across to pass from the air in the alveolus to the haemoglobin inside an R.B.C.?
1. Two
2. Three
3. Four
4. Five
________ is a sound producing organ.
1. Larynx
2. Pharynx
3. Tonsils
4. Trachea
The maximum volume of gas that is inhaled during breathing in addition to T.V. is ________.
1. residual volume
2. I.R.V.
3. G.R.V.
4. vital capacity
________ muscles contract when the external intercostal muscles contract.
1. Internal abdominal
2. Jaw
3. Muscles in bronchial walls
4. Diaphragm
Movement of cytoplasm in unicellular organisms is called ________.
1. diffusion
2. cyclosis
3. circulation
4. thrombosis
Which of the following animals do not have closed circulation?
1. Earthworm
2. Rabbit
3. Butterfly
4. Shark
Diapedesis is performed by ________.
1. erythrocytes
2. thrombocytes
3. adipocytes
4. leucocytes
Pacemaker of the heart is ________.
1. SA node
2. AV node
3. His bundle
4. Purkinje fibers
Which of the following is without a nucleus?

Red blood corpuscle
Neutrophil
Basophil
Lymphocyte

Cockroach shows which kind of circulatory system?

Open
Closed
Lymphatic
Double

Diapedesis can be seen in ________ cell.

RBC
WBC
Platelet
neuron

Opening of the inferior vena cava is guarded by ________.

bicuspid valve
tricuspid valve
Eustachian valve
Thebesian valve

________ wave in ECG represents atrial depolarization.

P
QRS complex
Q
T

The fluid seen in the intercellular spaces in humans is ________.

blood
lymph
interstitial fluid
water

Match the respiratory surface to the organism in which it is found.

Respiratory surface	Organism
a. Plasma membrane	i. Insect
b. Lungs	ii. Salamander
c. External gills	iii. Bird
d. Internal gills	iv. Amoeba
e. Trachea	v. Fish

8.22 Check Your Concepts

The wall of an alveolus is only one cell thick, and the wall of a capillary beside it is also only one cell thick. Explain why a thicker, sturdier wall would be a disadvantage at this site, even though it would be stronger.
Assertion: Inspiration is an active process while quiet expiration is a passive one. Reason: During inspiration the diaphragm and external intercostal muscles contract, whereas during quiet expiration they simply relax and the stretched lung tissue recoils. Choose: (a) both assertion and reason are true and the reason explains the assertion; (b) both are true but the reason does not explain the assertion; (c) the assertion is true but the reason is false; (d) the assertion is false but the reason is true.
Arrange the following in the correct sequence, starting from the nostril, to trace the path of an inhaled molecule of oxygen: terminal bronchiole, nasal cavity, alveolus, larynx, primary bronchus, pharynx, trachea, secondary bronchus.
What would happen to a person’s breathing if the cartilage rings of the trachea were made of soft, flexible tissue instead of cartilage? Explain your answer with reference to the pressure changes during inspiration.
Draw a labelled diagram of the human respiratory system and mark on it the following: nasal cavity, pharynx, larynx, trachea, right main bronchus, left lung, diaphragm.
A student says, “Carbon monoxide is dangerous because it uses up all the oxygen in the room.” Explain why this statement is wrong, and give the correct reason for carbon monoxide poisoning.
Why is a person who has climbed rapidly to a high altitude likely to feel breathless, even though the percentage of oxygen in the air is exactly the same there as at sea level? Answer using the idea of partial pressure.
The enzyme carbonic anhydrase is found in the RBCs but not in the plasma. State two consequences of this fact for the transport of carbon dioxide.
Explain why the oxygen–haemoglobin dissociation curve shifting to the right in an actively contracting muscle is an advantage to that muscle.
Match the following:

Column A	Column B
a. Bohr effect	i. Movement of \(Cl^-\) into the RBC
b. Haldane effect	ii. Shift of the oxyhaemoglobin dissociation curve due to a change in \(ppCO_2\)
c. Hamburger’s phenomenon	iii. Reflex that ends inspiration when the lungs are stretched
d. Hering–Breuer reflex	iv. Oxyhaemoglobin acts as an acid and drives \(CO_2\) out of the blood

Why does the urge to breathe become irresistible when a person tries to hold their breath — and why is it the rise in carbon dioxide, rather than the fall in oxygen, that forces the next breath?
Assertion: A mature mammalian erythrocyte has no mitochondria. Reason: The absence of mitochondria means that the cell does not consume the oxygen it is carrying. Choose: (a) both assertion and reason are true and the reason explains the assertion; (b) both are true but the reason does not explain the assertion; (c) the assertion is true but the reason is false; (d) the assertion is false but the reason is true.
A patient’s blood report shows a normal RBC count but a low haemoglobin value. Can such a person be anaemic? Justify your answer.
Trace the fate of a worn-out red blood corpuscle after 120 days in circulation, naming the organs involved and the pigments formed. State where each part of the molecule finally goes.
A patient’s differential WBC count shows a marked rise in eosinophils. What two possibilities should the doctor consider, and why?
Distinguish between leucocytosis and leukemia, and explain why one is a sign of the body working correctly while the other is not.
Arrange the following events of blood clotting in their correct sequence: fibrin mesh traps blood cells; prothrombin converted to thrombin; damage to the blood vessel; fibrinogen converted to fibrin; activation of factor X; formation of the platelet plug.
Why does blood clot at a wound within seconds, but never inside a healthy blood vessel? Name the natural anticoagulant involved and state what happens to it during clotting.
What would be the effect on the heart’s pumping if the chordae tendinae of the left ventricle were to snap? Explain your reasoning using the pressure generated during ventricular systole.
Draw a labelled diagram of the internal structure of the human heart and mark: right atrium, left atrium, right ventricle, left ventricle, tricuspid valve, bicuspid valve, interventricular septum, chordae tendinae, papillary muscles, aorta, pulmonary trunk.
Explain why the impulse from the SA node is delayed at the AV node before it reaches the ventricles. What would go wrong if there were no such delay?
A heart removed from the body and supplied with oxygen and nutrients continues to beat. What property of the human heart does this demonstrate, and which tissue is responsible for it?
The ‘lubb’ and ‘dub’ sounds are not made by the heart muscle contracting. What actually produces each of them, and at which moment of the cardiac cycle does each occur?
Out of every 0.8 sec of the cardiac cycle, the ventricles contract for only 0.3 sec. Explain how this long resting period allows the heart to work continuously for a lifetime without fatigue.
Why are there no valves in the arteries, when the veins have valves at regular intervals? Answer with reference to blood pressure and gravity.
What is a portal vein? Explain the functional advantage of the hepatic portal vein carrying blood from the intestine to the liver before it reaches the heart.
The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. Does this break the definition of an artery and a vein? Justify your answer.
Explain, step by step, why the first Korotkoff sound heard while releasing the cuff of a sphygmomanometer marks the systolic pressure, and why the disappearance of the sounds marks the diastolic pressure.
Hypertension is often called a “silent” condition. Explain what this means, and why it makes routine measurement of blood pressure important.
The heart is full of blood at every moment, and yet a blockage of the coronary artery starves the heart muscle. Explain this apparent contradiction.
Assertion: The QRS complex of an ECG is much larger than the P-wave. Reason: The QRS complex represents the repolarization of the ventricles. Choose: (a) both assertion and reason are true and the reason explains the assertion; (b) both are true but the reason does not explain the assertion; (c) the assertion is true but the reason is false; (d) the assertion is false but the reason is true.
In a patient’s ECG, the P-R interval is found to be much longer than 0.16 sec. Which part of the heart’s conducting system would you suspect to be at fault, and why?
Lymph flows in one direction only and has no pump of its own. Explain how it is kept moving, and what happens to a limb whose lymphatic vessels become blocked.
Why does the body need lymphatic capillaries at all, when blood capillaries already reach every tissue? Give two distinct reasons.
Compare the respiratory surface of an earthworm with that of a human being. Both must be moist — explain how each organism keeps its respiratory surface moist, and why the human solution is placed inside the body.
Why does the trachea have ‘C’ shaped rings of cartilage? (30–40 words)
Why is respiration in insects called direct respiration? (30–40 words)
Why is gas exchange very rapid at the alveolar level? (30–40 words)
Name the organ which prevents the entry of food into the trachea while eating. (30–40 words)
Why is it advantageous to breathe through the nose rather than through the mouth? (80–100 words)
Identify the incorrect statement and correct it. (80–100 words)

A respiratory surface should have a large surface area.
A respiratory surface should be kept dry.
A respiratory surface should be thin, may be 1 mm or less.

Given below are the characteristics of some modified respiratory movements. Identify them. (80–100 words)

Spasmodic contraction of the muscles of expiration and forceful expulsion of air through the nose and mouth.
An inspiration followed by many short convulsive expirations accompanied by a facial expression.

Write a note on blood plasma. (80–100 words)
Explain blood clotting in short. (80–100 words)
Describe the pericardium. (80–100 words)
Describe the valves of the human heart. (80–100 words)
What is the role of the papillary muscles and chordae tendinae in the human heart? (80–100 words)
Explain in brief the factors affecting blood pressure. (80–100 words)
Smita was working in a garage with the doors closed and the automobile’s engine running. After some time she felt breathless and fainted. What would be the reason? How can she be treated? (150–200 words)
Shreyas went to a garden on a wintry morning. When he came back, he found it difficult to breathe and started wheezing. What could be the possible condition and how can he be treated? (150–200 words)
Why can you feel a pulse when you keep a finger on the wrist or neck, but not when you keep it on a vein? (150–200 words)
Which blood vessel of the heart will have the maximum content of oxygen, and why? (150–200 words)
How is blood kept moving in the large veins of the legs? (150–200 words)
Describe the histological structure of an artery, a vein and a capillary. (150–200 words)
What is blood pressure? How is it measured? Explain the factors affecting blood pressure. (150–200 words)
Describe human blood and give its functions. (150–200 words)
Give scientific reasons.

Closed circulation is more efficient than open circulation.
The human heart is called myogenic and autorhythmic.
A person who has undergone a heart transplant needs a lifetime supply of immunosuppressants.
Arteries are thicker than veins.
The left ventricle is thicker than all the other chambers of the heart.

Distinguish between:

Open and closed circulation.
Artery and vein.
Blood and lymph.
Blood capillary and lymph capillary.
Intrinsic and extrinsic process of clotting.

8.23 Practice With Numericals

A healthy adult has a tidal volume of 500 ml, an inspiratory reserve volume of 2800 ml, an expiratory reserve volume of 1100 ml and a residual volume of 1200 ml. Calculate (a) the vital capacity and (b) the total lung capacity.
A person breathes 12 times per minute with a tidal volume of 500 ml. If the dead space is 150 ml, calculate the volume of fresh air that actually reaches the alveoli in one minute.
The stroke volume of a healthy adult is 70 ml and the heart rate is 72 beats per minute. Calculate the cardiac output. If the same person’s heart rate rises to 120 beats per minute during exercise while the stroke volume increases to 100 ml, calculate the new cardiac output and the factor by which it has increased.
A patient’s cardiac output is 4800 ml/min and the heart rate is 60 beats per minute. Calculate the stroke volume.
One cardiac cycle is completed in 0.8 sec. Calculate the heart rate per minute. If a patient’s cardiac cycle takes 1.2 sec, calculate the heart rate and state whether this is tachycardia or bradycardia.
In an ECG, the R-R interval is measured as 0.6 sec. Calculate the heart rate in beats per minute and comment on whether it lies within the normal range.
A person’s blood pressure is recorded as 145/95 mmHg. Calculate the pulse pressure, and compare the reading with both the normal value and the threshold of hypertension.
An adult male has 5 litres of blood with an RBC count of 5.4 million/mm³. Calculate the total number of red blood corpuscles in his body. (1 litre = 10⁶ mm³)
If a red blood corpuscle lives for 120 days, calculate approximately what fraction of a person’s total RBCs must be replaced every single day in order to keep the count steady.
The blood of a healthy adult contains 4 to 6 litres in all, of which plasma is 55%. Calculate the volume of plasma in a person having 5.2 litres of blood.
A person’s cardiac output is 5 litres per minute and the total blood volume is 5 litres. Calculate how many times the entire blood volume passes through the heart in one hour.
A man’s pulse rate is 68 and cardiac output is 5500 cm³. Find the stroke volume.
If the duration of the atrial systole is 0.1 sec and that of complete diastole is 0.4 sec, then how does one cardiac cycle complete in 0.8 sec?