← Back to Subjects ororo.world

Chapter 8: The Circulatory System

Nearly four hundred years ago, doctors taught that blood was manufactured fresh in the liver out of food, carried outwards to the organs, and used up there like fuel. William Harvey, an English physician, decided to test that idea by measuring.

He estimated how much blood the heart pushes out with each beat, multiplied it by the number of beats in an hour, and arrived at a quantity far larger than the body could possibly manufacture from a day’s meals. If blood were being made and then consumed, the sums did not work. The same blood, he reasoned, must be going round and coming back.

Then he showed it on a living arm. He tied a bandage around a volunteer’s upper arm, tight enough to press on the shallow vessels near the skin but not on the deeper ones. Below the band the veins swelled into cords, with small knots spaced along them. Harvey placed a fingertip on one vein and slid it along, emptying a stretch of the vessel. That empty stretch filled again only from the far side, never from the side nearer the heart. Something inside the vein was allowing traffic in one direction only.

In 1628 he published the argument. He never saw the tiny vessels that link arteries to veins; Marcello Malpighi found those in 1661, using a microscope Harvey never had.

Diagram

Fig. 8.0-1 — Harvey’s Bandage Demonstration Line drawing of a human forearm and upper arm with a bandage tied around the upper arm; show the swollen surface veins below the band with small raised knots along them, a fingertip pressing and sliding along one vein, and the emptied stretch of vessel; label bandage, swollen vein, knot marking the position of a valve, emptied stretch of vein, and an arrow marked direction of blood flow towards the heart.

NoteCuriosity Corner

Q1. Blood is never blue, yet every diagram you will meet in this chapter shows some vessels in blue. Why is that colour used, and why do the veins on the back of your hand look bluish? Q2. A red blood cell throws away its nucleus before it enters your blood. How can a cell become better at its job by losing parts? Q3. Cut a finger while chopping vegetables and the bleeding stops by itself in a few minutes. What builds the plug that seals the wound? Q4. Why must a patient’s blood group be tested before blood from a donor is given, when all human blood looks alike? Q5. Why is the food absorbed from your intestine carried first to the liver, instead of going straight to the heart and out to the body?

8.1 Need For Transport Inside The Body

Paramecium is a single-celled organism that lives in pond water. Every part of it lies within a few micrometres of the water outside. Oxygen seeps in and wastes seep out simply by diffusion — the slow spreading of molecules from where they are crowded to where they are scarce. Over such tiny distances, diffusion is fast enough to keep the cell alive.

Your body is built on a different scale. The distance from your lungs to the cells of your toes is about a metre, and diffusion is hopeless over a distance like that. Left to diffusion alone, a molecule of oxygen would take years to travel from a lung to a toe.

A large body therefore needs a delivery service — a fluid that is kept moving, so that materials are carried the long distances at speed. That moving fluid does two jobs at once. It works as a distributing system, supplying substances to the organs, and as a collecting system, picking substances up from them. Between them, the two jobs reach every cell in the body, including the remotest one.

Q. If diffusion is too slow for a body as large as ours, does it play any part in us at all?

A. It plays the last part. The circulating fluid carries materials to within a fraction of a millimetre of a cell, and diffusion finishes the journey across that final short gap.

What Must Be Moved, And Where

Almost every organ system in your body depends on this transport, as Fig. 8.1-1 shows.

  • The digestive system digests food and absorbs the products of digestion — simple sugars such as glucose, amino acids, vitamins and mineral salts. These have to reach every cell in the body.
  • The respiratory system draws in air, and the oxygen taken up in the lungs has to reach all parts of the body. In the same way, the carbon dioxide collected from the whole body has to be carried back to the lungs and breathed out.
  • Extra water, excess salts and nitrogenous wastes such as urea are produced all over the body. They have to be carried to the excretory system and thrown out.
  • Hormones are secreted by the endocrine glands directly into the circulating fluid, and are carried through the body to act wherever they are required.

Diagram

Fig. 8.1-1 — The Body’s Delivery And Collection Routes Simple line drawing of a human body outline enclosing a single continuous loop that stands for the circulating fluid; draw arrows entering the loop from the lungs, the small intestine and an endocrine gland, arrows leaving the loop towards a group of body cells, and arrows returning from the body cells to the loop and onward to the lungs and the kidney; label lungs, small intestine, endocrine gland, kidney, body cells, oxygen, digested food, hormones, carbon dioxide, urea, and mark the direction of flow round the loop with arrowheads.

When a person is seriously ill and cannot swallow, fluid and medicines are put straight into a vein through a drip. The doctor does not have to place the medicine near the organ that needs it. Once it is in the circulating blood, it is carried everywhere, and a complete round of the body takes only about a minute at rest. The same route explains why an infection in one part of the body can make the whole body feverish.

The Two Circulating Fluids

All this carrying is done by two fluids that keep moving through the body: the blood, which is pumped through the heart and blood vessels, and the lymph, which drains slowly through a separate set of vessels of its own. Transport is not their only work. Both fluids also defend the body, and blood in addition seals wounds and spreads heat, as you will see in section 8.4.

Where You Are Zoom in from the whole organism and transport is a problem of organ systems working together. Zoom in one step further and it becomes a problem of single cells, each needing oxygen delivered to its own membrane.

Press the tip of your thumb firmly against the desk for about ten seconds and watch it turn pale. Release it, and the normal colour floods back within a second or two. The colour is not made in the thumb. It arrives there, and stops arriving the moment you block the flow.

8.2 The Fluids Of Our Body

The transport described in section 8.1 is carried out by fluids that keep moving. Before following any one of them, it helps to know how many there are and where each of them lies.

8.2.1 Blood, Tissue Fluid And Lymph

There are three principal fluids in the human body.

  1. Blood, which is contained in the heart and in the blood vessels — the arteries, veins and capillaries of the circulatory system. You will meet the three kinds of vessel in section 8.8.8.
  2. Tissue fluid, which occupies the narrow spaces between the cells inside an organ. It lies outside any vessel, bathing the cells directly.
  3. Lymph, which is contained within the lymph vessels and the lymphatic organs, such as the spleen and the tonsils.

These three are not three separate substances. They are the same material at three points along one route, as Fig. 8.2-1 shows. Blood flowing through a capillary leaks part of its fluid out into the spaces around the cells, where it becomes tissue fluid; from those spaces the fluid drains into lymph vessels, where it is called lymph; and the lymph is finally poured back into the blood. Section 8.5 takes up the blood in detail, and section 8.10 returns to tissue fluid and lymph.

Diagram

Fig. 8.2-1 — How Blood, Tissue Fluid And Lymph Are Related Schematic circulation diagram: lungs at the top, heart at the centre, arteries shown in red leading to a capillary bed among a group of body cells, veins shown in blue returning to the heart; show fluid leaving the capillary bed into the spaces between the body cells, blindly ending lymph capillaries shown in yellow collecting that fluid, passing through a lymph node and a lymphatic duct which empties into a large vein close to the heart. Label lungs, heart, pulmonary artery, pulmonary vein, arteries, blood capillaries, veins, tissue fluid, lymph capillaries, lymph node, lymphatic duct. Include a key: red — oxygenated blood, blue — deoxygenated blood, yellow — lymph.

Red And Blue In Diagrams

Fig. 8.2-1 follows a convention you will see in every diagram in this chapter: vessels carrying oxygenated blood — blood that has picked up oxygen in the lungs — are printed red, and vessels carrying deoxygenated blood — blood that has handed its oxygen over to the tissues and is loaded with carbon dioxide — are printed blue.

The code is convenient, but only the red half of it is true to life. Oxygenated blood really is bright red. Deoxygenated blood is dark red, never blue.

Q1. Blood is never blue, yet every diagram you will meet in this chapter shows some vessels in blue. Why is that colour used, and why do the veins on the back of your hand look bluish?

A. Red and blue are only a code — red for oxygen-rich blood, blue for oxygen-poor. Real deoxygenated blood is dark red. Skin scatters the light coming back from a vein unevenly, so the vessel reads as bluish against the paler skin.

Fluids That Do Not Circulate

Not every fluid in the body is on the move. Synovial fluid fills the cavities of the skeletal joints, where it acts as a lubricant. The vitreous humour fills the large chamber of the eyeball behind the lens, and the aqueous humour fills the small chamber in front of it. Each of these stays where it is, in one organ, and takes no part in transport.

Tissue Fluid Or Lymph? Tissue fluid lies in the spaces between the cells of an organ, outside any vessel. The very same fluid, once it has drained into a lymph vessel, is called lymph. The difference is not what the fluid is made of, but where it happens to be.

8.2.2 Closed And Open Circulatory Systems

Look again at Fig. 8.2-1 and follow the blood the whole way round. At no point does it leave the vessels; it goes from the heart into arteries, through capillaries, back along veins and into the heart again. Blood circulation of this kind is called a closed blood circulatory system.

Not every animal is built this way. In insects, the blood is pumped out of the heart into open spaces in the body, where it washes over the organs directly before draining back. This is an open blood circulatory system, and it manages without any capillaries at all.

The difference is not a small one. When blood is confined to narrow tubes, it can be driven at pressure, moved quickly, and steered towards whichever organ needs it most. When it simply floods a body cavity, it does none of these things well. Table 8.1 sets the two side by side.

Table 8.1 — Closed Circulatory System Compared With Open Circulatory System | Feature | Closed system | Open system | |—|—|—| | Path of the blood | Stays inside blood vessels throughout | Leaves the vessels and flows through open body spaces | | Contact with body cells | Indirect, through the thin walls of capillaries | Direct, the blood washes over the organs | | Capillaries | Present | Absent | | Blood pressure | Comparatively high | Low | | Speed of flow | Rapid | Slow | | Control over supply | Blood can be directed to particular organs | Little control over where the blood goes | | Examples | Human beings and other vertebrates, earthworm | Insects such as the cockroach, prawns and crabs |

A Different Name The fluid in an open system is not usually called blood at all, but haemolymph — a name that admits it is doing the work of blood and lymph together, since it bathes the cells directly instead of handing fluid over to a second system.

8.3 Properties Of Blood

Blood is a tissue, although not the kind you may be picturing. Its cells are not cemented into a fixed sheet; they are carried along in a liquid. Before taking blood apart to see what it is made of, it is worth noticing a few plain properties of the fluid itself.

  • Never stationary. Blood is always in motion, out of the heart into the arteries and back to the heart through the veins. It has to be. A transport fluid that stops is no longer transporting anything, and the cells waiting at the far end of the route would very soon be short of oxygen. What keeps it moving is the pumping of the heart, which you will study in section 8.8.
  • Colour. Blood is a somewhat thick fluid, distinctly thicker and stickier than water. Blood drawn from an artery is bright red; blood drawn from a vein is dark red.
  • Volume. An average adult human being has 5 to 6 litres of blood — five or six of the one-litre bottles you drink water from, and about 7 per cent of the whole body weight. A smaller person has less, since the volume goes roughly with body size.

Q. The same blood is bright red in an artery and dark red in a vein. Has the blood itself changed?

A. Only its load has. The red pigment inside the red blood cells is bright red while it is carrying oxygen and dark red once it has handed the oxygen over to the tissues. You will meet the pigment in section 8.5.3.2.

Saltish, And Slightly Alkaline

Most of us have tasted our own blood at some time — from a cut on the lip, or from bleeding gums. It tastes saltish, and it does so for a simple reason: common salt is dissolved in the liquid part of blood, along with other salts you will meet in section 8.5.1.

Blood is also slightly alkaline. Its pH stays between 7.35 and 7.45. On the pH scale in Fig. 8.3-1, the value 7 is neutral — neither acidic nor alkaline; values below 7 are acidic and values above 7 are alkaline. Blood therefore sits just to the alkaline side of neutral, and it is held there very tightly indeed. The whole band it is allowed to wander through is only about a tenth of a pH unit wide.

Diagram

Fig. 8.3-1 — Where Blood Sits On The pH Scale Horizontal pH scale strip numbered 0 to 14 with the neutral point 7 marked clearly; label the region below 7 acidic and the region above 7 alkaline; mark a narrow band from 7.35 to 7.45 with a pointer labelled normal range of human blood; for comparison mark lemon juice at about pH 2 and a baking soda solution at about pH 9 on the same scale.

Why So Tight? Enzymes, the proteins that run the reactions inside your cells, work properly only within a narrow pH range. A drift of a few tenths of a unit either way would slow many of them at once, which is why the body spends so much effort holding the pH of blood steady.

Where Blood Sits Zoom in and blood is molecules dissolved in a liquid, with cells afloat in it. Zoom out one step and that liquid and those cells together count as a tissue — a fluid tissue — and that tissue is what fills the organs of the circulatory system.

8.4 Functions Of Blood

The work blood does in the body falls under two broad headings: transport and protection. Section 8.1 showed why the transport is needed. This section deals with what blood carries, how it manages to carry it, and how the same fluid also defends you.

8.4.1 Transport By Blood

Blood carries digested food, oxygen, carbon dioxide, excretory substances, hormones and body heat. Its six transport functions are these.

  1. Transport of digested food from the alimentary canal to the tissues. The substances carried are the simple products of digestion — simple sugars such as glucose, amino acids, vitamins and mineral salts.
  2. Transport of oxygen from the lungs to the tissues. This is done by the red blood cells, using the red pigment inside them.
  3. Transport of carbon dioxide from the tissues back to the lungs, where it is breathed out.
  4. Transport of excretory material from the tissues to the liver, the kidney or the skin, either to be eliminated or to be rendered harmless.
  5. Distribution of hormones, which the endocrine glands pour directly into the blood, to the parts of the body where they act.
  6. Distribution of heat, which keeps the temperature of the body uniform.
How The Gases Are Carried

Oxygen dissolves very poorly in water, and blood is mostly water. If the plasma had to carry oxygen in simple solution, it could not carry nearly enough. The red blood cells solve the problem with haemoglobin, a red pigment that oxygen can attach itself to.

Where oxygen is plentiful, as in the lungs, haemoglobin picks it up and forms oxyhaemoglobin, a compound of haemoglobin with oxygen.

haemoglobin + oxygen \(\rightleftharpoons\) oxyhaemoglobin

\[Hb + O_2 \rightleftharpoons HbO_2\]

The double arrow is the important part. Oxyhaemoglobin is an unstable compound, so the reaction runs backwards just as readily as forwards. In a tissue that is using up oxygen, oxygen is scarce, the compound breaks up again, and the oxygen is released to the cells that need it. FC 8.1 sets out the whole relay.

Flowchart

FC 8.1 — The Oxygen Relay Oxygen breathed into the lungs → oxygen diffuses into the blood → haemoglobin in red blood cells picks it up as oxyhaemoglobin → blood carries oxyhaemoglobin to a respiring tissue → oxygen is scarce there, so oxyhaemoglobin breaks up → oxygen diffuses into the tissue cells → haemoglobin, now free, returns in the blood to the lungs (cycle repeats)

Carbon dioxide travels the other way, from the tissues to the lungs, and it travels by two routes at once. A part of it combines with haemoglobin to form carbaminohaemoglobin.

haemoglobin + carbon dioxide \(\rightleftharpoons\) carbaminohaemoglobin

\[Hb + CO_2 \rightleftharpoons HbCO_2\]

The larger part of it, however, is simply carried in solution in the plasma, mostly in the form of hydrogencarbonate (bicarbonate) ions. Table 8.2 compares the two journeys.

Table 8.2 — Transport Of Oxygen Compared With Transport Of Carbon Dioxide | Feature | Oxygen | Carbon dioxide | |—|—|—| | Picked up at | The lungs | The respiring tissues | | Released at | The respiring tissues | The lungs | | Compound formed with haemoglobin | Oxyhaemoglobin | Carbaminohaemoglobin | | Main route of travel | Almost entirely on haemoglobin, inside the red blood cells | Mostly in solution in the plasma, as hydrogencarbonate ions | | Effect on colour of blood | Blood becomes bright red | Blood becomes dark red |

Haemoglobin Binds oxygen firmly but loosely, forming an unstable compound → the compound holds together where oxygen is plentiful and falls apart where oxygen is scarce → oxygen is automatically collected at the lungs and delivered at the tissues, with no switch to be thrown.

Spreading The Heat

Heat is not produced evenly in the body. Your liver and your working muscles make far more of it than your fingers or your ear lobes do. Blood flowing through a warm organ is warmed by it, and carries that warmth away to cooler parts. Because the same blood passes through every region in turn, the temperature of the whole body is evened out instead of the deep organs cooking while the outlying parts freeze.

8.4.2 Protection By Blood

Blood does not only supply the body; it also guards it. It does so in three ways.

  1. Wherever a blood vessel is cut, the blood forms a clot — a solid plug that seals the break. The clot serves to prevent further loss of blood, and to prevent the entry of disease-causing germs through the opening. The steps by which a clot is built are taken up in section 8.6.
  2. The white blood corpuscles protect the body from disease by engulfing bacteria that have got in. This is described in section 8.5.4.3.
  3. Blood produces antitoxins and antibodies — substances that neutralise the poisons made by germs, or destroy the germs themselves. You will meet them in section 8.5.4.4.

8.5 Composition Of Blood

Let a sample of blood stand undisturbed for a while, or spin it in a centrifuge, and it separates into two layers. Blood is a mixture of a liquid and the cells floating in it.

  1. Plasma — the fluid part, which makes up 55 to 60 per cent of the blood.
  2. Cellular elements — the red cells, the white cells and the platelets, which make up 40 to 45 per cent of the blood.

8.5.1 Plasma — The Liquid Part Of Blood

Plasma is a light-yellow, alkaline liquid. The red colour of blood belongs entirely to the cells suspended in it, not to the plasma. Its composition is set out in Fig. 8.5-1.

  • Water — 90 to 92 per cent
  • Proteins — 7 to 8 per cent
  • Inorganic salts — 1 per cent
  • Other substances — traces

Diagram

Fig. 8.5-1 — What Blood Is Made Of Two panels side by side. Left panel: a test tube of blood that has been allowed to separate, showing an upper straw-yellow column labelled plasma, 55 to 60 per cent and a lower dark red column labelled cellular elements, 40 to 45 per cent, with a thin layer between them labelled white cells and platelets. Right panel: a bar or pie divided to show the composition of plasma alone, labelled water 90 to 92 per cent, proteins 7 to 8 per cent, inorganic salts 1 per cent, other substances in traces.

The inorganic salts are mainly sodium chloride and sodium bicarbonate — the salts that give blood its saltish taste. Among the other substances dissolved in plasma are glucose, amino acids, fibrinogen, hormones and urea. Notice how many of them are the cargo of section 8.4: food going out to the cells, hormones going to their targets, urea coming back from them.

One of the plasma proteins, fibrinogen, is the material out of which a clot is built. If blood is allowed to clot and the fibrinogen is used up in the process, what remains of the plasma is a clear straw-coloured liquid called serum.

NoteDefinition

Serum The plasma of the blood from which the protein fibrinogen has been removed.

Plasma Or Serum? Plasma is the liquid part of blood as it flows in your vessels, fibrinogen and all. Serum is what is left of that liquid after clotting has taken the fibrinogen out of it. Plasma can clot; serum cannot.

8.5.2 The Cellular Elements

The cellular elements are also called the formed elements of blood, meaning simply that they are shaped structures which can be seen under magnification. There are three categories, shown in Fig. 8.5-2.

  1. Red blood cells, or erythrocytes
  2. White blood cells, or leukocytes
  3. Blood platelets, or thrombocytes

Diagram

Fig. 8.5-2 — A Smear Of Human Blood Drawing of a stained blood smear as seen under the microscope: many pale-centred circular red blood cells filling the field, two or three larger white blood cells with darkly stained nuclei of different shapes, and a small cluster of tiny irregular platelets. Label red blood cell, white blood cell, blood platelet. Include one red blood cell drawn in vertical section alongside, to show that the disc is hollowed on both faces.

8.5.3 Red Blood Cells

Red blood cells are the oxygen carriers of the body. Their other name, erythrocytes, comes from the Greek word erythros, meaning red.

8.5.3.1 Shape And Size Of A Red Blood Cell

A red blood cell is a minute disc, flattened and hollowed on both faces so that it is thin in the centre and thick and rounded at the rim. A disc of this shape is described as biconcave.

It is also very small — about 7 micrometres (µm) across. One micrometre is one-thousandth of a millimetre, so about 140 of these cells laid edge to edge would span a single millimetre.

Both the shape and the size are worth pausing on, because each of them buys the cell something.

  • The small size, together with the hollow on either face, gives the cell a large surface for its volume. Oxygen enters and leaves through that surface, so a large surface means quick loading and quick unloading.
  • The small size also lets the cell pass through the finest capillaries in the body, some of which are so narrow that the red cells must travel through them in single file.

The Biconcave Disc Hollowed on both faces → far more surface for the same volume of cell, and no point inside the cell lies far from that surface → oxygen loads and unloads through a short distance across a wide area, so a red cell fills and empties quickly.

8.5.3.2 Haemoglobin And The Carriage Of Gases

Inside a red blood cell is a colourless spongy body called the stroma, and it is the stroma that holds the haemoglobin. Haemoglobin is a respiratory pigment — a coloured substance whose business is to pick up and release a respiratory gas.

Haemoglobin is built of two parts: haemin, which contains iron, and a protein called globin. The iron is the reason a person who is short of iron in the diet cannot make enough haemoglobin, and the reason a doctor asks about iron when a patient’s haemoglobin level is low.

You have already met what haemoglobin does with the two respiratory gases in section 8.4.1 — oxyhaemoglobin with oxygen, and carbaminohaemoglobin with the small share of carbon dioxide that travels on the pigment. What matters here is how thoroughly it does the first job, and Fig. 8.5-3 shows that.

Diagram

Fig. 8.5-3 — How Much Oxygen Blood Carries Simple vertical bar chart. Vertical axis labelled percentage of haemoglobin carrying oxygen, marked 0 to 100. Two bars: the first labelled blood leaving the lungs, drawn to 97 to 99 per cent; the second labelled blood returning in the veins, drawn to about 75 per cent. Mark the gap between the two bar tops with a bracket labelled oxygen handed over to the tissues, about 23 per cent.

Reading Fig. 8.5-3 The vertical axis does not show how much oxygen is in the blood in grams. It shows what percentage of the haemoglobin present is carrying oxygen at that moment. Read the first bar. In the lungs, haemoglobin becomes 97 to 99 per cent loaded — very nearly every molecule of the pigment has taken up oxygen. Read the second bar. Blood coming back from the tissues in the veins is still about 75 per cent loaded. Now read the gap between them. Only about 23 per cent of the oxygen was handed over during one trip round the body. So venous blood is not empty of oxygen. It is a partly used store, and the reserve it still carries is what the body draws on when a tissue suddenly starts working harder.

Q. If veins carry oxygen too, why are they called deoxygenated?

A. Because the name records a comparison, not an absence. Blood in a vein has given up part of its oxygen and picked up carbon dioxide, so it is less oxygenated than blood in an artery — not without oxygen.

Haemoglobin And Carbon Monoxide

Haemoglobin has a very strong affinity for carbon monoxide, far stronger than its affinity for oxygen. With carbon monoxide it forms carboxyhaemoglobin (HbCO), and this compound is stable — unlike oxyhaemoglobin, it does not readily break up again. Every haemoglobin molecule locked up in this way is a molecule that can no longer carry oxygen, so the oxygen-carrying capacity of the blood falls. This is carbon monoxide poisoning, and it can be fatal.

Why A Room Must Be Ventilated Carbon monoxide is produced whenever a fuel burns without enough air — a coal or charcoal brazier, a wood fire, a kerosene stove, a gas water heater. In a closed room with no ventilation the gas builds up, and because it has no smell and no colour, a sleeping person gets no warning. Deaths of this kind happen every winter. The remedy is not a better fuel but an open window: a brazier or a fuel-burning heater always needs a supply of fresh air and an escape route for the fumes.

8.5.3.3 Deficient, But More Efficient

A mature mammalian red blood cell is a strange sort of cell. As it circulates in the blood it has no nucleus, no mitochondria and no endoplasmic reticulum. Judged as a cell, it is missing a great deal. Judged as an oxygen carrier, every one of those losses is a gain.

  1. Loss of the nucleus. With no nucleus taking up room in the middle, the cell caves in on both faces and becomes biconcave, which raises its surface area for the volume it occupies. A flattened cell also takes up less space, so more red cells can be packed into the same volume of blood.
  2. Loss of mitochondria. Cellular respiration using oxygen takes place in mitochondria. A cell without them cannot use oxygen for itself, and meets its own small energy needs without oxygen. So every molecule of oxygen a red blood cell picks up at the lungs is delivered to the tissues unconsumed — the carrier does not eat the cargo.
  3. No endoplasmic reticulum. This leaves the cell more flexible, so it can bend and fold its way through capillaries narrower than itself.

Q2. A red blood cell throws away its nucleus before it enters your blood. How can a cell become better at its job by losing parts?

A. Because its job is carrying, not living. Losing the nucleus lets it cave in on both faces and gain surface; losing mitochondria means it burns none of the oxygen it carries; losing the endoplasmic reticulum leaves it flexible enough to bend through the narrowest capillaries.

8.5.3.4 Formation, Life And Destruction Of Red Blood Cells

In an adult, red blood cells are made in the red bone marrow, chiefly in the ribs, the breast bone, the vertebrae and the ilium of the hip girdle. It is not so in every stage of life. In the embryo they are produced in the liver and the spleen, and in a child the marrow of all the bones makes them until about five years of age.

A red blood cell does have a nucleus while it is being formed. As it matures, the nucleus is lost — the cell is said to become enucleated — and only then does it enter the blood.

The average life of a red blood cell is about 120 days. Old and worn-out cells are destroyed in the spleen, the liver and the bone marrow. Their iron is not wasted: it is retained in the liver and used again, while the rest of the haemoglobin is broken down and excreted as the bile pigment bilirubin, which is what colours bile.

The numbers involved are large. In a normal adult about 2 million red blood cells are destroyed every second, which works out at roughly 1 per cent of all the erythrocytes in the body every day. Exactly as many have to be built in the same time, or the count would drift.

Flowchart

FC 8.2 — The Life Of A Red Blood Cell Red bone marrow: cell formed with a nucleus → nucleus lost, cell becomes biconcave and enucleated → cell enters the blood → circulates and carries oxygen for about 120 days → cell grows old and fragile → destroyed in the spleen, liver and bone marrow → iron retained in the liver and reused for new haemoglobin (returns to the marrow); remainder converted to bilirubin and excreted in the bile

8.5.3.5 The Number Of Red Blood Cells

Red blood cells are counted in a very small volume — one cubic millimetre (mm³) of blood, which is about the size of a grain of coarse salt. An adult human male has about 5 million red blood cells per mm³ of blood, and an adult female slightly fewer, about 4.5 million.

The count is not fixed for life, nor even for the day.

  • New-born infants have a larger number, about 6 to 7 million per mm³.
  • The count falls by about 5 per cent during sleep, and rises during physical activity, in pregnancy, and in states of emotional upset.
  • People living at a height of 4,200 metres and above have red blood cell counts nearly 30 per cent higher than people at sea level. The air there holds less oxygen in the same volume, so the body compensates by making more carriers — which is also why athletes sometimes train at altitude.

An abnormally increased number of red blood cells is called polycythaemia; an abnormally decreased number is called erythropenia.

8.5.4 White Blood Cells

White blood cells, or leukocytes, differ from red blood cells in two obvious ways: they have a nucleus, and they contain no haemoglobin. They are also far fewer — usually about 4000 to 8000 per mm³ of blood, against several million red cells in the same volume.

Their name is a little misleading. Leukocytes spend about 90 per cent of their time in the tissue fluid and the lymph, and only about 10 per cent in the blood, so ‘white cells’ suits them better than ‘white blood cells’. The blood is not their home; it is their road.

Most white cells are amoeboid. They can push out pseudopodia — temporary finger-like extensions of the cell — and use them to squeeze between the cells of a capillary wall and out into the tissues. This squeezing out is called diapedesis, from dia, across, and pedesis, oozing out. Fig. 8.5-4 shows it happening.

Diagram

Fig. 8.5-4 — Diapedesis And Phagocytosis Four-stage strip labelled A to D. A: a length of blood capillary in section with red blood cells inside and one white blood cell against the wall; a bacterium lies in the tissue outside. B: the white blood cell pushing a pseudopodium between two cells of the capillary wall. C: the white blood cell now outside the capillary, moving towards the bacterium. D: the white blood cell flowing round the bacterium and enclosing it. Label blood capillary, red blood cell, white blood cell, pseudopodium, bacterium; caption stages A to C as diapedesis and stage D as phagocytosis.

8.5.4.1 The Five Types Of White Blood Cells

White blood cells are sorted by their appearance after staining into two major categories, and within those into five distinct types. The categories are set out in FC 8.3 and the types compared in Table 8.3.

Flowchart

FC 8.3 — Classification Of White Blood Cells White blood cells (leukocytes) → branch 1: granular, cytoplasm containing granules and a nucleus usually constricted into lobes → neutrophils, eosinophils, basophils; branch 2: non-granular, cytoplasm without granules and a single large nucleus → lymphocytes, monocytes

Table 8.3 — The Five Types Of White Blood Cells | Type and normal share | Category | Distinguishing features | Function | Site of production | |—|—|—|—|—| | Neutrophils, 55–70% | Granular | Nucleus with 3 or 4 lobes; granular cytoplasm; stain with neutral dyes | Engulf bacteria by phagocytosis | Bone marrow | | Eosinophils, 1–3% | Granular | Nucleus with 2 lobes; large granules that stain dark red with the acid dye eosin | Act against parasites and take part in allergic reactions; their number rises in allergies | Bone marrow | | Basophils, 0.5–1% | Granular | Large, indistinctly lobed nucleus; granules stain with basic dyes such as methylene blue | Release histamine, which produces inflammation and widens blood vessels, and heparin, which slows clotting | Bone marrow | | Lymphocytes, 20–35% | Non-granular | Smallest of the white cells; a single large nucleus filling most of the cell | Produce antibodies | Bone marrow, and lymphatic organs such as the spleen and tonsils | | Monocytes, 3–8% | Non-granular | Largest of the white cells; large kidney-shaped nucleus | Ingest germs; at the site of an infection they transform into macrophages | Bone marrow |

Five Types, Most To Fewest Never Let Monkeys Eat Bananas — Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils, in falling order of how many of each you have. Use it for the order only; the work each type does still has to be learnt.

Diagram

Fig. 8.5-5 — The Five Types Of White Blood Cell Five separate line drawings of stained white blood cells, in a row, each labelled: neutrophil with a three-lobed nucleus and fine granules; eosinophil with a two-lobed nucleus and large coarse granules; basophil with a large indistinctly lobed nucleus and dark granules; lymphocyte, small, almost filled by a single round nucleus with a thin rim of cytoplasm; monocyte, the largest, with a kidney-shaped nucleus. Draw all five to the same scale so their relative sizes can be compared.

8.5.4.2 Origin And Life Of White Blood Cells

White blood cells are produced in the red bone marrow and in the lymph nodes, and sometimes even in the liver and the spleen. Their average life is about two weeks, though it varies a great deal from one type to another: neutrophils live only a few hours, and about 125 billion of them are produced every day to keep the supply up. Old and worn-out white cells are destroyed in the same way as red cells are.

Two conditions are named after disturbances in this population. Leukaemia is a cancer of the tissue that forms white blood cells; the number of white cells increases manifold, at the cost of the red cells. A person with leukaemia is therefore short of oxygen-carrying cells as well as burdened with abnormal white ones. It is treated with medicines that destroy the abnormal cells, and in some cases by replacing the diseased marrow with healthy marrow from a donor; many people, children in particular, recover. Leukopenia is an abnormal decrease in the number of white blood cells.

8.5.4.3 How White Blood Cells Defend The Body

The white cells are the body’s defence force, and they work in three ways.

  1. Phagocytosis. Most white cells, and the neutrophils in particular, engulf solid particles, especially bacteria. The cell flows around the particle and takes it inside, where it is destroyed. Because the body makes more of these cells when it is under attack, a white cell count that has risen to about 50,000 or more per mm³ is a sign of infection somewhere in the body.
  2. Inflammation. Inflammation is the reaction of the tissues to injury and to a local invasion by germs. The inflamed spot shows increased local heat, redness, swelling and pain. Monocytes and neutrophils leave the blood vessels there by diapedesis and fight the germs, and they also clear away the body’s own damaged cells by phagocytosis. Pus is made mainly of dead white blood cells together with the tissue cells the bacteria destroyed.
  3. Formation of antibodies. The lymphocytes produce antibodies, which kill the germs or neutralise the poisons the germs make.
  1. A thorn pricks your finger and pushes bacteria through the skin.
  2. The damaged tissue reacts. Blood vessels near the spot widen, so more blood arrives — the redness, the warmth and the swelling you can see and feel.
  3. Neutrophils and monocytes in that blood press against the capillary walls and squeeze out through them by diapedesis.
  4. Once in the tissue, they crawl towards the bacteria, using pseudopodia.
  5. Each white cell flows around a bacterium and engulfs it. Inside the cell, the bacterium is destroyed.
  6. White cells die in the fight, along with the tissue cells the bacteria have killed. The mixture collects as pus.
  7. With the bacteria gone, the vessels return to normal and the tissue is repaired.

8.5.4.4 Antibodies And Antitoxins

When disease-causing germs get into the blood stream, many of them produce poisonous substances called toxins. The lymphocytes answer by producing antibodies, which circulate freely in the blood plasma.

Antibodies are specific: an antibody produced against one toxin acts against that toxin and not against another. Antibodies that work in this way, by neutralising or detoxifying the poisonous effect of a toxin, are the antitoxins you met in section 8.4.2.

Antibodies often persist long after the disease itself has been overcome, and a person who has recovered is then immune to that disease — the antibodies are already in place if the germ returns. This is the principle on which vaccination rests. A vaccine deliberately introduces a weakened germ, or a substance taken from a germ, so that the body produces the matching antibodies in advance. If the real germ enters later, the antibodies are ready and the disease never takes hold.

8.5.5 Blood Platelets

Blood platelets are minute oval or round structures with no nucleus, floating in the blood. An adult has about 200,000 to 400,000 of them per mm³ — far more than the white cells, far fewer than the red.

They are not whole cells in the ordinary sense. They are budded off from giant cells called megakaryocytes in the red bone marrow, each bud being completely wrapped in a piece of membrane as it separates. Their life span is short, about 5 to 9 days, and they are destroyed mainly in the spleen, a lymphatic organ in the abdomen.

Platelets matter because of what they do at a wound. At the site of an injury they disintegrate and release a chemical substance called thrombokinase, and that release is the first step of clotting, which section 8.6 takes up in full.

8.5.6 The Three Kinds Of Blood Cells Compared

Table 8.4 brings the three cellular elements together, so that the differences between them can be seen at a glance.

Table 8.4 — Erythrocytes, Leukocytes And Thrombocytes Compared | Parameter | Erythrocytes | Leukocytes | Thrombocytes | |—|—|—|—| | Shape | Biconcave, oval, disc-like | Irregular, amoeboid | Minute, oval or round | | Nucleus | Absent in the mature cell | A well-developed nucleus | Absent | | Pigment | Haemoglobin present | No pigment | No pigment | | Life span | About 120 days | About 2 weeks on average | About 5 to 9 days | | Count per mm³ | 4.5 to 5 million | 4000 to 8000 | 200,000 to 400,000 | | Function | Transport of the respiratory gases, oxygen and carbon dioxide, with the help of haemoglobin | Defence — phagocytosis of germs, and production of antibodies and antitoxins | Initiate the clotting of blood, preventing excessive loss of blood and the entry of germs |

8.6 Clotting Of Blood

Cut a blood vessel and blood escapes from it. Within a few minutes, though, a solid plug forms over the wound and the flow stops. Nothing about that is inevitable — if it did not happen, a person with even a small injury would go on bleeding until they bled to death.

The process that seals the break is called coagulation, or clotting, and it is the chief work of the platelets.

8.6.1 The Steps Of Clotting

Clotting is not one event but a series of them, each step producing the substance that sets off the next.

  1. The injured tissue cells and the platelets disintegrate at the site of the wound, and in doing so release thrombokinase, which is also called thromboplastin.
  2. Thrombokinase acts as an enzyme. With the help of the calcium ions present in the plasma, it converts prothrombin, an inactive protein of the plasma, into thrombin, its active form. Vitamin K, a fat-soluble vitamin, is essential for the production of prothrombin in the first place.
  3. Thrombin, again in the presence of calcium ions, acts on the soluble fibrinogen of the plasma and converts it into insoluble fibrin. Fibrin is a solid substance that comes out as microscopic threads. These threads are sticky, and they criss-cross into a network, or mesh, over the wound.
  4. Blood cells are caught in this network. The mesh then shrinks and squeezes out the remaining plasma as a clear liquid — serum — and the solid mass left behind is the clot, also called a thrombus.

Notice how the sequence is put together. Each substance sits ready in the plasma in a harmless, inactive form, and only an injury sets the first one working. FC 8.4 shows the whole chain, and Fig. 8.6-1 shows what it looks like at a cut vessel.

Flowchart

FC 8.4 — Main Reactions In Blood Clotting Injured tissue cells and platelets disintegrate at the wound → release thrombokinase (thromboplastin) → thrombokinase, with calcium ions, acts on prothrombin (inactive) → thrombin (active) → thrombin, with calcium ions, acts on fibrinogen (soluble) → fibrin (insoluble, in threads) → threads form a sticky network over the wound → blood cells trapped in the network → network shrinks and squeezes out serum → clot (thrombus)

Diagram

Fig. 8.6-1 — Clotting At A Cut Blood Vessel Three-stage sequence drawn as successive views of the same short length of a cut blood vessel, one below the other. Stage 1: the vessel wall broken, red blood cells and platelets escaping through the gap; label vessel is damaged, blood vessel, red blood cell, platelet. Stage 2: platelets collecting and sticking together across the gap; label platelet plug. Stage 3: a mesh of fine threads across the gap with red blood cells caught in it; label fibrin threads and clot. Mark an arrow of time from stage 1 to stage 3.

NoteDefinition

Clot A clot is the solid mass formed at a wound, consisting of blood cells entangled in a network of insoluble fibrin threads.

Q3. Cut a finger while chopping vegetables and the bleeding stops by itself in a few minutes. What builds the plug that seals the wound?

A. The platelets do. They disintegrate at the cut and release thrombokinase, which sets off a chain ending in threads of fibrin. The threads mesh across the wound and trap blood cells, and that trapped mass is the clot.

What Is Left Behind

It is worth being clear about what has become of the blood once a clot has formed. The blood has been divided in two, as FC 8.5 shows: the solid clot, and a clear liquid left over.

Flowchart

FC 8.5 — The Fate Of Blood In Clotting Blood → branch 1, corpuscles; branch 2, plasma → the plasma yields fibrin and serum → corpuscles together with fibrin form the clot → serum remains as the clear liquid outside the clot

Fibrinogen Or Fibrin? Fibrinogen is the soluble protein dissolved in circulating plasma; you cannot see it. Fibrin is what thrombin turns it into — insoluble, and drawn out into visible threads. One is the raw material, the other is the finished mesh.

The Letter K Vitamin K was named by the Danish scientist Henrik Dam, who found it while studying why some animals bled easily. He called it the Koagulationsvitamin, the coagulation vitamin, and the K has stayed with it ever since.

8.6.2 Clotting In A Test Tube

Blood does not need a wound to clot. Draw some into a test tube and leave it standing, and it will clot there in the ordinary way. The fresh clot at first fills the tube, made of fibrin strands with corpuscles and serum among them. Left a little longer, the mesh shrinks and the serum it squeezes out collects above the clot as a clear yellowish liquid, as in Fig. 8.6-2.

Diagram

Fig. 8.6-2 — Clotting Of Blood In A Test Tube Two test tubes drawn side by side, highly diagrammatic. Left tube: freshly coagulated blood filling the whole tube, drawn to show fibrin strands with corpuscles entangled among them and serum in between; label fibrin threads, corpuscles, serum. Right tube: the same blood after standing for some time, with a shrunken solid clot at the bottom and a clear layer above it; label clot and serum.

This simple observation clears up a common misunderstanding. Blood clotting is often said to depend on the blood being exposed to air. It does not. What sets clotting off is contact with a rough or damaged surface — which is what a torn vessel wall provides, and which is also what a deposit of cholesterol on the inner lining of a blood vessel provides. A clot forming inside an intact vessel for that reason is a serious matter, because there is no wound there for it to seal, and the vessel it blocks is carrying blood some organ needs.

Blood collected for transfusion would be useless if it clotted in the bag. The collection bag therefore contains sodium citrate, which locks up the calcium ions in the plasma. Look again at steps 2 and 3 of the sequence above: calcium ions are needed at both. Remove them and the chain cannot proceed, so the blood stays fluid until it is needed. Adding calcium back would let it clot normally.

8.6.3 When Clotting Fails

Because clotting depends on a chain of substances, the failure of any one of them slows or stops the whole process.

Haemophilia is a genetic disorder in which blood does not clot properly, because one of the blood-clotting proteins is missing or faulty. A person with haemophilia bleeds for much longer than usual from an injury, and may bleed into joints and muscles after quite a minor knock.

Clotting also becomes slow when the number of platelets falls to an abnormally low count, since it is the platelets that start the sequence. Coagulation then takes far longer than it should, and bleeding — a haemorrhage — may go on for a dangerous length of time. A sharp fall in the platelet count happens in certain infections, dengue among them, which is why doctors treating a person with dengue keep checking the platelet count.

8.7 Blood Transfusion And Blood Groups

A person who has lost a great deal of blood — in an accident, or during a surgical operation — cannot wait for the body to replace it. Blood taken from a healthy person is introduced into one of the patient’s veins instead. This is called blood transfusion. The person who gives the blood is the donor, and the person who receives it is the recipient.

One condition has to be met first. The type of blood being transfused must match, or be compatible with, the type of blood of the recipient. For a long time nobody knew that such types existed, and transfusions succeeded or killed for no reason anyone could see.

In 1901, in Vienna, Karl Landsteiner took blood from himself and from several colleagues in his laboratory and separated each sample into red cells and serum. He then mixed the serum of each person with the red cells of each of the others, in every combination, and looked at the mixtures.

Most combinations stayed evenly cloudy. Some, however, turned grainy: the red cells clumped together into visible specks. The clumping was not random. Landsteiner could sort his colleagues into groups such that blood within a group always mixed peacefully and blood across certain groups always clumped.

Human blood, he concluded, is not one substance but several kinds, and a transfusion works only when the kinds agree. It was the first explanation of why transfusion had been so unpredictable a procedure, and it made safe transfusion possible for the first time.

8.7.1 The ABO System

The red blood cells of every human being carry particular proteins on their surface. These surface proteins are called antigens, and in this system there are two of them: antigen A and antigen B.

The plasma carries the matching antibodies. The rule connecting the two is short and worth memorising as a rule rather than as a table: your plasma contains the antibody against the antigen you do not have. It could hardly be otherwise. An antibody against an antigen present on your own red cells would attack them.

Depending on which antigens a person’s red cells carry, human blood falls into four groups, set out in Table 8.5.

Table 8.5 — The Four Blood Groups Of The ABO System | Blood group | Antigens on the red blood cells | Antibodies in the plasma | |—|—|—| | A | Antigen A | Antibody B | | B | Antigen B | Antibody A | | AB | Antigen A and antigen B | No antibody | | O | No antigen | Both antibody A and antibody B |

There are several systems of blood grouping. Two of them matter most in practice — the ABO system described here, and the Rh system described in section 8.7.3.

Antigen Or Antibody? The antigen sits on the surface of the red blood cell. The antibody floats free in the plasma. A blood group is named after its antigens, never after its antibodies — group A means antigen A on the cells, and the antibody in that plasma is B.

Finding The Group A drop of blood is mixed on a slide with a serum containing antibody A, and another drop with a serum containing antibody B. Wherever the matching antigen is present, the cells clump into visible grains. Which drops clump tells you the group.

8.7.2 Matching Donor And Recipient

Now the danger of a mismatch becomes clear. Suppose blood of group A is given to a person of group B. That recipient’s plasma contains antibody A, and the incoming cells are covered in antigen A. The antibodies fasten on to the donated cells and make them clump together, and clumps of cells block the smallest blood vessels instead of passing along them. What was meant to save the patient harms them instead.

Work through the four groups with that in mind and the possible transfusions fall out on their own, as Fig. 8.7-1 shows.

Diagram

Fig. 8.7-1 — Who Can Give Blood To Whom Four labelled circles arranged as a diamond — O at the left, A at the top, B at the bottom, AB at the right. Draw arrows from O to A, from O to B, from O to AB, from A to AB and from B to AB, each arrowhead pointing to the recipient. Add a short looping arrow at each circle showing that every group can receive blood of its own group. Label O as universal donor and AB as universal recipient.

Blood of group O carries no antigen at all, so there is nothing on its cells for any recipient’s antibodies to fasten on to. It can therefore be given to a person of any group, and a person of group O is called a universal donor.

Blood of group AB carries no antibody in its plasma, so a person of that group has nothing with which to attack any donated cells. Such a person can receive blood of every group, and is called a universal recipient. A person of group A can receive from A and O only, and a person of group B from B and O only. Table 8.6 collects the whole picture.

Table 8.6 — Summary Of ABO Compatibility | Blood group of donor | To recipient A | To recipient B | To recipient AB | To recipient O | |—|—|—|—|—| | A | Yes | No | Yes | No | | B | No | Yes | Yes | No | | AB | No | No | Yes | No | | O | Yes | Yes | Yes | Yes |

Q4. Why must a patient’s blood group be tested before blood from a donor is given, when all human blood looks alike?

A. Because the difference is chemical, not visible. Red cells carry antigens, and plasma carries antibodies against the antigens it lacks. Give the wrong group and the recipient’s antibodies clump the donated cells, blocking small vessels.

8.7.3 The Rh System

The blood of most people contains another substance altogether, called the Rh factor. The name is short for Rhesus, after the rhesus monkey (Macaca mulatta), the animal in whose blood the factor was first found.

A person whose red cells carry it is Rh-positive, written Rh+ve or Rh\(^+\); a person whose red cells lack it is Rh-negative, written Rh−ve or Rh\(^-\). What Rh-positive people have on the surface of their red cells, and Rh-negative people do not, is the D antigen.

The Rh factor behaves differently from the ABO antigens in one important respect. An Rh-negative person does not carry ready-made antibodies against it. Those antibodies have to be made, and they are made only after a first exposure — which is why the danger lies in the second transfusion, not the first. FC 8.6 follows the sequence.

Flowchart

FC 8.6 — Why The Second Transfusion Is The Dangerous One Rh-negative person receives Rh-positive blood → the D antigen is recognised as foreign → over about two weeks the recipient produces antibodies against it (the person is now sensitised) → a second transfusion of Rh-positive blood is given → the antibodies already present attack the transfused red cells at once → a severe reaction follows, which may even be fatal

The pattern is much like the development of an allergy: harmless on first meeting, violent on the next.

The Rh Factor In Pregnancy

The same sensitisation can happen without any transfusion. If an Rh-negative woman carries an Rh-positive child — which can happen when the father is Rh-positive — some of the baby’s red cells may reach her blood, usually around the time of birth, and her body may then make antibodies against the D antigen.

The first Rh-positive child is not affected, because the antibodies are made only afterwards. A later Rh-positive child, however, may be harmed by antibodies that cross from the mother’s blood and destroy the baby’s red cells.

This is now prevented as a matter of routine. An Rh-negative woman is given an injection of anti-D immunoglobulin during pregnancy and again shortly after the birth of an Rh-positive baby. It clears the baby’s red cells from her circulation before her body can learn to recognise them, so she is never sensitised and later pregnancies are unaffected. It is the reason the Rh group of every expectant mother is checked early, and one of the clearest examples of a piece of laboratory biology turning into ordinary medical practice.

Because both systems matter, a transfusion is matched for both. Blood is described as A positive, O negative, and so on — and the truly universal donor is a person of group O who is also Rh-negative, since that blood carries neither the A and B antigens nor the D antigen.

8.8 The Blood Circulatory System

The circulatory system is made of three things: the blood, which you have now studied; the blood vessels, which are the pipes; and the heart, which is the pump. This section deals with the pump and the pipes.

8.8.1 Position And Covering Of The Heart

Almost everyone will tell you that the heart is on the left side of the chest. It is not. The heart lies right in the centre of the chest, between the two lungs and above the diaphragm.

What misleads us is its shape. The heart is roughly triangular, and its narrow lower end is tipped towards the left. During working, the contraction is most powerful at that narrow end, so the thump you feel is felt on the left — and we conclude, wrongly, that the whole organ is there.

The heart of an adult human being is about the size of your closed fist, some 12 centimetres (cm) in length and 9 cm in width. It is protected by a double-walled membranous covering called the pericardium. Between its two walls is the lubricating pericardial fluid, which lets the beating heart slide instead of rubbing. It reduces friction during the heart beat and cushions the organ against mechanical injury.

8.8.2 The Chambers Of The Heart

The heart consists of four chambers: two upper atria (singular, atrium — also called auricles) and two lower ventricles. Fig. 8.8-1 shows the heart from the front, and Fig. 8.8-2 shows it cut open.

Diagram

Fig. 8.8-1 — Front View Of The Human Heart External front view of the human heart with the great vessels attached. Label superior vena cava, inferior vena cava, aortic arch, ascending aorta, descending aorta, pulmonary trunk, right pulmonary artery, left pulmonary artery, right pulmonary veins, left pulmonary veins, right atrium, left atrium, right ventricle, left ventricle, right coronary artery, left coronary artery, right coronary vein, left coronary vein. Straight, non-crossing lead lines.

The walls of the four chambers are not alike, and the difference is worth pausing on, because it is a plain case of a structure matching the work it does.

The atria have thin walls. Their job is only to receive blood arriving from elsewhere and push it into the ventricles immediately below them — a journey of a couple of centimetres, which needs very little force.

The ventricles have thick, muscular walls, because they have to drive blood over long distances. Even between the two ventricles there is a difference. The right ventricle pumps blood only as far as the lungs, which are next door. The left ventricle pumps blood to the farthest points in the body — down to the toes and up to the brain, the latter against gravity. So the wall of the left ventricle is the thickest of the four.

Diagram

Fig. 8.8-2 — Internal Structure Of The Human Heart Diagrammatic longitudinal section of the human heart with the front wall removed, showing all four chambers and the associated blood vessels. Label right atrium, left atrium, right ventricle, left ventricle, interventricular septum, tricuspid valve (3 flaps), bicuspid or mitral valve (2 flaps), semilunar (pocket) valves of the pulmonary artery, semilunar (pocket) valves of the aorta, chordae tendineae, papillary muscles, superior (anterior) vena cava, inferior (posterior) vena cava, pulmonary artery to right lung, pulmonary artery to left lung, pulmonary veins from right lung, pulmonary veins from left lung, aorta. Show the wall of the left ventricle drawn distinctly thicker than that of the right ventricle.

8.8.3 Blood Vessels Entering And Leaving The Heart

Blood Vessels Entering The Heart

The right atrium receives two large vessels.

  1. The anterior vena cava, also called the superior vena cava or precaval, brings deoxygenated blood from the anterior or upper regions of the body — the head, the chest and the arms.
  2. The posterior vena cava, also called the inferior vena cava or postcaval, brings blood from the posterior or lower region of the body — the abdomen and the legs.

The left atrium receives four pulmonary veins, two from each lung. These bring oxygenated blood back from the lungs.

Blood Vessels Leaving The Heart

Two large blood vessels arise from the ventricles.

  1. The pulmonary artery arises from the right ventricle and carries deoxygenated blood to the lungs for oxygenation.
  2. The aorta arises from the left ventricle and carries oxygenated blood to supply all parts of the body.
The Heart’s Own Blood Supply

The heart is a muscular organ working every second of your life, and like any other organ it needs its own supply of oxygen and food. It does not take these from the blood rushing through its chambers. Two coronary arteries, a right and a left, arise from the base of the aorta and run over the surface of the heart, as you can see in Fig. 8.8-1, dividing into finer and finer branches that supply the heart muscle itself. Coronary veins collect the blood from the heart wall and pour it into the right atrium.

If one of these arteries, or one of its branches, is blocked, the region of heart muscle it supplied is starved of blood and that patch of muscle dies. This is myocardial infarction, called a heart attack in ordinary language. A partial shortage that has not killed the muscle causes chest pain on exertion, known as angina pectoris.

Q. The heart is full of blood at every moment. Why does it need arteries of its own?

A. Because the blood inside the chambers is only passing through. It touches the inner lining, but the thick muscle of the wall is far too deep for oxygen to reach by diffusion from the cavity. The muscle has to be supplied from outside, like any other organ.

8.8.4 The Valves Of The Heart

Blood in the heart must move in one direction only — atrium to ventricle, ventricle to artery, and never back. Four valves see to it.

  1. The right atrio-ventricular valve lies at the opening between the right atrium and the right ventricle. It has three thin, triangular, leaf-like flaps, or cusps, and is therefore called the tricuspid valve. The tips of its flaps are held in position by tendinous cords called chordae tendineae, which arise from muscular projections of the ventricle wall known as the papillary muscles.
  2. The left atrio-ventricular valve lies in the same position on the left side of the heart. It has two cusps, and is therefore called the bicuspid valve, also known as the mitral valve.
  3. The pulmonary semilunar valves are set at the opening of the right ventricle into the pulmonary artery. They are pocket-shaped and three in number.
  4. The aortic semilunar valves are set at the point where the aorta leaves the left ventricle. These too are pocket-shaped and three in number.

Which Valve Is Where LAB RAT — Left Atrium, Bicuspid; Right Atrium, Tricuspid. The left side takes two cusps, the right side three.

Chordae Tendineae Tough cords tie the free tips of the cusps to the ventricle wall → when the ventricle contracts, the pressure below pushes the flaps up but the cords hold them → the valve shuts flat instead of turning inside out into the atrium → blood cannot go backwards.

8.8.5 Circulation Of Blood Through The Heart

Follow one full beat, with Fig. 8.8-3 beside you.

It begins with the contraction of the two atria. At this moment the ventricles are relaxing and are empty, so blood from the atria passes down into them easily through the tricuspid and bicuspid valves, which are open.

Next the ventricles contract and the atria relax. Blood in the ventricles is now under pressure and tends to push back up into the atria. It does not get there. The flaps of the two cuspid valves are pushed up, tightened and puffed out, and they close the passage — held from turning over by the chordae tendineae, much as the cords of a parachute hold its canopy. The only route left open to the blood is forward: into the pulmonary artery from the right ventricle, and into the aorta from the left ventricle. The pocket-like semilunar valves at the mouths of these two vessels face away from the ventricles, so the escaping blood presses them flat against the vessel wall and passes freely between them.

Then the ventricles dilate. Blood already in the pulmonary artery and the aorta now tends to fall back towards the emptying ventricles. It runs into the pockets of the semilunar valves, fills them, and in filling them closes the passage. FC 8.7 sets out the whole sequence with the valves at each stage.

Diagram

Fig. 8.8-3 — The Two Main Phases Of The Heart Beat Two sectional views of the heart side by side, A and B, drawn in the same style as Fig. 8.8-2, with arrows showing the direction of blood flow in each. A — atrial systole and ventricular diastole: atria shown contracting, ventricles relaxing and dilating, tricuspid and bicuspid valves open, semilunar valves closed. B — ventricular systole and atrial diastole: ventricles shown contracting, atria relaxing, tricuspid and bicuspid valves closed, semilunar valves open. Label in both, atrium, ventricle, tricuspid valve, bicuspid valve, semilunar valves of pulmonary artery, semilunar valves of aorta, vena cava, pulmonary vein, pulmonary artery, aorta, chordae tendineae, and mark clearly which valves are open and which are closed.

Flowchart

FC 8.7 — One Full Beat, Valve By Valve Atria contract, ventricles relaxed (atrial systole) — tricuspid and bicuspid valves open, semilunar valves closed → blood passes from atria into ventricles → ventricles contract, atria relax (ventricular systole) — tricuspid and bicuspid valves close, semilunar valves open → blood is driven into the pulmonary artery and the aorta → ventricles relax (ventricular diastole) — semilunar valves fill and close → all four chambers relaxed together (joint diastole), atria filling from the veins → cycle repeats

8.8.6 The Heart Beat And The Cardiac Cycle

Each heart beat consists of two main steps: the atria contract first, and the contraction of the ventricles follows. A contraction phase is called systole, and the relaxing, expansion phase that follows it is called diastole.

Each full beat of the human heart lasts about 0.85 seconds, divided as follows.

  • Systole of the atria — 0.15 second
  • Systole of the ventricles — 0.30 second
  • All chambers in the relaxed state — 0.40 second

At the end of ventricular systole the ventricles begin to relax, which is ventricular diastole. The atria have been relaxing already, in atrial diastole, and for a short period both the atria and the ventricles are relaxed together — the joint diastole. This whole sequence of events, from one contraction of the atria to the next, is called the cardiac cycle.

Look at those three figures again. Almost half of every beat is rest. A heart that contracted continuously would exhaust itself within hours; this one takes 0.40 second off in every 0.85, which is how it manages to go on beating for a lifetime without tiring.

Systole Or Diastole? Systole is contraction, diastole is relaxation — and each is stated for a named chamber. Atrial systole and ventricular diastole happen at the same instant, so the phrase ‘the heart is in systole’ means nothing until you say which chambers you mean.

8.8.6.1 The Heart Sounds

Place a stethoscope on your chest and you hear two sounds in quick succession, ‘lubb’ and ‘dup’, followed by a gap before the next pair. Both are made by valves shutting.

The first sound, ‘lubb’, is produced when the atrio-ventricular valves — the tricuspid and the bicuspid — close sharply at the start of ventricular systole. The second sound, ‘dup’, is produced at the beginning of ventricular diastole, when the semilunar valves at the roots of the aorta and the pulmonary artery close. The gap that follows is the joint diastole, when nothing is shutting.

8.8.6.2 Rate Of Heart Beat

The rate of the heart beat differs from one species to another, and even between individuals of the same species. Some approximate values, in beats per minute at rest, are given below.

  • Whale — 15
  • Elephant — 25
  • Horse — 40
  • Adult men — 64 to 72
  • Adult women — 72 to 80
  • Cat — 120
  • Newborn infant — 140
  • Rat — 250
  • Sparrow, in vigorous activity — 800 to 900

Read the list from the top downwards. The animals are arranged from the largest body to the smallest, and the numbers climb steadily as you go: 15 for a whale, 250 for a rat. That is the pattern to fix first — the smaller the animal, the faster its heart.

Why should size decide the rate? A small animal has a large surface for its volume, so it loses body heat faster than a large animal does. Heat has to be generated and distributed faster to replace what is lost, and a faster heart rate is what distributes it.

Now look at the two human entries near the middle. A newborn infant beats at about 140, an adult at about 70 — the same species, twice the rate. Size is part of it, but so is growth: a growing body has a higher rate of metabolism, and the faster heart keeps the supply of materials to the tissues, and the removal of wastes from them, in step with that demand.

One entry is not comparable with the rest. The sparrow’s figure is measured in vigorous activity, not at rest, so it cannot be set directly against the others. When you read any table of measurements, check the conditions before you compare the numbers.

8.8.7 The Pacemaker

Nothing outside the heart tells it when to beat. The command arises inside the heart itself, at a patch of specialised tissue called the pacemaker, or the sino-atrial node (SAN), which lies in the wall of the right atrium near the opening of the superior vena cava.

From there the impulse has to reach the ventricles, and it does so along a route of its own, shown in Fig. 8.8-4. It travels first to the atrio-ventricular node (AVN), found near the interauricular septum close to the tricuspid valve. From the AVN a bundle of muscle fibres called the bundle of His runs into the interventricular septum, and its branches, the Purkinje fibres, spread along the walls of the ventricles. Together these make up a system that generates an impulse and conducts it to every part of the heart, so that the chambers contract in the right order and not all at once.

Diagram

Fig. 8.8-4 — The Conducting System Of The Heart Sectional view of the heart in the style of Fig. 8.8-2, with the conducting tissue drawn as a distinct network. Label sino-atrial node (SAN) in the wall of the right atrium near the opening of the superior vena cava, atrio-ventricular node (AVN) near the interauricular septum, bundle of His in the interventricular septum, Purkinje fibres branching along the walls of both ventricles, and also label right atrium, left atrium, right ventricle, left ventricle, tricuspid valve, mitral valve, superior vena cava, inferior vena cava. Show the direction in which the impulse spreads with small arrows.

Sometimes the natural pacemaker works faultily, and the heart then beats too slowly or irregularly. An artificial pacemaker — a small battery-powered device that sends out regular electrical impulses — can be fitted for such a person, and it takes over the timing that the SAN can no longer keep.

8.8.8 The Blood Vessels

The blood vessels are branched tubes running from the heart to every part of the body and back. They are of three kinds: arteries, capillaries and veins.

8.8.8.1 Arteries

NoteDefinition

Artery An artery is a blood vessel which carries blood away from the heart towards any organ.

An artery has three characteristics.

  • Thick muscular walls, as Fig. 8.8-5 shows.
  • A narrow lumen — the central bore of the tube.
  • Blood that flows in spurts, each spurt corresponding to a contraction of the ventricles.

These three go together. Blood leaving the ventricle arrives in surges and at high pressure, and a thick, elastic, muscular wall is what stands up to it. The wall of a blood vessel is built in three layers: an outer layer of connective tissue, a middle layer of smooth muscle and elastic fibres, and an inner lining, the endothelium, a single layer of flat cells.

Diagram

Fig. 8.8-5 — Transverse Section Of An Artery And A Vein Two transverse sections drawn side by side and to the same scale, one of an artery and one of a vein. In each, show the three layers of the wall and the central space. Label external layer of connective tissue (tunica externa), middle layer of smooth muscle and elastic fibres (tunica media), endothelium (tunica interna) and lumen in both. Draw the artery with a thick wall, a thick middle layer and a narrow lumen, and the vein with a thin wall, a thin middle layer and a wide lumen.

The smallest and final branch of an artery is called an arteriole. Arterioles are highly muscular and can change their diameter many times over, and each arteriole then breaks up into capillaries.

8.8.8.2 Veins

NoteDefinition

Vein A vein is a blood vessel which carries blood away from an organ towards the heart.

A vein has these characteristics.

  • Thin muscular walls.
  • A wider lumen.
  • Blood that flows uniformly, not in spurts.
  • Thin pocket-shaped valves along its length, whose openings face in the direction of the heart. These prevent the backflow of blood.

The valves matter most where the blood has to climb. Blood returning from your legs is at low pressure and has gravity against it. It is pushed along mainly by the squeezing of the surrounding muscles as you walk, and the valves make that squeezing useful: each squeeze drives blood on towards the heart, and the valves shut behind it so that it cannot slide back. Fig. 8.8-6 shows one such valve open and closed.

Diagram

Fig. 8.8-6 — Valves In A Vein Two longitudinal views of a short length of vein, side by side. Left: blood flowing towards the heart, the pocket-shaped valve flaps pressed flat against the wall, labelled valve open, with arrows showing flow in the direction of the heart. Right: blood tending to flow backwards, filling the pockets so that the flaps meet in the middle, labelled valve closed, with arrows labelled blood presses the valve. Mark the direction of blood flow towards the heart in both views.

When the valves in the veins of the legs stop closing properly, blood collects instead of moving on, and the veins swell and become twisted and visible under the skin. These are varicose veins. They are commoner in people whose work keeps them standing still for long hours, because standing still gives the leg muscles no chance to do their squeezing. Walking about, and raising the legs when resting, both help — for the same reason that the valves exist at all.

Where the capillaries join up again, the smallest common branch formed is called a venule. Venules join to form larger veins. Compared with arterioles, venules are larger and have a much weaker muscular coat.

8.8.8.3 Capillaries

A capillary is a very narrow tube, about 8 micrometres in diameter — narrow enough that red blood cells pass along it in single file. Its wall is not built in three layers like the wall of an artery or a vein. It consists of a single layer of flat squamous epithelial cells, the endothelium, and it has no muscle at all, as Fig. 8.8-7 shows.

Diagram

Fig. 8.8-7 — Arteriole, Capillary Network And Venule Part A: an arteriole branching into a fine network of capillaries which reunite to form a venule; label arteriole, arterial capillaries, venous capillaries, venule, and mark the direction of blood flow. Part B: a short length of a single capillary drawn enlarged, cut open at one end to show that its wall is one cell thick; label endothelium, single layer of squamous epithelial cells, and lumen.

That single thin wall is the whole point of a capillary. Everything the blood carries has to get out to the cells, and everything the cells give up has to get in, and both happen across this wall. Its functions are these.

  1. To allow the outward diffusion of oxygen into the intercellular fluid, and from there into the tissue cells.
  2. To allow the inward diffusion of carbon dioxide from the intercellular fluid.
  3. To allow the inward and outward diffusion of substances such as glucose, amino acids, urea and hormones.
  4. To allow leukocytes to squeeze out through the capillary wall by amoeboid movement.

The amount of blood reaching a capillary network is not fixed. The arterioles feeding it can widen — vasodilation — or narrow — vasoconstriction — and so increase or decrease the supply to that part of the body. You can see the result in your own skin. Walk in the hot sun and your face turns pink, because more blood is being sent to the skin, where its heat can escape. In severe cold the face turns bluish, because less blood is being sent there and heat is being kept in.

The total number of capillaries in the body is almost impossible to picture. If all of them were laid end to end, they are estimated to stretch about 100,000 kilometres, and their combined wall surface would be more than 500 square metres — a wall thinner than a sheet of tissue paper, with the area of a large house plot.

8.8.8.4 Arteries Compared With Veins

Table 8.7 sets the two kinds of vessel side by side. Read it as a set of consequences rather than a list: nearly every difference follows from the pressure of the blood each has to carry.

Table 8.7 — Differences Between Arteries And Veins | Feature | Arteries | Veins | |—|—|—| | Definition | Carry blood away from the heart and into an organ | Carry blood away from an organ and towards the heart (except the hepatic portal vein) | | Branching pattern | Branch progressively, decreasing in size | Unite progressively, increasing in size | | Smallest units | The smallest artery breaks up into arterioles | The smallest vein arises from venules | | Wall | Thick and more muscular | Thin and less muscular | | Elasticity | Walls elastic | Walls non-elastic | | Lumen | Narrower | Wider | | Valves | No valves in the inner lining | Valves present in the inner lining, preventing backward flow | | Change of diameter | Can constrict or dilate to control blood flow | Cannot constrict | | Position in the body | Usually placed deeper | Usually more superficial, nearer the skin surface | | When empty | Do not collapse | Collapse | | Flow of blood | In jerks, under great pressure | Continuous, under very little pressure | | Kind of blood carried | Fully oxygenated blood, except the pulmonary artery | Partially deoxygenated blood laden with carbon dioxide, except the pulmonary vein |

8.9 The Two Blood Circulations

Follow a single drop of blood from the moment it enters the heart until it arrives back at the same point, and you will find that it has passed through the heart not once but twice. Its round trip is made of two separate journeys: a short one to the lungs and back, and a long one to the rest of the body and back. Because the blood goes through the heart twice in one complete round, circulation in the human body is called double circulation. Fig. 8.9-1 shows the general plan.

Diagram

Fig. 8.9-1 — General Plan Of Double Circulation Simple schematic of the human double circulation. At the top, a capillary bed labelled lungs; at the bottom, a capillary bed labelled body organs; between them, the heart drawn as two halves side by side, the right half shaded to show deoxygenated blood and the left half shaded to show oxygenated blood. Draw the upper loop from the right half of the heart to the lungs and back to the left half, labelled pulmonary (lung) circulation, and the lower loop from the left half of the heart to the body and back to the right half, labelled systemic (body) circulation. Mark the direction of flow with arrowheads throughout.

Why should the blood be sent back to the pump halfway through? Two reasons. Passing through the fine capillaries of the lungs costs the blood almost all its pressure, and it could not be driven round the whole body on what is left; it needs a fresh push. And returning it to a separate side of the heart keeps the oxygenated blood from mixing with the deoxygenated blood, so that what leaves for the body is fully loaded.

8.9.1 Pulmonary And Systemic Circulation

  1. The pulmonary circulation is the short one, and it pertains to the lungs. It starts in the pulmonary artery arising from the right ventricle, which soon divides into two branches, one entering each lung. The pulmonary veins collect the oxygenated blood from the lungs and carry it back to the left atrium of the heart.
  2. The systemic circulation is the long one, and it pertains to the major circulation through the body. It starts with the aorta, which arises from the left ventricle. The aorta arches back and continues behind as the dorsal aorta, sending arteries to the various body parts and their tissues. From there the blood is collected by veins and poured back into the heart.

Flowchart

FC 8.8 — The Double Circuit Right atrium → right ventricle → pulmonary artery → lungs (blood becomes oxygenated) → pulmonary veins → left atrium [pulmonary circulation complete] → left ventricle → aorta → arteries to the body organs → capillaries of the organs (blood becomes deoxygenated) → veins → anterior and posterior vena cava → right atrium [systemic circulation complete, cycle repeats]

Following One Drop Of Blood 1. The drop arrives at the heart along the posterior vena cava, deoxygenated, and enters the right atrium. 2. The atrium contracts and the drop passes through the tricuspid valve into the right ventricle. 3. The ventricle contracts. The tricuspid valve shuts behind the drop, and it is driven through the pulmonary semilunar valve into the pulmonary artery. 4. It reaches the capillaries of a lung. Carbon dioxide diffuses out into the air sac and oxygen diffuses in. The drop is now oxygenated and bright red. 5. It returns along a pulmonary vein to the left atrium. That is one pass through the heart, and the pulmonary circulation is complete. 6. From the left atrium it passes through the bicuspid valve into the left ventricle. 7. The thick left ventricle contracts and drives it through the aortic semilunar valve into the aorta — the hardest push of the whole journey, and the one that has to last all the way to the toes. 8. It travels along arteries and arterioles into the capillaries of a leg muscle, gives up its oxygen to the muscle cells and takes on their carbon dioxide. It is deoxygenated once more. 9. Venules collect it, veins carry it upwards, and the valves in those veins stop it slipping back down. 10. It enters the posterior vena cava and arrives at the right atrium, where step 1 began. One full round; two passes through the heart.

8.9.2 The Hepatic Portal System

One set of veins does not behave as veins are supposed to. The veins beginning in the stomach and the intestines do not carry their blood directly to the posterior vena cava. They join instead into a single large vessel, the hepatic portal vein, which enters the liver — and inside the liver it does something no ordinary vein does. It breaks up into capillaries again. Those capillaries then reunite to form a new vein, the hepatic vein, which joins the posterior vena cava. FC 8.9 follows the detour, and Fig. 8.9-2 shows it in place.

Flowchart

FC 8.9 — The Hepatic Portal Route Capillaries in the walls of the stomach and intestine, where digested food is absorbed → small veins unite to form the hepatic portal vein → hepatic portal vein enters the liver and breaks up into capillaries → liver cells act on the absorbed food → capillaries reunite to form the hepatic vein → hepatic vein joins the posterior vena cava → right atrium of the heart

A vein of this kind has a name of its own.

NoteDefinition

Portal Vein A portal vein is a vein which starts with capillaries and also ends in capillaries.

Hepatic Vein Or Hepatic Portal Vein? The hepatic portal vein goes into the liver, carrying absorbed food from the stomach and intestine. The hepatic vein comes out of the liver and joins the posterior vena cava. One is the way in, the other the way out; only the portal vein begins and ends in capillaries.

Why The Detour Is Worth It

All the food absorbed from the stomach and the intestines is thus brought to the liver before it reaches any other organ. Nothing arrives anywhere else until the liver has dealt with it. Three things happen there.

  1. Glycogenesis. Excess sugar is converted and stored in the liver in the form of glycogen, so that the level of sugar reaching the rest of the body stays steady after a heavy meal.
  2. Deamination. Excess amino acids are broken down. The nitrogen-containing part is separated off in the form of ammonia, urea or uric acid, and the remaining part is stored as glycogen.
  3. Detoxification. If any poisonous or harmful material has been absorbed along with the food, it is rendered harmless in the liver.

Q5. Why is the food absorbed from your intestine carried first to the liver, instead of going straight to the heart and out to the body?

A. Because the liver checks it first. Excess sugar is stored as glycogen, excess amino acids are broken down, and any harmful substance absorbed with the food is made harmless — before that blood is allowed to reach the rest of the body.

Diagram

Fig. 8.9-2 — Double Circulation And The Hepatic Portal System Highly diagrammatic whole-body scheme. Show the heart at the centre, the lungs above it, and below it the stomach, the intestine and the liver. Draw the pulmonary circulation from the heart to the lungs and back. Draw the aorta leading to an intestinal artery supplying the capillaries of the stomach and intestine; from those capillaries draw the hepatic portal vein passing into the liver, breaking into capillaries within the liver, and reuniting as the hepatic vein which joins the posterior vena cava on its way back to the heart. Label lungs, heart, anterior vena cava, posterior vena cava, pulmonary artery, pulmonary vein, aorta, stomach, intestine, intestinal artery, liver, hepatic portal vein, hepatic vein, and mark the hepatic portal vein with the note all absorbed food is brought to the liver first. Show the direction of flow with arrowheads.

8.9.3 Main Blood Vessels Of The Heart, Liver And Kidney

You are not required to learn the name of every vessel in the body — only the main vessels entering and leaving the heart, the liver and the kidney. They are collected here, and their positions are shown in Fig. 8.9-3.

The Heart

Entering the heart:

  1. Anterior vena cava — from the upper parts of the body into the right atrium. It is formed by the union of the jugular vein, which drains the head, and the subclavian vein, which drains the arm.
  2. Posterior vena cava — from the lower parts of the body into the right atrium.
  3. Pulmonary veins — two from each lung, into the left atrium.

Leaving the heart:

  1. Pulmonary artery — from the right ventricle to the lungs.
  2. Aorta — from the left ventricle to all other parts of the body.
The Liver

Entering the liver:

  1. Hepatic artery — from the aorta into the liver.
  2. Hepatic portal vein — from the stomach and intestine into the liver.

Leaving the liver:

  1. Hepatic vein — from the liver into the posterior vena cava.

The liver is unusual in receiving blood from two vessels. The hepatic artery brings it oxygen, as an artery does for any organ; the hepatic portal vein brings it the absorbed food to be processed.

The Kidney

Entering the kidney:

  1. Renal artery — from the aorta into the kidney.

Leaving the kidney:

  1. Renal vein — from the kidney into the posterior vena cava.

Diagram

Fig. 8.9-3 — Major Arteries And Veins In The Human Body Front view of a human body outline showing the main blood vessels, arteries drawn in red and veins in blue, with a key given. Label carotid artery, jugular vein, subclavian artery, subclavian vein, anterior vena cava, posterior vena cava, aorta, pulmonary artery, pulmonary vein, heart, left lung, liver, hepatic artery, hepatic vein, hepatic portal vein, gut, mesenteric artery, left kidney, renal artery, renal vein, iliac artery, iliac vein. Mark the direction of blood flow with arrowheads. Straight, non-crossing lead lines.

8.9.4 The Pulse

Every time the ventricles contract, blood is forced into the arteries under pressure, and the elastic muscular wall of an artery stretches to take it. As the wave passes, the wall springs back. Where an artery runs close to the surface, that alternate stretching and recoil can be felt from outside.

NoteDefinition

Pulse The pulse is the alternate expansion and elastic recoil of the wall of an artery during ventricular systole.

The most convenient place to feel it is the radial artery of the wrist, on the same side as the thumb. Because one expansion of the artery corresponds to one contraction of the ventricles, counting the pulse is an indirect way of counting the heart beat. The pulse rate rises with physical exercise, and also under certain emotions.

Rest your fingertips — not your thumb, which has a pulse of its own — on the wrist just below the base of the thumb, and count the beats for one minute. Sit quietly for two minutes and count again. Then climb a flight of stairs briskly and count once more. Note the three numbers and compare them.

8.9.5 Blood Pressure

Blood pressure is the pressure which the blood flowing through the arteries exerts on their walls. It is not one number but two, because the pressure rises and falls with every beat.

  • The systolic pressure is the upper limit, recorded at the moment when fresh blood is being pushed through the artery by the contraction of the ventricles.
  • The diastolic pressure is the lower limit, recorded once that wave has passed over.

Blood pressure is measured in millimetres of mercury, written mm Hg — the unit comes from the height of a mercury column that the pressure can support. For adults, the normal systolic pressure is 100 to 140 mm Hg and the normal diastolic pressure is 60 to 80 mm Hg. The two are written together as one figure over the other.

A sustained rise above 140/90 mm Hg is called hypertension, or high blood pressure. It is measured with an instrument called a sphygmomanometer, the cuff-and-dial device you will have seen wrapped around a person’s upper arm at a clinic.

A Long Name Sphygmomanometer looks impossible until it is taken apart. Sphygmos is Greek for pulse and manos for thin or rare, and a manometer is any instrument that measures pressure. So the word means, quite literally, a pulse-pressure meter.

8.10 Tissue Fluid And The Lymphatic System

Section 8.2.1 named the three fluids of the body and promised that two of them would be taken up later. That is the business of this section.

8.10.1 Tissue Fluid

No cell in your body is ever touched by blood. Blood stays inside its vessels, and the cells of an organ lie outside them. Something has to cross the gap.

As blood flows through the capillaries of a tissue, part of the plasma, together with some leukocytes, leaks out through the capillary wall. This escaped fluid fills the narrow spaces between the cells and bathes them directly. It is the tissue fluid, also called the intercellular or extracellular fluid, and Fig. 8.10-1 shows it being formed.

Tissue fluid is the go-between. The cells absorb oxygen and other required substances from it, and give out carbon dioxide and other wastes into it. Every exchange between your blood and your cells passes through this thin film of fluid.

Diagram

Fig. 8.10-1 — Formation And Drainage Of Tissue Fluid Enlarged view of a single blood capillary running between a group of body cells, with a blind-ending lymph capillary alongside. At the arterial end of the capillary, show fluid passing out through the wall into the spaces around the cells; label this tissue fluid. Show exchange arrows between the tissue fluid and the body cells, marked oxygen and food in, carbon dioxide and wastes out. At the venous end, show most of the fluid passing back into the capillary. Show the remainder entering the blind end of the lymph capillary, labelled lymph. Label blood capillary, arterial end, venous end, body cells, tissue fluid, lymph capillary.

8.10.2 Lymph And Lymph Vessels

Most of the tissue fluid seeps back into the blood capillaries at their far end and rejoins the blood directly. What is left over cannot be allowed to collect in the tissues, and it is picked up instead by a second set of minute channels, the lymph vessels. The same fluid, once it is inside these channels, is called lymph.

NoteDefinition

Lymph Lymph is the tissue fluid that has drained into the lymph vessels — a colourless fluid containing leukocytes, mostly lymphocytes, but no red blood cells and no platelets.

There is no second pump for this system. Lymph is moved along by the contraction of the muscles surrounding the vessels, which squeeze them as you move about; here is one more benefit of physical exercise. On the way, the lymph vessels drain into lymph nodes, from which fresh lymph channels arise. These finally pour the lymph into the major veins of the upper body, close to where they enter the right atrium — and so the fluid is back in circulation. FC 8.10 follows the whole route and Fig. 8.10-2 shows where the main vessels lie.

Flowchart

FC 8.10 — From Blood To Lymph And Back To Blood Plasma leaks out of a blood capillary → tissue fluid bathing the cells → branch 1, most of it is reabsorbed into the blood capillary at its venous end; branch 2, the remainder enters the blind-ending lymph capillaries as lymph → lymph vessels → lymph nodes → larger lymph channels, the thoracic duct and the right lymphatic duct → subclavian veins → anterior vena cava → right atrium of the heart

Diagram

Fig. 8.10-2 — Distribution Of The Main Lymph Vessels In The Human Body Front view of a human body outline showing the lymphatic system. Label right lymphatic duct opening into the right subclavian vein, thoracic duct opening into the left subclavian vein, right atrium of the heart, posterior vena cava, spleen, lymph glands, lymph nodes shown in the neck, armpits and groin, and the ileum with the note lacteals drain into the lymphatics. Draw the lymph vessels as a network of fine channels linking the nodes, with arrowheads showing that the flow is one way, towards the veins near the heart.

What Lymph Is Made Of
  1. Cellular part — only leukocytes, and mostly lymphocytes among them. There are no red blood cells and no blood platelets in lymph.
  2. Non-cellular part — water, 94 per cent; solids such as proteins, fats, carbohydrates, enzymes and antibodies, 6 per cent.

Table 8.8 sets lymph beside the blood it came from.

Table 8.8 — Blood Compared With Lymph | Feature | Blood | Lymph | |—|—|—| | Colour | Red | Colourless or pale yellow | | Cells present | Red blood cells, white blood cells and platelets | Only leukocytes, mostly lymphocytes | | Proteins | Abundant, including fibrinogen | Fewer | | Vessels it flows in | Heart, arteries, capillaries and veins | Lymph capillaries, lymph vessels and lymph ducts | | Direction of flow | Circulates out from the heart and back to it | One way only, from the tissues towards the heart | | What drives the flow | Pumping of the heart | Squeezing by the surrounding muscles | | Main function | Transport of gases, food, wastes, hormones and heat | Drainage, absorption of fats and defence |

The Work Lymph Does
  1. Nutritive. It supplies nutrition and oxygen to those parts of the body which the blood vessels do not reach.
  2. Drainage. It drains away the excess tissue fluid and the metabolites from the tissue spaces, and returns proteins from those spaces to the blood.
  3. Absorption. Fats from the intestine are absorbed through the lymphatics — specifically through the lacteals, the lymph vessels lying in the intestinal villi.
  4. Defence. The lymphocytes and monocytes carried in the lymph defend the body, and the lymphatics remove bacteria from the tissues.

That last function explains something you may have felt yourself. When there is a boil or an injury in a limb, painful swellings sometimes appear in the groin or in the armpit. Those are lymph nodes at work. The nodes tend to localise an infection and prevent it from spreading to the body as a whole, so the swelling is a protective sign rather than a fresh problem.

Lymph Node A node is a small chamber packed with lymphocytes, through which lymph must filter slowly → bacteria carried in from the tissues are trapped and destroyed inside it → the infection is held at the node instead of travelling on into the bloodstream.

In lymphatic filariasis, thread-like parasitic worms spread by mosquito bites settle in the lymph vessels and block them. The lymph can no longer drain out of the limb, so fluid collects in the tissues and the leg or arm swells greatly — the condition long known as elephantiasis. It shows, by its absence, what the drainage function of lymph is doing every day in a healthy body. The disease is treated with medicines that kill the worms, and controlled by reducing mosquito breeding.

8.10.3 The Spleen And The Tonsils

The lymphatic system has organs as well as vessels. The largest of them is the spleen — a large lymphatic organ about the size of a clenched fist, reddish brown in colour, lying in the abdomen behind the stomach and above the left kidney.

The spleen does four things.

  1. It acts as a blood reservoir. In an emergency such as a haemorrhage, physical or emotional stress, or carbon monoxide poisoning, the spleen releases the blood stored in it into the blood stream.
  2. It produces lymphocytes.
  3. It destroys worn-out red blood cells, a function it shares with the liver.
  4. In the embryo, the spleen produces red blood cells.

The tonsils are lymph glands too, a pair of them lying on the sides of the throat at the back of the mouth. Placed where air and food both pass, they meet incoming germs early, and they respond as lymphatic tissue does — which is why they swell and become sore during a throat infection.

Spleen Or Liver? Both destroy worn-out red blood cells. The spleen is a lymphatic organ: it also stores blood and makes lymphocytes. The liver is not lymphatic: it keeps the iron from the broken-down cells and turns the rest of the pigment into bile pigment.

8.11 Summary

  1. Diffusion is fast enough only over very short distances, so a body as large as ours must move materials by fluids that are kept flowing.
  2. The circulating fluid works both as a distributing system, supplying substances to the organs, and as a collecting system, picking substances up from them.
  3. There are three principal fluids in the body: blood, in the heart and blood vessels; tissue fluid, in the spaces between the cells; and lymph, in the lymph vessels and lymphatic organs. They are the same material at three points along one route.
  4. Human blood always stays inside vessels — a closed blood circulatory system. In insects the blood flows through open body spaces — an open blood circulatory system.
  5. Blood is never stationary. It is bright red from an artery and dark red from a vein, measures 5 to 6 litres in an average adult, tastes saltish, and is slightly alkaline, with a pH of 7.35 to 7.45.
  6. Blood transports digested food, oxygen, carbon dioxide, excretory materials, hormones and heat, and protects the body by clotting, by engulfing bacteria and by producing antibodies and antitoxins.
  7. Oxygen is carried on haemoglobin as unstable oxyhaemoglobin, which breaks up in the tissues to release the oxygen. Carbon dioxide travels partly as carbaminohaemoglobin and mostly in solution in the plasma.
  8. Blood consists of plasma, 55 to 60 per cent, and cellular elements, 40 to 45 per cent.
  9. Plasma is water 90 to 92 per cent, proteins 7 to 8 per cent, inorganic salts 1 per cent, and other substances in traces. The salts are mainly sodium chloride and sodium bicarbonate.
  10. Serum is plasma from which the protein fibrinogen has been removed.
  11. The cellular elements are of three kinds: red blood cells (erythrocytes), white blood cells (leukocytes) and blood platelets (thrombocytes).
  12. A red blood cell is a biconcave disc about 7 micrometres across. Its shape gives a large surface for its volume, and its small size lets it pass through the finest capillaries in single file.
  13. Haemoglobin is made of an iron-containing part, haem, and a protein, globin, and is held in the stroma of the red cell.
  14. Haemoglobin binds carbon monoxide to form the stable compound carboxyhaemoglobin, which puts that haemoglobin out of use — this is carbon monoxide poisoning.
  15. Blood returning in the veins still carries about three-quarters of its oxygen; veins carry less oxygen than arteries, not none.
  16. A mature mammalian red blood cell has no nucleus, no mitochondria and no endoplasmic reticulum. These losses make it more efficient: it becomes biconcave, it cannot consume the oxygen it carries, and it is flexible enough for narrow capillaries.
  17. Red blood cells are made in the red bone marrow, and in the liver and spleen in the embryo. They live about 120 days and are destroyed in the spleen, liver and bone marrow; the iron is retained in the liver and the rest is excreted as the bile pigment bilirubin.
  18. An adult male has about 5 million red blood cells per mm\(^3\) and an adult female about 4.5 million. An abnormally high count is polycythaemia; an abnormally low one is erythropenia.
  19. White blood cells have a nucleus and no haemoglobin, number 4,000 to 8,000 per mm\(^3\), and squeeze out through capillary walls by diapedesis.
  20. Granular white cells are of three types — neutrophils, eosinophils and basophils. Non-granular white cells are of two types — lymphocytes and monocytes.
  21. White cells defend the body by phagocytosis, by inflammation and by producing antibodies. Pus is mainly dead white cells with the tissue cells destroyed by bacteria.
  22. Antibodies are specific to the toxin or germ they are made against, and acting against a poison they are called antitoxins. Antibodies persisting after a disease give immunity, which is the principle of vaccination.
  23. Platelets have no nucleus, number 200,000 to 400,000 per mm\(^3\), are budded off from megakaryocytes in the bone marrow, and release thrombokinase at a wound.
  24. In clotting, thrombokinase with calcium ions converts prothrombin into thrombin; thrombin with calcium ions converts soluble fibrinogen into insoluble fibrin; the fibrin mesh traps blood cells and squeezes out serum, leaving the clot. Vitamin K is needed for the production of prothrombin.
  25. Clotting is set off by contact with a rough surface, not by exposure to air. It is delayed in haemophilia and when the platelet count is abnormally low.
  26. In the ABO system there are four blood groups, named after the antigens on the red cells, with the matching antibodies in the plasma. Group O is the universal donor and group AB the universal recipient.
  27. The Rh factor is the D antigen on the red cells. An Rh-negative person makes antibodies against it only after a first exposure, so the danger lies in the second transfusion; the factor also matters in pregnancy.
  28. The heart lies centrally between the lungs and above the diaphragm, with its narrow end tipped to the left, and is enclosed in the double-walled pericardium containing pericardial fluid.
  29. The heart has four chambers. The atria have thin walls; the ventricles have thick walls, and the wall of the left ventricle is the thickest because it pumps blood to the whole body.
  30. The venae cavae enter the right atrium and four pulmonary veins the left atrium; the pulmonary artery leaves the right ventricle and the aorta the left ventricle. The coronary arteries, arising from the base of the aorta, supply the heart muscle itself.
  31. Four valves keep the flow one-way: the tricuspid, the bicuspid or mitral, the pulmonary semilunar and the aortic semilunar. The chordae tendineae and papillary muscles stop the cuspid valves from turning back into the atria.
  32. One cardiac cycle lasts about 0.85 second — atrial systole 0.15, ventricular systole 0.30 and joint diastole 0.40. The sound ‘lubb’ is the atrio-ventricular valves closing and ‘dup’ the semilunar valves closing.
  33. The smaller the animal, the faster its heart rate, and a growing young animal beats faster than an adult of the same species.
  34. The beat begins at the pacemaker, or sino-atrial node, in the wall of the right atrium, and the impulse is carried on by the atrio-ventricular node, the bundle of His and the Purkinje fibres.
  35. An artery carries blood away from the heart: thick elastic muscular wall, narrow lumen, no valves, blood in spurts. A vein carries blood from an organ towards the heart: thin wall, wide lumen, pocket-shaped valves, uniform flow.
  36. A capillary wall is a single layer of squamous cells with no muscle, and all exchange between blood and tissues takes place across it. The arterioles supplying a capillary bed control its blood flow by vasodilation and vasoconstriction.
  37. Circulation in the human body is double circulation: the short pulmonary circulation to the lungs and back, and the long systemic circulation to the body and back, so the blood passes twice through the heart in one complete round.
  38. A portal vein starts in capillaries and ends in capillaries. The hepatic portal vein brings all the absorbed food from the stomach and intestine to the liver, where glycogenesis, deamination and detoxification take place before the blood goes on to the rest of the body.
  39. The liver receives the hepatic artery and the hepatic portal vein and is drained by the hepatic vein; the kidney receives the renal artery and is drained by the renal vein.
  40. The pulse is the alternate expansion and elastic recoil of an artery wall during ventricular systole, and counting it is an indirect way of counting the heart beat.
  41. Blood pressure is the pressure blood exerts on the artery walls: normally 100 to 140 mm Hg systolic and 60 to 80 mm Hg diastolic. A sustained rise above 140/90 mm Hg is hypertension, measured with a sphygmomanometer.
  42. Tissue fluid is formed by plasma and leukocytes leaking out of the capillaries, and every exchange between blood and cells passes through it.
  43. Most tissue fluid returns to the blood capillaries; the rest enters the lymph vessels as lymph. Lymph contains only leukocytes, mostly lymphocytes, is 94 per cent water and 6 per cent solids, and is moved by the contraction of the surrounding muscles.
  44. Lymph is nutritive, drains excess tissue fluid and returns proteins to the blood, absorbs fats from the intestine through the lacteals, and defends the body; the lymph nodes localise infection.
  45. The spleen is the largest lymphatic organ. It acts as a blood reservoir, produces lymphocytes, destroys worn-out red blood cells, and produces red blood cells in the embryo. The tonsils are lymph glands at the sides of the throat.

Flowchart

FC 8.11 — The Whole Route, End To End Heart contracts → aorta → arteries → arterioles → capillaries of an organ → fluid leaks out as tissue fluid → exchange with the body cells → branch 1, most of the fluid returns to the capillaries → venules → veins → venae cavae → heart; branch 2, the remainder enters the lymph capillaries as lymph → lymph vessels → lymph nodes → lymph ducts → subclavian veins → heart (both branches return to the heart, and the cycle repeats)

8.12 Key Terms

  1. Angina pectoris — chest pain caused by an insufficient supply of blood to the heart muscle.
  2. Anterior vena cava — the large vein bringing deoxygenated blood from the head, chest and arms into the right atrium; also called the superior vena cava.
  3. Antibodies — substances produced by lymphocytes that kill or neutralise particular germs or their poisons.
  4. Antigens — proteins on the surface of red blood cells, which decide a person’s blood group.
  5. Antitoxins — antibodies acting against a poison, neutralising its effect.
  6. Aorta — the largest artery, which leaves the left ventricle and carries oxygenated blood to the whole body.
  7. Arteriole — the smallest and final branch of an artery, which breaks up into capillaries.
  8. Artery — a blood vessel that carries blood away from the heart towards an organ.
  9. Atria — the two thin-walled upper chambers of the heart, which receive blood; singular, atrium, also called auricle.
  10. Atrio-ventricular node (AVN) — the patch of conducting tissue near the interauricular septum that passes the impulse on towards the ventricles.
  11. Bicuspid valve — the two-cusped valve between the left atrium and the left ventricle; also called the mitral valve.
  12. Bilirubin — the bile pigment formed from the breakdown of haemoglobin in worn-out red blood cells.
  13. Blood — the red circulating fluid contained in the heart and blood vessels.
  14. Blood pressure — the pressure that blood flowing through the arteries exerts on their walls.
  15. Blood transfusion — the introduction of blood from a donor into a vein of a recipient.
  16. Bundle of His — the bundle of conducting muscle fibres running from the AVN into the interventricular septum.
  17. Capillary — a very narrow blood vessel whose wall is a single layer of cells, across which all exchange with the tissues takes place.
  18. Carbaminohaemoglobin — the compound formed when carbon dioxide combines with haemoglobin.
  19. Carboxyhaemoglobin — the stable compound formed when carbon monoxide combines with haemoglobin, putting it out of use.
  20. Cardiac cycle — the complete sequence of events in one heart beat, lasting about 0.85 second.
  21. Cellular elements — the red cells, white cells and platelets of blood; also called the formed elements.
  22. Chordae tendineae — tendinous cords that hold the cusps of the atrio-ventricular valves in position.
  23. Closed blood circulatory system — a circulation in which the blood remains inside blood vessels throughout.
  24. Clot — the solid mass of blood cells entangled in a mesh of fibrin threads that seals a wound.
  25. Coagulation — the process by which blood forms a clot; also called clotting.
  26. Coronary arteries — the two arteries arising from the base of the aorta that supply the heart muscle itself.
  27. Deoxygenated blood — dark red blood that has given up its oxygen to the tissues and is carrying carbon dioxide.
  28. Diapedesis — the squeezing out of white blood cells through the wall of a capillary into the tissues.
  29. Diastole — the relaxation phase of a chamber of the heart.
  30. Diastolic pressure — the lower of the two blood pressure readings, recorded after the pressure wave has passed.
  31. Donor — the person who gives blood in a transfusion.
  32. Dorsal aorta — the continuation of the aorta running backwards along the body.
  33. Double circulation — circulation in which blood passes twice through the heart in one complete round of the body.
  34. Endothelium — the single layer of flat cells forming the inner lining of a blood vessel.
  35. Enucleated — without a nucleus, as a mature mammalian red blood cell is.
  36. Erythrocytes — red blood cells, the oxygen carriers of the blood.
  37. Erythropenia — an abnormally low number of red blood cells.
  38. Fibrin — the insoluble threaded protein that forms the mesh of a clot.
  39. Fibrinogen — the soluble plasma protein from which fibrin is made.
  40. Globin — the protein part of haemoglobin.
  41. Haem — the iron-containing part of haemoglobin, to which oxygen attaches; often written haemin.
  42. Haemoglobin — the red respiratory pigment of red blood cells, which carries oxygen.
  43. Haemophilia — a genetic disorder in which blood does not clot properly because a clotting protein is missing or faulty.
  44. Haemorrhage — loss of blood from a vessel that has not been sealed.
  45. Hepatic artery — the artery carrying oxygenated blood from the aorta into the liver.
  46. Hepatic portal vein — the vein carrying absorbed food from the stomach and intestine into the liver.
  47. Hepatic vein — the vein carrying blood out of the liver into the posterior vena cava.
  48. Hypertension — a sustained rise of blood pressure above 140/90 mm Hg; high blood pressure.
  49. Inflammation — the reaction of a tissue to injury or local invasion by germs, marked by heat, redness, swelling and pain.
  50. Jugular vein — the vein draining blood from the head, which joins the subclavian vein to form the anterior vena cava.
  51. Lacteals — the lymph vessels inside the intestinal villi, through which fats are absorbed.
  52. Leukaemia — a cancer of the tissue that forms white blood cells.
  53. Leukocytes — white blood cells, the defenders of the body.
  54. Leukopenia — an abnormally low number of white blood cells.
  55. Lumen — the central bore, or space, inside a tube such as a blood vessel.
  56. Lymph — tissue fluid that has drained into the lymph vessels, containing leukocytes but no red cells or platelets.
  57. Lymph nodes — small swellings along the lymph vessels in which bacteria are trapped and destroyed.
  58. Lymph vessels — the channels that collect tissue fluid and return it to the blood near the heart.
  59. Megakaryocytes — the giant cells of the bone marrow from which platelets are budded off.
  60. Myocardial infarction — the death of a patch of heart muscle whose coronary blood supply has been blocked; a heart attack.
  61. Open blood circulatory system — a circulation in which blood leaves the vessels and flows through open body spaces, as in insects.
  62. Oxygenated blood — bright red blood that has taken up oxygen in the lungs.
  63. Oxyhaemoglobin — the unstable compound of haemoglobin with oxygen, which readily gives the oxygen up to the tissues.
  64. Pacemaker — the sino-atrial node, which starts each heart beat.
  65. Papillary muscles — the muscular projections of the ventricle wall from which the chordae tendineae arise.
  66. Pericardium — the double-walled membranous covering of the heart, containing pericardial fluid.
  67. Phagocytosis — the engulfing of solid particles, especially bacteria, by a white blood cell.
  68. Plasma — the light-yellow liquid part of blood, in which everything else is carried.
  69. Polycythaemia — an abnormally high number of red blood cells.
  70. Portal vein — a vein that starts in capillaries and also ends in capillaries.
  71. Posterior vena cava — the large vein bringing blood from the abdomen and legs into the right atrium; also called the inferior vena cava.
  72. Prothrombin — the inactive plasma protein that is converted into thrombin during clotting.
  73. Pulmonary artery — the artery carrying deoxygenated blood from the right ventricle to the lungs.
  74. Pulmonary circulation — the short circuit of blood from the heart to the lungs and back.
  75. Pulmonary veins — the four veins bringing oxygenated blood from the lungs into the left atrium.
  76. Pulse — the alternate expansion and elastic recoil of an artery wall during ventricular systole.
  77. Purkinje fibres — the branches of the bundle of His that spread the impulse through the ventricle walls.
  78. Pus — a mixture of dead white blood cells and tissue cells destroyed at the site of an infection.
  79. Recipient — the person who receives blood in a transfusion.
  80. Renal artery — the artery carrying blood from the aorta into the kidney.
  81. Renal vein — the vein carrying blood from the kidney into the posterior vena cava.
  82. Respiratory pigment — a coloured substance, such as haemoglobin, that carries respiratory gases.
  83. Rh factor — the D antigen on the surface of red blood cells, present in Rh-positive and absent in Rh-negative people.
  84. Semilunar valves — the pocket-shaped valves at the openings of the pulmonary artery and the aorta.
  85. Serum — blood plasma from which the protein fibrinogen has been removed.
  86. Sino-atrial node (SAN) — the patch of tissue in the wall of the right atrium where each heart beat begins.
  87. Sphygmomanometer — the instrument used to measure blood pressure.
  88. Spleen — the largest lymphatic organ, lying behind the stomach; a blood reservoir that also makes lymphocytes and destroys worn-out red cells.
  89. Stroma — the colourless spongy body of a red blood cell, which holds the haemoglobin.
  90. Subclavian vein — the vein draining blood from the arm, which joins the jugular vein to form the anterior vena cava.
  91. Systemic circulation — the long circuit of blood from the heart to the body organs and back.
  92. Systole — the contraction phase of a chamber of the heart.
  93. Systolic pressure — the higher of the two blood pressure readings, recorded during ventricular contraction.
  94. Thrombin — the active enzyme that converts fibrinogen into fibrin.
  95. Thrombocytes — blood platelets, which start the clotting of blood.
  96. Thrombokinase — the substance released by damaged tissue and platelets that sets off clotting; also called thromboplastin.
  97. Thrombus — another name for a clot.
  98. Tissue fluid — the fluid that leaks out of capillaries and bathes the cells of an organ.
  99. Tonsils — a pair of lymph glands at the sides of the throat.
  100. Toxins — poisonous substances produced by disease-causing germs.
  101. Tricuspid valve — the three-cusped valve between the right atrium and the right ventricle.
  102. Universal donor — a person of blood group O, whose blood can be given to any ABO group.
  103. Universal recipient — a person of blood group AB, who can receive blood of any ABO group.
  104. Vasoconstriction — the narrowing of the arterioles supplying a part, which reduces its blood flow.
  105. Vasodilation — the widening of the arterioles supplying a part, which increases its blood flow.
  106. Vein — a blood vessel that carries blood away from an organ towards the heart.
  107. Ventricles — the two thick-walled lower chambers of the heart, which pump blood out of it.
  108. Venule — the smallest vein, formed where capillaries reunite.

8.13 Points To Ponder

  1. A mature red blood cell gives up its nucleus and so can neither repair itself nor divide, and it is worn out within about 120 days. Weigh the gain against the loss: would a red blood cell that kept its nucleus be, on the whole, a better oxygen carrier or a worse one?
  2. Almost half of every cardiac cycle is spent with all four chambers relaxed. Suppose a heart could be built that contracted continuously, with no relaxing phase at all. Apart from the muscle tiring, what else would stop such a heart from working?
  3. Insects manage with an open circulatory system, no capillaries and blood at low pressure. What limits would that arrangement place on how large an animal can grow, and on how active it can be?
  4. Every drop of blood leaving the intestine is sent through the liver before it is allowed anywhere else. What would follow, meal by meal, if the hepatic portal vein emptied straight into the posterior vena cava instead?
  5. Blood has a pump; lymph has none, and moves only when the muscles around its vessels squeeze them. What does that predict about a person who has to lie still for many weeks, and why might a doctor ask such a person to move their legs regularly?
  6. In deciding whether a transfusion is safe, it is usually the antigens on the donor’s red cells that matter rather than the antibodies in the donor’s plasma. Work out why the donor’s antibodies count for so much less.

8.14 Objective Questions

Multiple Choice Questions

Select the most appropriate option in each case.

  1. The blood cell which, when mature, has no nucleus, no mitochondria and no endoplasmic reticulum: (1 mark)
    1. Lymphocyte
    2. Erythrocyte
    3. Monocyte
    4. Megakaryocyte
  2. The compound formed when haemoglobin combines with carbon monoxide: (1 mark)
    1. Oxyhaemoglobin
    2. Carbaminohaemoglobin
    3. Carboxyhaemoglobin
    4. Haemin
  3. The valve which has two cusps: (1 mark)
    1. Tricuspid valve
    2. Bicuspid valve
    3. Pulmonary semilunar valve
    4. Aortic semilunar valve
  4. The clear liquid left behind after blood has clotted: (1 mark)
    1. Plasma
    2. Lymph
    3. Serum
    4. Tissue fluid
  5. The blood vessel which begins in capillaries and also ends in capillaries: (1 mark)
    1. Hepatic artery
    2. Hepatic vein
    3. Hepatic portal vein
    4. Renal vein
  6. The largest of the white blood cells, which transforms into a macrophage at the site of infection: (1 mark)
    1. Neutrophil
    2. Basophil
    3. Lymphocyte
    4. Monocyte
  7. The part of one cardiac cycle during which all four chambers of the heart are relaxed lasts: (1 mark)
    1. 0.15 second
    2. 0.30 second
    3. 0.40 second
    4. 0.85 second
  8. Lymph differs from blood in containing no: (1 mark)
    1. Lymphocytes
    2. Proteins
    3. Red blood cells
    4. Water
Assertion And Reason

In each of the following, choose the answer from the codes given below. (1) A is True and R is False (2) A is False and R is True (3) Both A and R are True (4) Both A and R are False

  1. Assertion (A): The wall of the left ventricle is thicker than the wall of the right ventricle. (1 mark) Reason (R): The left ventricle pumps blood only as far as the lungs.
  2. Assertion (A): A mature mammalian red blood cell is an efficient oxygen carrier although it lacks several organelles. (1 mark) Reason (R): Having no mitochondria, the cell cannot consume the oxygen it is carrying.
  3. Assertion (A): A person of blood group AB is called the universal recipient. (1 mark) Reason (R): The plasma of blood group AB contains both antibody A and antibody B.
  4. Assertion (A): Veins have pocket-shaped valves in their inner lining. (1 mark) Reason (R): Blood in the veins flows at low pressure and often has to travel against gravity.
Match The Following
  1. Match the items in Column A with those in Column B and rewrite the correct pairs. (5 marks)
Column A Column B
a. Sino-atrial node i. Absorption of fats
b. Lacteals ii. Hold the cusps of a valve in position
c. Megakaryocytes iii. Pacemaker
d. Sphygmomanometer iv. Platelets are budded off from these
e. Chordae tendineae v. Measurement of blood pressure
Fill In The Blanks
  1. Complete the following statements. (5 marks)
    1. The vitamin essential for the production of prothrombin is ………… .
    2. The smallest and final branch of an artery is called a(n) ………… .
    3. The heart is enclosed in a double-walled covering called the ………… .
    4. The heart sound ‘dup’ is produced by the closing of the ………… valves.
    5. Fats are absorbed from the intestine through the ………… present in the villi.
True Or False
  1. State whether each of the following is true or false. (4 marks)
    1. Serum is plasma that still contains fibrinogen.
    2. The pulmonary vein carries oxygenated blood.
    3. Clotting of blood begins because the blood has been exposed to air.
    4. An adult female has slightly fewer red blood cells per mm\(^3\) of blood than an adult male.
Name The Following
  1. Name the following. (6 marks)
    1. The respiratory pigment of the red blood cells.
    2. The smallest of the white blood cells.
    3. The giant cells of the bone marrow from which platelets arise.
    4. The vein which drains blood from the arm and joins the jugular vein.
    5. The node at which each heart beat begins.
    6. The bile pigment formed when worn-out red blood cells are broken down.
Odd One Out
  1. Pick the odd one out in each group and give a reason for your choice. (4 marks)
    1. Neutrophil, eosinophil, basophil, lymphocyte
    2. Tricuspid valve, bicuspid valve, semilunar valve, pericardium
    3. Hepatic artery, renal artery, pulmonary artery, hepatic portal vein
    4. Synovial fluid, vitreous humour, aqueous humour, lymph

8.15 Check Your Concepts

  1. Define the following terms: (i) diapedesis (ii) portal vein. (2 marks)

  2. State two structural differences between a red blood cell and a lymphocyte. (2 marks)

  3. Name the two circulations that together make up double circulation, and state the chamber of the heart in which each begins. (2 marks)

  4. Give a reason why veins possess valves in their inner lining while arteries do not. (2 marks)

  5. What is meant by the term ‘universal donor’? Name the blood group concerned and give the reason for the name. (2 marks)

  6. Differentiate between plasma and serum, stating the basis of difference. (3 marks)

  7. Explain why the wall of the left ventricle is thicker than the wall of the right ventricle. (3 marks)

  8. State the role of calcium ions and of vitamin K in the clotting of blood. (3 marks)

  9. Why is a mature mammalian red blood cell described as deficient but more efficient? Give two reasons. (3 marks)

  10. Describe how lymph is kept moving in the lymph vessels, and state any two functions of lymph. (3 marks)

  11. Differentiate between an artery and a vein on the basis of (i) wall (ii) lumen (iii) valves (iv) pressure of the blood flowing through them. (4 marks)

  12. Write the main steps in the coagulation of blood in their correct sequence. (4 marks)

  13. What would happen in each of the following cases, and why? (i) The chordae tendineae of the tricuspid valve snap. (ii) The aortic semilunar valves fail to close during ventricular diastole. (4 marks)

  14. Trace the path of a drop of blood from the posterior vena cava until it enters the aorta, naming in order every chamber and every valve it passes through. (5 marks)

  15. Describe the hepatic portal system and explain why it is significant for the body. (5 marks)

8.16 Figure-Based Questions

  1. Study Fig. 8.5-3, which shows a mature human red blood cell in surface view and in section, and answer the following. (4 marks)
    1. Name the shape of the cell as seen in the sectional view.
    2. State one advantage of this shape to the cell in carrying out its function.
    3. Give the approximate diameter of the cell.
    4. Name any two organelles absent from this cell.
  2. Study Fig. 8.8-2, the internal structure of the human heart, and answer the following. (5 marks)
    1. Name the valve lying between the right atrium and the right ventricle, and state the number of cusps it has.
    2. Name the structures which hold the cusps of this valve in position, and state the muscles from which they arise.
    3. Name the blood vessel which leaves the left ventricle.
    4. Which chamber of the heart has the thickest wall? Give a reason.
  3. Fig. 8.8-5 shows transverse sections of an artery and a vein drawn side by side. (5 marks)
    1. State two features by which you would identify which section is the artery.
    2. Name the three layers of the wall, from the outside inwards.
    3. Redraw the section of the artery, neatly and to a good size, and label its three layers and its lumen. Use straight lead lines that do not cross.
  4. Fig. 8.9-2 shows double circulation together with the hepatic portal system. (4 marks)
    1. Name the vessel which carries the absorbed food from the intestine to the liver.
    2. Why is this vessel described as a portal vein?
    3. Name the vessel which carries blood from the liver to the posterior vena cava.
    4. Name any two processes carried out in the liver on the food brought to it.
  5. Fig. 8.10-1 shows the formation and drainage of tissue fluid around a capillary. (4 marks)
    1. Name the fluid occupying the spaces between the body cells.
    2. State where most of this fluid goes at the venous end of the capillary.
    3. What is the fluid called once it has entered the blind-ending vessel shown alongside?
    4. Name any two substances that pass from this fluid into the body cells.