Chapter 15: Excretion and Osmoregulation
In the Netherlands during the Second World War, a young physician named Willem Kolff watched patients die of kidney failure while he could do nothing. Their kidneys had stopped clearing waste from the blood, and the waste itself was killing them. Kolff reasoned that if the blood could simply be led out of the body, cleaned, and returned, the patient might survive until the kidneys recovered.
Working with whatever wartime shortages allowed, he wound long tubes of cellophane sausage casing around a rotating wooden drum and dipped the drum in a cleansing fluid. Blood flowed through the tubing; waste molecules seeped out through the thin cellophane wall into the fluid, while the blood cells and proteins, being too large, stayed behind. In 1945 a patient in kidney failure was treated with his machine and recovered — the first clear success of an artificial kidney.
Kolff had not invented a new principle. He had copied one. Every healthy kidney already does exactly this, roughly a hundred and eighty litres a day, using a filter so fine that it holds back protein while letting salts, sugar and urea pass. What Kolff’s clumsy drum showed the world was how remarkable the original organ is.
Diagram
Fig. 15.1 – A rotating-drum artificial kidney beside a human kidney, both shown filtering blood, with arrows for “blood in”, “waste out”, “cleaned blood returned”
This chapter will answer each of these questions as you work through it.
15.1 Excretion and Excretory Products
Every second, thousands of chemical reactions are running inside your body. Sugars are being broken down, proteins are being built, worn-out molecules are being dismantled. The sum total of all these chemical processes taking place in the body of an organism is called metabolism.
Metabolism has two halves that work together. Catabolic processes break large molecules down and release energy, while anabolic processes use that energy to build larger molecules from smaller ones. A cell is therefore always demolishing and constructing at the same time.
What Counts as a Waste Product
No chemical factory is perfectly efficient, and the body is no exception. Metabolism produces a variety of by-products, and some of these must be removed from the body. Such by-products are called metabolic waste products.
These waste products are not all alike. They may be fluid or gaseous, organic or inorganic. Because they differ so much in their nature, they cannot all leave by the same route — depending on the type, they are eliminated through various organs of the body.
Excretion Is Not the Same as Egestion
The removal of metabolic waste products from the body is called excretion. This is a precise term, and it is worth separating it carefully from something that looks similar.
Digestive wastes are made up mainly of unabsorbed or undigested substances. Such material passes through the gut but never actually enters the body’s cells. Metabolic wastes are different — they are produced inside body cells, and so they must first be carried out of the cell, then out of the body.
The Excretory Wastes of the Human Body
It helps to actually list what leaves the human body as waste. Fluids such as water are excreted; so are gaseous wastes like \(CO_2\); and so are the nitrogenous wastes — ammonia, urea and uric acid. Creatinine is another nitrogenous waste, and minerals and salts of sodium, potassium, calcium and so on are excreted whenever they are present in excess.
These substances leave by three main routes: urine, faeces and sweat. Which route a substance takes depends on its chemical nature and on which organ handles it.
Some wastes are coloured, because they come from the breakdown of coloured molecules. Pigments formed by the breakdown of haemoglobin include bilirubin, which is excreted through faeces, and urochrome, which is eliminated through urine and gives urine its familiar pale yellow tint.
The body also disposes of things it never made itself. Pigments present in foodstuff — the deep red of beetroot, for example — pass out of the body, as do excess vitamins, excess hormones, and drugs that have been taken. Spices we consume contain volatile substances, and because these evaporate easily they are excreted through the lungs.
Where Nitrogenous Waste Comes From
The body has a storage strategy for two of the three major food groups. Excess carbohydrates can be stored, and excess fats can be stored. Amino acids cannot — the body has no storage form for them.
So when amino acids are present in excess, they must be dealt with immediately. They are broken down by a process called deamination. In this process the amino group (\(-NH_2\)) is separated from the amino acid, and ammonia is formed.
This is where the problem begins. Ammonia is highly toxic. An organism therefore has only two options: excrete the ammonia as it is, or convert it into a less toxic form — urea or uric acid — before excretion.
Notice the logic here. The waste is nitrogenous because it comes from the amino group of amino acids, and the whole later story of excretion in animals is really the story of what each animal does with that ammonia.
Flowchart
FC 15.1 – Fate of excess amino acids: excess amino acid → deamination in liver → amino group removed → ammonia formed → branch: plenty of water available → excreted as ammonia; limited water → converted to urea in liver (ornithine/urea cycle) → excreted; very little water → converted to uric acid (inosinic acid pathway) → excreted
Habitat, Not Ancestry, Decides the Waste
You might expect closely related animals to excrete the same waste. Surprisingly, there is no clear correlation between the phylogenetic relationship of organisms and their major excretory products. What does correlate is habitat.
The clearest proof is an animal that changes habitat during its own lifetime. The tadpole of a frog lives in water and excretes ammonia, while adult frogs, which spend much of their time on land, are mostly ureotelic. The animal’s genes have not changed — its surroundings have.
The same pattern appears among reptiles. Some terrestrial turtles excrete uric acid, whereas others excrete urea or ammonia, depending on how dry their habitat is.
Animals can therefore be broadly classified into three types on the basis of the nitrogenous waste they produce. Ammonia is the basic product of deamination, but it is highly toxic and has to be diluted immediately. If there is no access, or only limited access, to water, then conversion of ammonia becomes necessary. Thus the availability of water plays the key role in deciding an organism’s mode of excretion. These are mainly of three types.
Ammonotelism
The elimination of nitrogenous wastes in the form of ammonia is called ammonotelism, and such animals are called ammonotelic.
Ammonia is basic in nature. If it were retained in the body, it would disturb the pH of the body — and even a slight increase in pH is dangerous. Enzymes work only within narrow pH ranges, so a rise in pH would disturb all enzyme-catalysed reactions in the body. It would also make the plasma membrane unstable, which threatens the very boundary of every cell.
Ammonia is readily soluble in water, and it needs a large quantity of water to dilute it and reduce its toxicity. Roughly 1 gm of ammonia needs about 300–500 ml of water for elimination — a huge volume for a small amount of waste.
Against this cost stands one clear advantage: ammonotelism is an energy-saving mechanism of excretion, because no chemical conversion is required. The ammonia is simply released. Hence all animals that have plenty of water available for dilution of ammonia excrete their nitrogenous wastes in the form of ammonia.
Ammonotelism is found in aquatic invertebrates, bony fishes, and aquatic or larval amphibians. Animals without a proper excretory system are also ammonotelic — for example, Protozoa, where waste simply diffuses out across the cell surface.
Because ammonia is so soluble, it does not need a specialised exit. Ammonotelic animals excrete ammonia through the general body surface (skin), through gills, and through kidneys.
Ureotelism
The elimination of nitrogenous wastes in the form of urea, \(H_2N-CO-NH_2\), is called ureotelism.
Urea is less toxic than ammonia and also less water-soluble. Both properties matter. Being less toxic, it can be concentrated to some extent in the body without harm; being less soluble, it does not drag as much water out with it. Because of this, urea requires much less water for elimination — compared with ammonia, about 100 times less water in humans, and several hundred times less in the camel, kangaroo rat and shark.
Since it is less toxic and less water-soluble, ureotelism suits animals that need to conserve water to some extent. It is therefore common in terrestrial animals, as they must conserve \(H_2O\). It takes about 50 ml of \(H_2O\) for the removal of 1 gm of \(NH_2\) in the form of urea.
Mammals, cartilaginous fishes (sharks and rays), many aquatic reptiles and most of the adult amphibians are ureotelic. In all of these, the conversion happens in the liver: ammonia is converted to urea by operating the ornithine cycle, also called the urea cycle. This conversion is not free — 3 ATP molecules are used to produce one molecule of urea.
Uricotelism
The elimination of nitrogenous wastes in the form of uric acid, \(C_5H_4O_3N_4\), is called uricotelism.
Uric acid is the least toxic of the three nitrogenous wastes. Because it does so little damage, it can be retained in the body for some time in a concentrated form. It is also the least soluble in water, which means it does not need much water to carry it out — a minimum of about 5–10 ml for 1 gm, and in some cases practically no water at all, since it can be voided as a semi-solid paste.
For these two reasons, animals that need to conserve more water follow uricotelism. In the liver of birds, ammonia is converted into uric acid by the ‘inosinic acid pathway’.
Birds, some insects, many reptiles and land snails are uricotelic, because they need to conserve water. However, this water saving comes at a price — they have to spend more energy than ureotelic animals do.
Creatinine — A Waste That Reports on the Kidney
One nitrogenous waste in the list deserves separate attention, because doctors use it as a measuring instrument.
Plasma creatinine is produced from the catabolism of creatinine phosphate during skeletal muscle contraction. Creatinine phosphate itself is useful — it provides a ready source of high energy phosphate for a contracting muscle — and creatinine is what remains after it is broken down.
Normally, blood creatinine levels remain steady, because the rate at which muscles produce it matches the rate at which the kidneys excrete it in urine. This steadiness is the key. If the kidneys begin to fail, production continues unchanged while removal slows down, so creatinine piles up in the blood. Hence a level above normal is an indication of poor renal function.
Homeostasis and Osmoregulation
So far we have treated excretory organs as disposal units. They are much more than that.
Excretory organs play an important role in the maintenance of a constant internal environment of the body, called homeostasis. Maintaining that constancy requires osmoregulation — the process of controlling solute concentrations and water balance in the body.
There is a striking way of stating what this means. It can be rightly said that the composition of blood — and therefore of the whole internal environment — is determined not by what the mouth ingests, but by what the excretory organs retain. You may drink a litre of salty water, but your blood composition is decided by how much of that salt and water your kidneys choose to keep.
Osmoconformers and Osmoregulators
Animals deal with the osmotic problem in two broad ways. Some are isoosmotic to their surroundings — their body fluid concentration simply matches the outside — and these are called osmoconformers. Others control their internal environment independently of the external environment, and these are called osmoregulators.
Marine organisms are mostly osmoconformers, because their body fluids and the external environment are isoosmotic in nature. When the outside and the inside already match, there is no gradient to fight, and no energy needs to be spent maintaining a difference.
Freshwater forms and terrestrial organisms are osmoregulators. A freshwater animal is surrounded by water far more dilute than its body fluids, and a land animal is surrounded by no water at all, so neither can afford to let the internal environment follow the external one.
Tolerance to Changing Salinity
Whether conformers or regulators, most organisms can tolerate only a narrow range of salt concentrations. Such organisms are called stenohaline organisms — the prefix steno means narrow.
Those that are capable of handling wide changes in salinity are called euryhaline organisms, for example barnacles and clams. These animals live where a river meets the sea, or in the intertidal zone, where salinity may change dramatically twice a day. Their tolerance is a direct adaptation to that instability.
Excretory Structures Across the Animal Kingdom
Not every animal needs an organ to excrete. In the simplest bodies, the waste has such a short distance to travel that diffusion alone is enough.
Unicellular forms have contractile vacuoles, which collect waste products and discharge them outside the cell. Excretion in sponges takes place by diffusion of waste material into the water, which is then discharged through the osculum.
True organs of excretion are found in those animals that show bilateral symmetry. The most common type is a simple or branching tube that opens to the exterior through pores called nephridiopores. Once a body becomes large and three-dimensional, its inner cells are no longer in contact with the outside, and a tube is the simplest way to bring the outside in.
Two Major Types of Nephridia
Protonephridia are a network of dead-end tubes called flame cells. Because the tubes are closed at one end, fluid must be drawn into them by the beating of cilia rather than flowing in from a body cavity. These are found mostly in animals that lack a true body cavity, for example Platyhelminthes. Protonephridia are also found in rotifers, some annelids and Amphioxus. The arrangement of flame cells and excretory canals is shown in Fig. 15.2.
Diagram
Fig. 15.2 – Excretory system in platyhelminthes, with the whole-body view labelled (excretory canal, excretory pore) and an enlarged flame cell labelled (nucleus, cilia, flame cell, waste fluid, tubule, tube cell)
Metanephridia are unbranched coiled tubes that connect to the body cavity through funnel-like structures called nephrostomes. Here the tube is open at both ends: body fluid enters the nephridium through the nephrostome and gets discharged through the nephridiopore. Earthworms are the standard example.
Excretory Organs in Other Groups
In most of the insects, excretion takes place by a set of blind-ended tubules called malpighian tubules. These lie in the body cavity and empty into the gut, which is why an insect’s nitrogenous waste leaves along with its faeces as a dry, uric-acid-rich material — a striking water-saving design, as seen in Fig. 15.3.
Diagram
Fig. 15.3 – Insect excretion, showing the abdomen of the insect with an enlarged view labelled malpighian tubules, midgut, hindgut, rectum, and dry waste with uric acid
Crustaceans have green glands as excretory organs. Members of the phylum Echinodermata do not have any specialised excretory organs at all — waste materials directly diffuse into the water, or are excreted through the tube feet.
Mammalian kidneys are a collection of functional units called nephrons, which are well designed to extract metabolic waste. The rest of this chapter is largely the story of how a nephron does that job.
Flowchart
FC 15.2 – Excretory structures across animal groups: Protozoa → contractile vacuole; Porifera → diffusion, discharged through osculum; Platyhelminthes → protonephridia with flame cells; Annelida (earthworm) → metanephridia with nephrostome and nephridiopore; Insecta → malpighian tubules; Crustacea → green glands; Echinodermata → no specialised organ, diffusion or tube feet; Mammalia → kidney with nephrons
15.2 Excretory System in Human Being
The human excretory system is built around four organs: a pair of kidneys, a pair of ureters, a single urinary bladder and a single urethra. The kidneys make the urine, the ureters carry it away, the bladder stores it, and the urethra releases it. Study Fig. 15.4 as you read, and locate each part in turn.
Diagram
Fig. 15.4 – The human excretory system, labelled: right and left kidney, hilus, renal artery, renal vein, abdominal aorta, inferior vena cava, ureters, urinary bladder, urethra; one kidney shown in section
The Kidneys — Position and Shape
A pair of bean-shaped kidneys are present on either side of the vertebral column, extending from the level of the 12th thoracic vertebra to the 3rd lumbar vertebra. They are not floating loosely — they sit high in the abdomen, tucked against the back wall.
The kidneys are present behind the peritoneum, the membrane that lines the abdominal cavity. Because of this position they are called retroperitoneal. This is worth pausing on: most abdominal organs hang within the peritoneal cavity, but the kidneys lie behind it, pressed against the back muscles, which is why a surgeon can reach a kidney from the back without opening the abdominal cavity.
The dimensions of each kidney are about \(10 \times 5.5 \times 3\) cm. The average weight is about 150 g in males and 135 g in females.
The outer surface of the kidney is convex and the inner surface is concave, giving it the familiar bean shape. The notch on the inner concave surface is called the hilus. The renal artery enters, and the renal vein as well as the ureter leave the kidney, through the hilus — so this single notch is the gateway for everything entering and leaving.
Each kidney has almost 1 million functional units called nephrons. The whole of the rest of this section is essentially an examination of one such unit.
Ureters
A pair of ureters arise from the renal pelvis of each kidney. Each ureter is a long muscular tube, 25–30 cm in length, and its muscular wall matters — urine is actively pushed along by waves of contraction, not simply allowed to trickle down.
The ureters open into the urinary bladder by separate openings, which are not guarded by valves. This sounds like a design flaw, since urine could flow backwards towards the kidney when the bladder fills. The body solves the problem mechanically instead.
The ureters pass obliquely through the wall of the urinary bladder. As the bladder fills, its stretching wall compresses these slanting sections and squeezes them shut. This helps in prevention of backflow of urine due to compression of the ureters while the bladder is filled.
Urinary Bladder
The urinary bladder is a median pear-shaped sac. It is a hollow muscular organ, situated in the pelvic cavity posterior to the pubic symphysis.
At the base of the bladder there is a small inverted triangular area called the Trigone. At the apex of this triangle is the opening of the urethra, and at the two points of the base of the triangle are the openings of the ureters. If you can picture that triangle, you can picture the entire plumbing of the bladder — two pipes in at the top corners of the base, one pipe out at the apex.
The wall of the bladder is built in layers. The urinary bladder is covered externally by peritoneum. Inner to the peritoneum is a muscular layer, formed by the detrusor muscles, which consist of three layers of smooth muscle — longitudinal, circular and longitudinal respectively. The innermost layer is made up of transitional epithelium, and it helps the bladder to stretch.
Transitional epithelium is the key to the bladder’s behaviour. Its cells can slide over one another and flatten as the organ fills, so the lining expands without tearing and springs back when the bladder empties.
Urethra and Its Sphincters
The urethra is a tubular structure arising from the urinary bladder and opening to the exterior of the body. Between the urinary bladder and the urethra there are two urethral sphincters, and they are deliberately of two different kinds.
The internal sphincter is made up of smooth muscles and is involuntary in nature. The external sphincter is made up of skeletal muscles and is voluntary in nature. The involuntary one responds automatically to the filling of the bladder; the voluntary one is under the control of the conscious brain, which is what allows a person to decide when to urinate.
If this valve is not functioning properly during inflammation of the bladder, it can lead to kidney infection, because organisms are then able to travel upwards from the bladder.
What Each Organ Does
Now that the parts are in place, their roles can be stated together.
Kidneys bring about the separation and elimination of nitrogenous waste, excess water and toxic substances from the body. They maintain homeostasis by way of their role in osmoregulation and in the regulation of pH of body fluids.
The kidneys are not only excretory organs, however — they also work as endocrine and metabolic organs. They produce calcitriol and renin. Erythropoietin secreted by the kidneys is essential for the production of RBCs.
Ureters transport urine from the renal pelvis to the urinary bladder. The urinary bladder is a temporary storage organ for urine, and it helps to expel urine, a process called micturition. The urethra is a passage way for discharging urine from the body; in males, it acts as a urinogenital organ, since it carries both urine and semen, though never at the same time.
Micturition
The bladder cannot simply be emptied whenever it happens to be full; the timing has to be controlled. This is achieved by a reflex working together with the conscious brain.
The average capacity of the urinary bladder is 700 ml. When the urinary bladder is almost half filled, stretch receptors in its wall transmit impulses to the spinal cord. This initiates the conscious desire to expel urine.
The micturition reflex centre of the spinal cord then transmits impulses to the wall of the urinary bladder and to the internal urethral sphincter. The bladder muscles contract and the muscles of the internal urethral sphincter relax — the two events must happen together, since squeezing a closed bag would achieve nothing.
Finally, the external sphincter receives impulses from the conscious centre of the brain and relaxes. This leads to the expulsion or elimination of urine from the bladder. The reflex prepares everything, but the last gate is opened by a conscious decision, and the whole sequence is summarised in FC 15.3.
Flowchart
FC 15.3 – Micturition reflex: bladder fills to about half of 700 ml capacity → stretch receptors in bladder wall stimulated → impulses to spinal cord → conscious desire to urinate → micturition reflex centre sends impulses → bladder (detrusor) muscles contract and internal urethral sphincter relaxes → impulses from conscious centre of brain → external urethral sphincter relaxes → urine expelled
The Three Protective Coverings
Each kidney is covered by 3 layers of tissue, arranged from outside inwards.
The outermost, the renal fascia, is made up of a thin layer of fibrous connective tissue. It anchors the kidney to the abdominal wall as well as to the surrounding tissue, holding the organ in place.
The middle layer is a mass of fatty tissue called the adipose capsule. It protects the kidneys by shock absorption — a soft cushion between a delicate organ and the hard structures of the back.
The innermost layer, the renal capsule, is a smooth transparent fibrous membrane that is continuous with the outer layer of the ureters. It acts as a barrier against the spread of infections into the kidney.
The longitudinal section (L.S.) of the kidney shows two distinct regions within this capsule, as seen in Fig. 15.5.
Diagram
Fig. 15.5 – L.S. of kidney, labelled: renal cortex, renal medulla, renal pyramid, renal papilla of pyramid, renal column, minor calyx, major calyx, renal pelvis, ureter
Inside the Kidney — Cortex and Medulla
Histologically, the kidney is divisible into two regions, the renal cortex and the renal medulla.
The renal cortex is the outer or peripheral region. It is red coloured and granular in appearance, and that granular look has a cause — the cortex contains Malpighian bodies, convoluted tubules and blood vessels, all packed closely together.
The renal medulla is the inner region of the kidney, pale red in colour and striated in appearance. Here too the appearance follows the contents: the medulla mainly consists of Loops of Henle and collecting ducts, which run in parallel straight lines and so give a striped look.
All these are arranged in a conical manner to form renal pyramids. The cortex extends into the medulla as columns of Bertini, also called renal columns, which lie between the pyramids. The narrow tip of a pyramid is called the renal papilla.
From Papilla to Ureter
There are several pyramids in each kidney, and their tips act as taps that empty into a collecting system.
The renal papillae open into a minor calyx. Minor calyces merge together to form major calyces, and major calyces unite together to form the renal pelvis. The renal pelvis, also called the renal sinus, is a funnel-shaped area in the region of the medulla of the kidney. The renal pelvis continues as the ureter, which leaves the kidney through the hilus.
Read that sequence as a funnel of funnels: many small openings gather into a few larger ones, which gather into a single wide space, which narrows into one tube. It is the same drainage logic that streams and rivers follow.
Flowchart
FC 15.4 – Path of urine after it is formed: collecting duct → renal papilla of pyramid → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra → outside the body
The Nephron — Structural and Functional Unit
Nephrons are the structural and functional units of the kidney. The phrase means that a nephron is both the smallest building block of the organ and the smallest part that can perform the organ’s complete job — a single nephron, on its own, can filter blood and make a drop of urine.
Each nephron consists of a thin-walled tube about 4–6 cm long, the renal tubule, and a bunch of capillaries, the glomerulus. The wall of the renal tubule is made up of a single layer of epithelial cells, which is what makes exchange across it possible at all.
The proximal end of the tubule is wide, blind and cup-like, and is called Bowman’s capsule. The distal end is open. The tubule is divisible into Bowman’s capsule, neck, proximal convoluted tubule (PCT), Loop of Henle (LoH), distal convoluted tubule (DCT) and collecting tubule (CT).
Flowchart
FC 15.5 – Parts of the nephron in order: Bowman’s capsule → neck → proximal convoluted tubule (PCT) → descending limb of Loop of Henle → ascending limb of Loop of Henle → distal convoluted tubule (DCT) → collecting tubule (CT) → collecting duct
The glomerulus is present in the cup-like cavity of Bowman’s capsule, and both together are known as the renal corpuscle or Malpighian body. Each Malpighian body is about 200 µm in diameter and consists of a Bowman’s capsule and a glomerulus. Study the arrangement carefully in Fig. 15.6.
Diagram
Fig. 15.6 – Bowman’s capsule and glomerulus, labelled: afferent arteriole, efferent arteriole, glomerulus, podocyte showing feet and slit pores in between, capillary showing pores, basement membrane, visceral wall, parietal wall, capsular space, PCT cell with microvillus, basal channel, intercellular space; with blood pressure 75 mm Hg, colloidal osmotic pressure 30 mm Hg and capsular hydrostatic pressure 15 mm Hg marked
Glomerulus
The glomerulus is a bunch of fine blood capillaries lying in the cup of Bowman’s capsule.
A small terminal branch of the renal artery, called the afferent arteriole, enters the cup cavity and undergoes extensive fine branching to form a network of several capillaries. This bunch is called the glomerulus. The capillary wall here is fenestrated, that is, it carries pores — an ordinary capillary would not let plasma escape fast enough.
All the capillaries then reunite and form an efferent arteriole that leaves the cup cavity. This is unusual: in most of the body a capillary bed drains into a venule, but here it drains into another arteriole, and that difference is deliberate.
The diameter of the afferent arteriole is greater than that of the efferent arteriole. Fluid entering through a wide pipe and leaving through a narrow one is forced to build up pressure inside, so this arrangement creates a high hydrostatic pressure in the glomerulus. That pressure is important for ultrafiltration, the first step of urine formation.
Bowman’s Capsule
Bowman’s capsule is a cup-like structure having a double wall. Both walls are composed of squamous epithelium. The outer wall is called the parietal wall and the inner wall is called the visceral wall. In between the two walls there is a space called the capsular space or urinary space — this is where the filtered fluid first collects.
The visceral wall is not made of ordinary flat cells. It consists of a special type of squamous cell called podocytes, having foot-like pedicels. These podocytes are in close contact with the walls of the capillaries of the glomerulus, wrapping around them like fingers gripping a rope.
There are small slits called filtration slits in between adjacent podocytes. Fluid leaving the capillary must pass through these slits to reach the capsular space, and their narrowness is what makes the filtration selective rather than a simple leak.
The parietal wall is thin, consisting of simple squamous epithelium, and it continues into the neck.
The Neck
The wall of the neck is made up of ciliated epithelium, and the beating cilia help move the filtrate forward. The lumen of the neck is called the urinary pole. The neck leads to the proximal convoluted tubule.
Proximal Convoluted Tubule
The PCT is the highly coiled part of the nephron. It is lined by cuboidal cells with a brush border (microvilli) and is surrounded by peritubular capillaries. It is the place of selective reabsorption.
Two structural features work together here. The microvilli of the brush border multiply the surface available for absorbing substances back. And due to the convolutions, that is the coiling, the filtrate flows slowly and remains in the PCT for a longer duration. More surface and more time together ensure the reabsorption of the maximum amount of useful molecules.
Loop of Henle
The Loop of Henle is a ‘U’ shaped tube consisting of a descending limb and an ascending limb, and the two limbs are deliberately unlike each other.
The descending limb is thin walled and permeable to water. It is lined with simple squamous epithelium. The ascending limb is thick walled and impermeable to water. It is lined with simple cuboidal epithelium.
The LoH is surrounded by capillaries called vasa recta. Its function is to operate the counter current system, a mechanism for osmoregulation. The regulation of salt and water balance in the body is called osmoregulation. The ascending limb of Henle’s loop leads to the DCT.
The difference in permeability between the two limbs is not an accident of structure; it is the whole basis of the kidney’s ability to make concentrated urine, and Section 15.4 will follow that mechanism step by step.
Distal Convoluted Tubule
The DCT is another coiled part of the nephron. Its wall consists of simple cuboidal epithelium.
The DCT performs tubular secretion, also called augmentation or active secretion, in which wastes are taken up from the surrounding capillaries and secreted into the passing urine. Notice that the direction of movement here is opposite to that in the PCT — material is being added to the filtrate rather than taken from it.
The DCT also helps in water reabsorption and in the regulation of pH of body fluids.
Collecting Tubule
The collecting tubule is a short, straight part of the DCT. It reabsorbs water and secretes protons, so it contributes both to water balance and to the acid–base balance of the blood. The collecting tubule opens into the collecting duct.
Two Types of Nephrons
There are two types of nephrons in the human kidney, and they are distinguished by how far the Loop of Henle reaches.
Cortical nephrons have a shorter loop of Henle, which extends very little into the medulla. Most of the nephrons are cortical nephrons.
A few nephrons have a longer loop of Henle that runs deep into the medulla. These are called juxtamedullary nephrons. Their long loops are the ones that build the concentration gradient in the medulla, so even though they are the minority, they carry the responsibility for producing concentrated urine.
Blood Supply to the Kidney
A small branch of the efferent arteriole forms the peritubular capillary network around the DCT, PCT and Henle’s loop of cortical nephrons. The same efferent arteriole also forms the loop-shaped vasa recta around Henle’s loop of juxtamedullary nephrons.
Nephrons are responsible for the elimination of waste and for osmoregulation, and hence they are richly supplied with blood. About one fourth of the cardiac output is supplied to the kidneys — a remarkable share for organs that together weigh less than 300 g. The full route the blood follows is set out in FC 15.6.
Flowchart
FC 15.6 – Blood supply to the kidney: renal artery → renal arteriole → afferent arteriole → glomerular capillary → efferent arteriole → peritubular capillaries network → renal veinule → renal vein
Juxta Glomerular Apparatus
At one small spot in every nephron, the tubule comes back and touches its own arteriole, and that meeting point becomes a control centre.
Some smooth muscle cells of the wall of the afferent arteriole are modified in such a way that their sarcoplasm is granular. These cells are called juxtaglomerular (JG) cells.
In each nephron, the beginning part of the DCT makes contact with the afferent arteriole of the same nephron. The cells in the wall of the DCT in this region are packed more densely than those in other regions of the DCT. This is called the macula densa.
The macula densa and the JG cells together form the Juxta Glomerular Apparatus (JGA), which plays an important role in blood pressure regulation within the kidney.
The arrangement is elegant when you see the purpose behind it. The macula densa samples the fluid that this nephron has just finished processing, while the JG cells sit on the vessel that supplies the same nephron. The nephron can therefore adjust its own blood supply according to its own output — a self-correcting loop. Section 15.5 will show how the JGA uses this information to trigger the renin–angiotensin–aldosterone system.
Diagram
Fig. 15.7 – Nephron and peritubular capillaries network, labelled: afferent arteriole, efferent arteriole, glomerulus, glomerular (Bowman) capsule, proximal convoluted tubule, distal convoluted tubule, ascending and descending limbs of Loop of Henle, peritubular capillaries, collecting duct, calyx, from renal artery, to renal vein
15.3 Urine Formation
Urine is not simply drained out of the blood in one step. The process of urine formation is completed in three successive steps — ultrafiltration or glomerular filtration, selective reabsorption, and tubular secretion or augmentation.
The order matters, and so does the logic behind it. The kidney first throws out almost everything small enough to leave the blood, then carefully takes back whatever the body still needs, and finally adds a few last wastes that filtration missed. It is a strategy of “remove everything, then rescue the useful”, which sounds wasteful but is far more reliable than trying to pick out each waste molecule individually. Fig. 15.8 shows all three steps operating along a single nephron.
Diagram
Fig. 15.8 – Process of urine formation, showing a nephron with afferent and efferent arterioles, glomerulus, glomerular capsule, capsular space, PCT, descending and ascending limbs of the nephron loop, DCT, collecting tubule, collecting duct, peritubular capillaries and vasa recta; arrows marked for glomerular filtration, tubular reabsorption and tubular secretion
Flowchart
FC 15.7 – Three steps of urine formation: blood in glomerulus → (1) ultrafiltration: plasma minus proteins moves from glomerulus into capsular space → glomerular filtrate → (2) selective reabsorption in PCT: useful substances move from tubular fluid back into blood → (3) tubular secretion in DCT and collecting tubule: wastes and \(H^+\) move from blood into tubular fluid → urine
a. Ultrafiltration / Glomerular Filtration
Filtration needs pressure, and the kidney generates that pressure by controlling the width of its vessels.
The diameter of the afferent arteriole is greater than that of the efferent arteriole, and the diameter of the capillaries is still smaller than both arterioles. Due to such difference in diameter, blood flows with greater pressure through the glomerulus. This is called the glomerular hydrostatic pressure (GHP), and normally it is about 55 mm Hg.
This pressure, however, does not act unopposed. Two forces push back against it.
The first is the osmotic pressure of blood, normally about 30 mm Hg. Plasma proteins stay behind in the capillary, and because they cannot leave, they hold water back osmotically. The second is the capsular pressure, normally about 15 mm Hg — the pressure of the fluid already sitting in the capsular space, which resists the entry of more fluid.
Whatever pressure survives after subtracting these two is what actually drives filtration. Hence the net or effective filtration pressure (EFP) is 10 mm Hg.
\[EFP = \text{Hydrostatic pressure in glomerulus} - (\text{Osmotic pressure of blood} + \text{Filtrate hydrostatic pressure})\]
\[10 \text{ mm Hg} = 55 - (30 + 15)\]
Here every term is a pressure measured in millimetres of mercury (mm Hg). GHP is the outward push of blood inside the glomerular capillaries, the osmotic pressure of blood is the inward pull exerted by plasma proteins, and the filtrate hydrostatic pressure — also called capsular pressure — is the outward-to-inward resistance offered by fluid already present in Bowman’s capsule.
What Passes and What Stays
The walls of the glomerular capillaries are extremely thin. Under the effect of high pressure, these walls become permeable to the major components of blood, except blood cells and macromolecules like protein. Thus plasma except proteins oozes out through the wall of the capillaries.
The size cut-off is the whole point. Blood cells and plasma proteins are too large to cross the fenestrated capillary wall, the basement membrane and the filtration slits between podocytes. Everything smaller passes through, and there is no attempt at chemical selection at this stage — hence the name ultrafiltration, filtration on the basis of molecular size alone.
About 600 ml of blood passes through each kidney per minute. The blood, or more precisely the plasma, flowing through the kidney is filtered as glomerular filtrate at a rate of 125 ml/min, which works out to 180 L/d.
That daily figure is worth holding on to. A person’s total blood volume is only about 5 litres, so 180 litres of filtrate a day means the entire plasma volume is filtered many times over each day — and almost all of it must be taken back, or a person would be fatally dehydrated within minutes.
The glomerular filtrate is also called deproteinized plasma or primary urine. It is alkaline in reaction, and it contains urea, amino acids, glucose, pigments and inorganic ions. Notice that useful substances such as glucose and amino acids are present in it — filtration has not distinguished them from wastes, and that job is left to the next step.
The glomerular filtrate passes through the filtration slits into the capsular space, and then reaches the proximal convoluted tubule.
b. Selective Reabsorption
If 180 litres are filtered in a day and only about 1.5 litres leave as urine, then reabsorption is by far the largest job the kidney performs.
The PCT is the place of reabsorption. It is highly coiled, so that the glomerular filtrate passes through it very slowly, giving the cells time to work. The columnar cells of the PCT are provided with microvilli, due to which the absorptive area increases enormously. Together, extra time and extra surface make the process of reabsorption very effective.
These cells perform both active reabsorption, which is ATP mediated, and passive reabsorption, which occurs by simple diffusion. The distinction between them decides which substances are recovered and how completely.
Substances of considerable importance to the body are called high threshold substances — glucose, amino acids, Vitamin C, \(Ca^{++}\), \(K^{+}\), \(Na^{+}\) and \(Cl^{-}\). These are absorbed actively, against the concentration gradient, because the body cannot afford to lose them merely because their concentration in the tubule has fallen low.
Low threshold substances like water, sulphates and nitrates are absorbed passively, simply following the gradients created by the active transport of other solutes.
In this way, about 99% of the glomerular filtrate is reabsorbed in the PCT and DCT. The pattern of reabsorption along the nephron is shown in Fig. 15.9.
Diagram
Fig. 15.9 – Reabsorption along the nephron, showing renal corpuscle (glomerular capsule, capsular space, glomerulus, afferent and efferent arterioles), PCT, thin and thick limbs of the nephron loop, ascending and descending limbs, DCT, collecting system, collecting duct and papillary duct, with arrows marking sites of reabsorption and cross-sections of each region
c. Tubular Secretion / Augmentation
Finally the filtrate reaches the distal convoluted tubule via the loop of Henle. Peritubular capillaries surround the DCT, keeping blood in close contact with the tubule at this stage.
The cells of the distal convoluted tubule and the collecting tubule actively absorb wastes like creatinine, and ions like \(K^{+}\) and \(H^{+}\), from the peritubular capillaries and secrete them into the lumen of the DCT and CT. This adds to what the filtrate already contains, thereby augmenting the concentration of urine and changing its pH from alkaline to acidic.
Recall that the glomerular filtrate was alkaline when it left Bowman’s capsule. The shift to acidic urine is entirely the work of this step.
The secretion of \(H^{+}\) ions in the DCT and CT is an important homeostatic mechanism for the pH regulation of blood. Every hydrogen ion pushed into the urine is a hydrogen ion removed from the blood, so the kidney is quietly defending blood pH with every drop of urine it makes. This process is called tubular secretion or augmentation.
Tubular secretion is the only mode of excretion in marine bony fishes and desert amphibians. In both these animals water is either unavailable or cannot be spared, so filtration is minimal and wastes are added directly to the tubular fluid instead.
15.4 Concentration of Urine
Under conditions like low water intake, or high water loss due to sweating, a human being can produce concentrated urine. The urine can be almost four times as concentrated as the blood — about 1200 mOsm/L against the blood’s 300 mOsm/L.
This is a remarkable ability, and it is not obvious how it is achieved. Water always moves down its own osmotic gradient, from a dilute solution to a concentrated one. To pull water out of the tubular fluid, the tissue outside the tubule must be more concentrated than the fluid inside it — and that concentrated tissue has to be built and maintained by the kidney itself. For this purpose, a mechanism called the countercurrent mechanism is operated in human kidneys.
Where the Countercurrent Mechanism Operates
The limbs of Henle’s loop of juxtamedullary nephrons, together with the vasa recta, operate the countercurrent mechanism. Recall that juxtamedullary nephrons are the few nephrons whose loops run deep into the medulla — it is precisely this depth that allows a gradient to be built from the cortex down to the inner medulla.
The mechanism involves the passage of fluid from the descending to the ascending limb of Henle’s loop. The flow of tubular fluid is in opposite directions through the two limbs, and this is exactly what gives the mechanism its name — counter meaning opposite, and current meaning flow. In the case of the vasa recta, blood flows from the ascending to the descending parts of itself.
The Two Limbs Do Different Jobs
The whole mechanism rests on one structural fact met earlier: the two limbs of the loop are not alike.
The wall of the descending limb is thin and permeable to water, whereas that of the ascending limb is thick and impermeable to water.
In the region of the descending limb, water diffuses from the tubular fluid into the tissue fluid, due to which the tubular fluid becomes concentrated. No salts are being added here — the fluid becomes concentrated purely because water is leaving it.
The ascending limb of Henle’s loop is thick walled, and its cells can reabsorb \(Na^{+}\) and \(Cl^{-}\) from the tubular fluid and release them into the tissue fluid. Here the reverse happens — salt leaves but water cannot follow, because the wall does not permit it.
Due to this, the tissue fluid around the descending limb becomes concentrated. That concentrated tissue fluid then makes more water move out from the descending limb into the tissue fluid by osmosis.
Follow the loop as a whole and the pattern becomes clear. As tubular fluid passes down through the descending limb, its osmolarity increases gradually due to water loss. On the other hand, its osmolarity progressively decreases as it flows up through the ascending limb, due to \(Na^{+}\) and \(Cl^{-}\) leaving the fluid.
Flowchart
FC 15.8 – Countercurrent multiplier in the loop of Henle: filtrate enters descending limb → water diffuses out into tissue fluid → tubular fluid becomes concentrated at the hairpin bend → fluid turns into ascending limb → \(Na^{+}\) and \(Cl^{-}\) pumped out, water cannot follow → tissue fluid of medulla becomes concentrated → this concentrated tissue fluid draws still more water out of the descending limb → gradient is multiplied along the length of the loop → dilute fluid leaves the ascending limb towards the DCT
Diagram
Fig. 15.10 – Concentration of urine, showing a juxtamedullary nephron and vasa recta running through cortex, outer medulla and inner medulla, with the osmolarity gradient marked as 300, 600, 900 and 1200 mOsmol L⁻¹, and arrows for movement of \(NaCl\), \(H_2O\) and urea
The Role of ADH
Besides the loop mechanism, whenever water retention is necessary, the pituitary secretes ADH (antidiuretic hormone).
ADH makes the cells in the wall of the collecting ducts permeable to water. Due to this, water moves from the tubular fluid into the tissue fluid, making the urine concentrated.
This is the point at which the two parts of the system come together. The loop of Henle builds the salty medulla; ADH decides whether the collecting duct is allowed to take advantage of it. Without ADH, the duct wall stays relatively impermeable and the fluid passes through unchanged, however salty the surrounding tissue may be.
Urea Recycling
Salt is not the only solute that makes the inner medulla concentrated. Urea contributes as well, through a neat circulation of its own.
The cells in the wall of the deep medullary part of the collecting ducts are permeable to urea. As concentrated urine flows through this region, urea diffuses from the urine into the tissue fluid, and from the tissue fluid into the tubular fluid flowing through the thin ascending limb of Henle’s loop.
This urea cannot pass out from the tubular fluid while flowing through the thick segment of the ascending limb, the DCT and the cortical portion of the collecting duct, because these regions are impermeable to it. So the urea is carried along, trapped inside the tubule, until it reaches the deep medulla once more.
Meanwhile, while the fluid is flowing through the collecting duct, water reabsorption is operating under the influence of ADH. Due to this, the urea concentration increases in the tubular fluid, and the same urea again diffuses into the tissue fluid in the deep medullary region.
Thus the same urea is transferred repeatedly between segments of the renal tubule and the tissue fluid of the inner medulla. This is called urea recycling. It is operated to achieve more and more water reabsorption from the tubular fluid, and thereby the excretion of small volumes of concentrated urine.
Flowchart
FC 15.9 – Urea recycling: urea in concentrated urine of deep medullary collecting duct → diffuses into tissue fluid of inner medulla → diffuses into tubular fluid of thin ascending limb → carried through thick ascending limb, DCT and cortical collecting duct, all impermeable to urea → reaches collecting duct → water reabsorbed under ADH, urea concentration rises → urea diffuses out again into inner medullary tissue fluid → cycle repeats
The Vasa Recta and Countercurrent Exchange
A problem now arises. Blood flows continuously through the medulla, and ordinary blood flow would wash the accumulated salt and urea away, destroying the very gradient the loop has built. The vasa recta prevent this.
An osmotic gradient is essential in the renal medulla for water reabsorption by the countercurrent multiplier system. This osmotic gradient is maintained by the vasa recta, which operate a countercurrent exchange system.
The vasa recta also have descending and ascending limbs. Blood that enters the descending limb of the vasa recta has a normal osmolarity of about 300 mOsm/L. As it flows down into the region of the renal medulla, where the tissue fluid becomes increasingly concentrated, \(Na^{+}\), \(Cl^{-}\) and urea molecules diffuse from the tissue fluid into the blood, and water diffuses from the blood into the tissue fluid.
Due to this, the blood becomes more concentrated, and it now flows through the ascending part of the vasa recta. This part runs through a region of the medulla where the tissue fluid is less concentrated. Due to this, \(Na^{+}\), \(Cl^{-}\) and urea molecules diffuse from the blood to the tissue fluid, and water from the tissue fluid to the blood.
The net result is that whatever the blood picked up on the way down, it gives back on the way up. This mechanism helps to maintain the osmotic gradient.
Longer Loops for Drier Habitats
The length of the loop decides how deep the gradient can extend, and therefore how concentrated the urine can become. So as to reabsorb water to maximum capacity, the loops of Henle are longer in desert mammals like the camel. Due to this, the camel excretes concentrated urine.
15.5 Composition of Urine
Normal urine is a pale yellow coloured, transparent liquid. This colour is due to the pigment urochrome, formed from the breakdown of haemoglobin. The composition of urine depends upon the food and fluid consumed by the individual.
That last sentence is more important than it looks. Urine has no fixed recipe. Its volume, its concentration and its salt content all change from hour to hour, because urine is the adjustable output through which the body corrects whatever the diet has upset.
Two Ways of Regulating Urine
How does the body adjust urine to suit its needs? There are two ways. One works by regulating water reabsorption through ADH, and the other by regulating electrolyte reabsorption through RAAS.
Keep the difference in mind as you read: one system moves water, the other moves salt. Since osmolarity depends on the ratio of the two, the body needs control over both.
Regulation by ADH
The hypothalamus of the brain has special receptors called osmoreceptors. These can detect a change in the osmolarity of blood — osmolarity being the measure of the total number of dissolved particles per litre of solution.
Suppose the osmolarity of blood increases for any reason — after eating namkeen, for instance, or because of sweating. In either case the effect is the same: water is lost from the body relative to the solutes, so the blood becomes more concentrated.
The osmoreceptors then trigger the release of Antidiuretic hormone (ADH) from the neurohypophysis, that is, the posterior pituitary. ADH stimulates the reabsorption of water from the last part of the DCT and the entire collecting duct, by increasing the permeability of their cells.
Two things follow. The volume of urine is reduced, and the osmolarity of the blood decreases as the recovered water dilutes it.
Once the osmolarity of the blood comes back to normal, the activity of the osmoreceptor cells decreases, which leads to a decrease in ADH secretion. This is called negative feedback — the response switches off the very stimulus that produced it.
Flowchart
FC 15.10 – ADH pathway: water loss (sweating, salty food) → blood osmolarity rises → osmoreceptors in hypothalamus stimulated → ADH released from neurohypophysis → last part of DCT and whole collecting duct become permeable to water → water reabsorbed → urine volume falls and blood osmolarity falls → osmoreceptor activity decreases → ADH secretion decreases (negative feedback)
In the case of haemorrhage or severe dehydration too, the osmoreceptors stimulate ADH secretion. ADH is therefore important in regulating water balance through the kidneys.
In the absence of ADH, diuresis — dilution of urine — takes place, and the person tends to excrete a large amount of dilute urine. This condition is called diabetes insipidus.
Regulation by RAAS
The second regulatory mechanism is RAAS, the Renin–Angiotensin–Aldosterone System, operated by the Juxta Glomerular Apparatus.
Whenever the blood supply to the afferent arteriole decreases — because of a change in blood pressure or blood volume, as in low BP or dehydration — the JGA cells release Renin.
Renin converts angiotensinogen, secreted by the hepatocytes in the liver, to Angiotensin I. Angiotensin converting enzyme further modifies Angiotensin I to Angiotensin II, which is the active form of the hormone.
Notice the design of this pathway. The kidney does not release the active hormone itself; it releases an enzyme that starts a cascade, and each step of that cascade offers a point at which the body — or a medicine — can intervene.
Angiotensin II Has Triple Function
- It constricts arterioles in the kidney, thereby reducing blood flow and increasing blood pressure.
- It stimulates PCT cells to enhance the reabsorption of \(Na^{+}\), \(Cl^{-}\) and water.
- It stimulates the adrenal cortex to release another hormone called aldosterone, which stimulates the DCT and collecting ducts to reabsorb more \(Na^{+}\) and water, thereby increasing blood volume and pressure.
All three actions point the same way — raising blood pressure and restoring blood volume — which is exactly what a kidney sensing poor blood supply needs.
Flowchart
FC 15.11 – RAAS pathway: blood pressure or blood volume falls → blood supply to afferent arteriole decreases → JG cells release renin → renin acts on angiotensinogen from liver hepatocytes → Angiotensin I → angiotensin converting enzyme → Angiotensin II → branch 1: constricts kidney arterioles, raising blood pressure; branch 2: stimulates PCT to reabsorb \(Na^{+}\), \(Cl^{-}\) and water; branch 3: stimulates adrenal cortex → aldosterone → DCT and collecting ducts reabsorb more \(Na^{+}\) and water → blood volume and pressure restored
Why Both ADH and RAAS Are Needed
Would only ADH, or only RAAS, be sufficient for homeostasis?
Both ADH and RAAS are essential, and the reason lies in what each one moves. Only ADH can lower the blood \(Na^{+}\) concentration, by way of water reabsorption in the DCT and collecting duct — bringing back water dilutes the sodium already present. RAAS, on the other hand, stimulates \(Na^{+}\) reabsorption, and thus maintains the osmolarity of body fluid.
If only water could be adjusted, the body could correct its volume but not its salt content; if only salt could be adjusted, the reverse would be true. Osmolarity is a ratio of solute to water, so control of both is needed to hold it steady.
Atrial Natriuretic Peptide — The Opposing Signal
The action of ADH and RAAS leads to an increase in blood volume and osmolarity. But no correcting mechanism should be allowed to overshoot, and so a third hormone exists to pull in the opposite direction.
A large increase in blood volume and pressure stimulates the atrial wall of the heart to produce atrial natriuretic peptide (ANP). The stretched atrium is, in effect, reporting that the blood volume has become too high.
ANP inhibits \(Na^{+}\) and \(Cl^{-}\) reabsorption from the collecting ducts, inhibits the release of renin, and reduces aldosterone and ADH release as well. Every one of these actions opposes what ADH and RAAS were doing.
This leads to a condition called natriuresis, meaning increased excretion of \(Na^{+}\) in the urine, along with diuresis.
Flowchart
FC 15.12 – ANP pathway: blood volume and pressure rise sharply → atrial wall stretched → ANP produced → inhibits \(Na^{+}\) and \(Cl^{-}\) reabsorption from collecting ducts, inhibits renin release, reduces aldosterone and ADH → natriuresis and diuresis → blood volume and pressure fall back to normal
Another Role of the Kidney
Kidneys also participate in the synthesis of calcitriol, the active form of Vitamin D, which is needed for the absorption of dietary calcium. A deficiency of calcitriol can lead to brittle bones.
This is a reminder that the kidney’s importance goes beyond waste removal. A person may eat plenty of calcium and still have weak bones if the kidney cannot activate Vitamin D for them.
During the day time it cannot go out to drink water. At the same time, its diet is very high in protein, so a large amount of nitrogenous waste is produced. Instead of diluting that waste, the kidneys now concentrate the urine in order to conserve water. This capacity to rapidly change the osmolarity of urine — dilute one hour, concentrated the next — is a classic example of adaptation.
15.6 Role of Other Organs in Excretion
The kidney is the main excretory organ, but it is not the only one. Two other organs — the skin and the lungs — remove wastes as a by-product of the work they were built to do.
Skin
The skin of many organisms is thin and permeable, and in such animals it helps in the diffusion of waste products like ammonia straight out of the body. Human skin is different: it is thick and impermeable, so nothing can simply diffuse across it. Whatever leaves through human skin must therefore be actively secreted by glands.
Human skin shows the presence of two types of glands, namely sweat glands and sebaceous glands, both of which can be seen in the longitudinal section of skin in Fig. 15.11.
Diagram
Fig. 15.11 – L.S. of skin, labelled: pores, epidermis, dermis, subcutaneous fatty tissue, sweat gland, oil (sebaceous) gland, hair, hair-erector muscle, blood vessels
Sweat Glands
Sweat glands are distributed all over the skin, and they are abundant in the palm and facial regions. These are simple, unbranched, coiled, tubular glands, and they open on the surface of the skin through an opening called the sweat pore.
Sweat is primarily produced for thermoregulation — as it evaporates, it carries heat away from the body surface. But the fluid it uses to do this is drawn from the tissue fluid, and so it also excretes substances such as water, \(NaCl\), lactic acid and urea.
Sebaceous Glands
Sebaceous glands are present at the neck of hair follicles. They secrete an oily substance called sebum.
Sebum forms a lubricating layer on the skin, making it softer, and it protects the skin from infection and injury. Its role is therefore protective rather than excretory, which makes it a useful contrast to the sweat gland beside it.
Lungs
Lungs are the respiratory organs, and their excretory role follows directly from that.
They help in the excretion of volatile substances like \(CO_2\) and water vapour produced during cellular respiration. Both of these are genuine metabolic wastes, and both happen to be gaseous, so the organ designed for gas exchange is the natural route for their removal.
The lungs also excrete volatile substances present in spices and other foodstuff, which is why some of what we eat can be detected on the breath rather than in the urine.
Flowchart
FC 15.13 – Routes by which wastes leave the human body: metabolic wastes in blood → branch: nitrogenous wastes, excess water, salts, creatinine → kidney → urine; volatile wastes, \(CO_2\), water vapour → lungs → exhaled air; water, \(NaCl\), lactic acid, urea → sweat glands of skin → sweat; bile pigments such as bilirubin → gut → faeces
15.7 Disorders and Diseases
A kidney that is not working properly announces the fact in the urine. Substances that should have been held back begin to appear, and substances that should have been removed begin to accumulate in the blood. Reading these signs is the beginning of diagnosis.
Warning Signs in the Urine
Excessive albumin in the urine, a condition called albuminuria, indicates injury to the endothelial-capsular membrane. This injury may be the result of increased blood pressure, or of injury or irritation of the kidney cells by substances such as toxins or heavy metals.
The reasoning behind this sign is worth spelling out. Albumin is a plasma protein, and the filtration barrier normally holds all proteins back. If albumin is escaping into the filtrate, the barrier itself has been damaged — the fault lies in the filter, not in the tubules.
The presence of excessive quantities of ketone bodies in urine may be caused by diabetes mellitus, starvation, or too little carbohydrate in the diet. All three have one thing in common: the body is being forced to burn fat instead of glucose for energy, and ketone bodies are the by-products of that fat breakdown.
The presence of leucocytes in urine indicates the possibility of infection of the kidney or of other urinary organs, since white blood cells gather wherever the body is fighting microbes.
1. Kidney Stones
Kidney stones are also called renal calculi. They may be formed in any portion of the urinary tract — from the kidney tubules right down to the external opening.
Stones form when substances dissolved in the urine become so concentrated that they begin to crystallise out. Depending on their composition, they are classified into the following types.
Calcium stones are usually calcium oxalate stones or calcium phosphate ones. These are the most familiar type.
Struvite stones are formed in response to bacterial infection caused by urea splitting bacteria. These grow quickly and become quite large, because the bacteria keep altering the chemistry of the urine in favour of crystal formation.
Uric acid stones occur in people who do not drink enough water, or who consume a high protein diet. Both conditions push uric acid concentration in the urine upward — one by reducing the solvent, the other by increasing the solute.
Cystine stones arise from a genetic disorder that causes the kidney to excrete too much of a certain amino acid. Here the fault is inherited and lies in the transport system of the tubule.
Symptoms: Intermittent pain below the rib cage, in the back and sideways. Hazy, brownish, reddish or pinkish urine. A frequent urge to pass urine. Pain during micturition.
Diagnosis: Uric acid content of the blood, colour of the urine, kidney X-ray and sonography of the kidney are the different diagnostic tests prescribed, depending on the symptoms.
Flowchart
FC 15.14 – Classification of kidney stones by composition: renal calculi → calcium stones (calcium oxalate or calcium phosphate) → struvite stones (from infection by urea splitting bacteria) → uric acid stones (low water intake or high protein diet) → cystine stones (genetic disorder of amino acid excretion)
2. Uremia
Blood normally contains 0.01 to 0.03% urea. If this rises above 0.05%, the condition is called uremia. It is harmful and may lead to kidney failure.
Note carefully what uremia measures. Urea is produced steadily by the liver, so a rising level in the blood means that removal has slowed, not that production has increased. Uremia is therefore a sign that filtration itself is failing.
3. Nephritis
Nephritis is inflammation of the kidneys, characterised by proteinuria. It is caused by increased permeability of the glomerular capsular membrane, which permits large amounts of protein to escape from the blood into the urine.
The consequences follow in a chain. Losing protein leads to a change in the colloidal osmotic pressure of blood — with fewer plasma proteins, the blood’s power to hold water osmotically falls. This leads to the movement of fluid from the blood into the interstitial spaces, and that accumulated fluid is reflected as edema, the swelling seen in such patients.
Recall from Section 15.3 that the osmotic pressure of blood, about 30 mm Hg, was one of the forces opposing filtration. Nephritis shows what happens when that force weakens throughout the circulation — fluid leaks out of capillaries everywhere in the body, not only in the kidney.
Flowchart
FC 15.15 – How nephritis produces edema: inflammation of kidney → increased permeability of glomerular capsular membrane → large amounts of protein escape from blood into urine (proteinuria) → colloidal osmotic pressure of blood falls → fluid moves from blood into interstitial spaces → edema
4. Renal Failure
Renal failure is a decrease or cessation of glomerular filtration. It is classified into two types.
a. Acute Renal Failure (ARF)
ARF is a sudden worsening of renal function, which most commonly happens after severe bleeding. When a large volume of blood is lost, the pressure driving filtration collapses, and filtration can stop within minutes.
There is a decrease in urine output, called oliguria or scanty urine — less than 400 ml/day, or less than 0.5 ml/kg/h in children. Other causes of ARF may include acute obstruction of both ureters, or nephrotoxic drugs. ARF can be detected biochemically by an elevated serum creatinine level.
b. Chronic Kidney Disease (CKD)
CKD is a progressive and generally irreversible decline in the glomerular filtration rate (GFR). It may be caused by chronic glomerulonephritis. It can be detected by reduced kidney size and by the possibility of anaemia.
The word irreversible marks the essential difference from ARF. In acute failure the nephrons may recover once the cause is removed; in chronic disease the nephrons are progressively lost and cannot be replaced, so the kidney shrinks.
Haemodialysis
When renal function falls below 5 to 7%, accumulation of harmful substances in the blood begins. In such a condition, the person has to go for artificial means of filtration of the blood.
In haemodialysis, blood is filtered outside the body using a dialysis unit. In this procedure the patient’s blood is removed, generally from the radial artery. It is passed through a cellophane tube that acts as a semipermeable membrane. The tube is immersed in a fluid called dialysate.
The composition of the dialysate is the key to the whole design. Dialysate is isosmotic to normal blood plasma. Hence only excess salts, if present in the plasma, pass through the cellophane tube into the dialysate — the salts the body still needs are in balance on both sides and stay where they are.
Waste substances being absent in the dialysate, they move from the blood into the dialysing fluid, following their own concentration gradients. The filtered blood is then returned to a vein, as shown in Fig. 15.12.
Diagram
Fig. 15.12 – Haemodialysis, labelled: patient, removed blood for cleaning, arterial pressure monitor, blood pump, heparin pump to prevent clotting, dialyser inflow pressure monitor, dialyser, fresh dialysate, used dialysate, air trap and air detector, venous pressure monitor, clean blood returning to patient
In this process it is essential that an anticoagulant like heparin is added to the blood while it is passing through the tube, since blood clots on contact with any surface that is not the lining of a vessel. Before resending the blood into the circulation, an adequate amount of anti-heparin is mixed in, so that the patient’s blood does not remain unable to clot.
The blood has to move slowly through the tube, and hence the process is slow.
Peritoneal Dialysis
In this method, the dialysing fluid is introduced into the abdominal cavity or peritoneal cavity. Here no artificial membrane is needed at all — the peritoneal membrane itself acts as the semipermeable dialysing membrane, and it is already richly supplied with blood.
Toxic wastes and extra solutes pass into the fluid. This fluid is drained out after a prescribed period of time, as shown in Fig. 15.13. Peritoneal dialysis can be repeated as per the need of the patient.
Diagram
Fig. 15.13 – Peritoneal dialysis, labelled: dialysate bag, catheter, peritoneum, peritoneal space, dialysate with waste products drained from peritoneal space, drainage bag
Because it needs no machine, it can be carried out at home, at work, or while travelling. But it is not as efficient as haemodialysis, since the peritoneal membrane cannot clear wastes as rapidly as a purpose-built dialyser.
What Dialysis Cannot Replace
Kidneys are associated with the secretion of erythropoietin, renin and calcitriol, and this is not possible using a dialysis machine.
A dialyser is a filter, and a filter can only remove. It cannot secrete a hormone, cannot sense a fall in blood pressure, and cannot activate Vitamin D. This is why a patient on long-term dialysis may still become anaemic, may still have difficulty controlling blood pressure, and may still develop weak bones.
5. Kidney Transplant
A kidney transplant is the organ transplant of a healthy kidney into a patient with end stage renal disease.
Kidney transplantation is classified as cadaveric (deceased donor) or living donor kidney transplant. Living donor kidney transplants are further classified as genetically related (living-related) or non-related (living non-related) transplants.
Flowchart
FC 15.16 – Classification of kidney transplants: kidney transplant → branch: cadaveric transplant (deceased donor); living donor transplant → branch: genetically related (living-related); non-related (living non-related)
15.8 Summary
Metabolism is the sum total of all chemical processes in the body, made up of catabolic (breaking down) and anabolic (building up) processes. Its by-products that must be removed are called metabolic waste products, and they may be fluid or gaseous, organic or inorganic.
Excretion is the elimination of metabolic wastes produced inside body cells. It is different from the removal of digestive wastes, which never entered the cells at all.
Human excretory wastes include water, \(CO_2\), ammonia, urea, uric acid, creatinine, and excess minerals and salts of sodium, potassium and calcium. They leave through urine, faeces and sweat. Bilirubin is excreted through faeces and urochrome through urine; excess vitamins, hormones, drugs and food pigments are also eliminated, and volatile substances from spices leave through the lungs.
Excess amino acids cannot be stored, so they undergo deamination, in which the amino group is removed and ammonia is formed. Ammonia is either excreted as such or converted to the less toxic urea or uric acid.
The mode of excretion depends on the availability of water in the habitat, not on phylogenetic relationship. A frog tadpole excretes ammonia while the adult frog is mostly ureotelic.
Ammonotelism — ammonia is highly toxic and basic, needs about 300–500 ml water per 1 gm, but saves energy. Seen in aquatic invertebrates, bony fishes, aquatic/larval amphibians and Protozoa; excreted through body surface, gills and kidneys.
Ureotelism — urea, \(H_2N-CO-NH_2\), is less toxic and less soluble, needing about 50 ml water for 1 gm of \(NH_2\). Formed in the liver by the ornithine/urea cycle at a cost of 3 ATP per molecule. Seen in mammals, cartilaginous fishes, many aquatic reptiles and most adult amphibians. Sharks retain urea to keep their blood isotonic to seawater.
Uricotelism — uric acid, \(C_5H_4O_3N_4\), is least toxic and least soluble, needing only about 5–10 ml water per 1 gm or none at all. Formed by the inosinic acid pathway in the liver of birds. Seen in birds, some insects, many reptiles and land snails, at a higher energy cost.
Spiders, scorpions and penguins excrete guanine, a mode called guanotelism.
Plasma creatinine comes from the catabolism of creatinine phosphate in skeletal muscle. Because its production and excretion are normally balanced, a raised blood level indicates poor renal function.
Excretory organs maintain homeostasis, the constant internal environment, which requires osmoregulation — the control of solute concentrations and water balance.
Osmoconformers are isoosmotic to their surroundings, and most marine organisms are osmoconformers. Osmoregulators control their internal environment independently, and freshwater and terrestrial organisms belong to this group. Marine birds such as the albatross use salt glands near the nostrils to secrete salt by active transport.
Stenohaline organisms tolerate only a narrow range of salinity; euryhaline organisms such as barnacles and clams tolerate wide changes.
Excretory structures across animal groups: contractile vacuoles in unicellular forms; diffusion through the osculum in sponges; protonephridia with flame cells in Platyhelminthes, rotifers, some annelids and Amphioxus; metanephridia with nephrostomes in earthworms; malpighian tubules in insects; green glands in crustaceans; no specialised organs in Echinodermata; and kidneys with nephrons in mammals.
The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. The kidneys are retroperitoneal, bean-shaped, about \(10 \times 5.5 \times 3\) cm, weighing about 150 g in males and 135 g in females, with a notch called the hilus.
Each ureter is 25–30 cm long and enters the bladder obliquely, which prevents backflow of urine. The bladder has the triangular trigone at its base, detrusor muscle in its wall and transitional epithelium as its lining. The urethra has an involuntary internal sphincter of smooth muscle and a voluntary external sphincter of skeletal muscle.
Micturition: the bladder holds about 700 ml; at about half filling, stretch receptors signal the spinal cord, the micturition reflex contracts the bladder and relaxes the internal sphincter, and the brain finally relaxes the external sphincter.
Each kidney is covered by renal fascia, adipose capsule and renal capsule. Internally it has a granular cortex containing Malpighian bodies and convoluted tubules, and a striated medulla containing Loops of Henle and collecting ducts arranged as renal pyramids, with columns of Bertini between them. Urine passes from renal papilla to minor calyx, major calyx, renal pelvis and ureter.
The nephron is the structural and functional unit of the kidney, about 4–6 cm long, consisting of Bowman’s capsule, neck, PCT, Loop of Henle, DCT and collecting tubule, together with the glomerulus. Bowman’s capsule and glomerulus together form the renal corpuscle or Malpighian body, about 200 µm in diameter.
The afferent arteriole is wider than the efferent arteriole, which raises hydrostatic pressure in the glomerulus for ultrafiltration. Podocytes of the visceral wall leave filtration slits between them. The PCT has a brush border for selective reabsorption; the descending limb of the loop is thin and water-permeable while the ascending limb is thick and water-impermeable; the DCT carries out tubular secretion and pH regulation; the collecting tubule reabsorbs water and secretes protons.
Cortical nephrons have short loops and are the majority; juxtamedullary nephrons have long loops running deep into the medulla and are surrounded by vasa recta. About one fourth of the cardiac output is supplied to the kidneys.
The Juxta Glomerular Apparatus is formed by the granular JG cells of the afferent arteriole and the macula densa of the DCT of the same nephron, and it regulates blood pressure within the kidney.
Urine formation has three successive steps — ultrafiltration, selective reabsorption and tubular secretion.
In ultrafiltration, glomerular hydrostatic pressure is about 55 mm Hg, opposed by blood osmotic pressure of about 30 mm Hg and capsular pressure of about 15 mm Hg, giving an effective filtration pressure of 10 mm Hg: \(10 \text{ mm Hg} = 55 - (30 + 15)\). About 600 ml of blood passes through each kidney per minute, and filtrate is formed at 125 ml/min, that is 180 L/d. The filtrate, also called deproteinized plasma or primary urine, is alkaline and contains urea, amino acids, glucose, pigments and inorganic ions.
In selective reabsorption, high threshold substances such as glucose, amino acids, Vitamin C, \(Ca^{++}\), \(K^{+}\), \(Na^{+}\) and \(Cl^{-}\) are absorbed actively against the gradient, while low threshold substances such as water, sulphates and nitrates are absorbed passively. About 99% of the glomerular filtrate is reabsorbed in the PCT and DCT.
In tubular secretion, cells of the DCT and collecting tubule actively move creatinine, \(K^{+}\) and \(H^{+}\) from the peritubular capillaries into the tubular fluid, concentrating the urine and changing its pH from alkaline to acidic. Secretion of \(H^{+}\) regulates blood pH. Tubular secretion is the only mode of excretion in marine bony fishes and desert amphibians.
Human urine can be about four times as concentrated as blood — 1200 mOsm/L against 300 mOsm/L — by the countercurrent mechanism operated by the loops of Henle of juxtamedullary nephrons and the vasa recta.
Water leaves the descending limb, concentrating the tubular fluid; \(Na^{+}\) and \(Cl^{-}\) leave the water-impermeable ascending limb, concentrating the medullary tissue fluid and diluting the tubular fluid. ADH from the pituitary makes the collecting duct permeable to water, so water moves out and the urine becomes concentrated.
Urea recycling between the deep medullary collecting duct, the inner medullary tissue fluid and the thin ascending limb raises medullary osmolarity and increases water reabsorption. The vasa recta act as a countercurrent exchange system that maintains this osmotic gradient. Desert mammals such as the camel have longer loops of Henle and excrete concentrated urine.
Normal urine is pale yellow and transparent, coloured by urochrome, and its composition depends on the food and fluid consumed.
ADH regulates water reabsorption. Osmoreceptors in the hypothalamus detect a rise in blood osmolarity and trigger ADH release from the neurohypophysis; ADH acts on the last part of the DCT and the whole collecting duct, reducing urine volume. Restored osmolarity switches ADH off — an example of negative feedback. Absence of ADH causes diuresis and the condition diabetes insipidus.
RAAS regulates electrolyte reabsorption. Reduced blood supply to the afferent arteriole makes JGA cells release renin, which converts liver angiotensinogen to Angiotensin I; angiotensin converting enzyme forms Angiotensin II. Angiotensin II constricts kidney arterioles, stimulates \(Na^{+}\), \(Cl^{-}\) and water reabsorption in the PCT, and stimulates the adrenal cortex to release aldosterone, which makes the DCT and collecting ducts reabsorb more \(Na^{+}\) and water.
Both systems are needed: ADH lowers blood \(Na^{+}\) concentration by reabsorbing water, while RAAS maintains osmolarity by reabsorbing \(Na^{+}\). A large rise in blood volume and pressure makes the atrial wall produce ANP, which inhibits \(Na^{+}\) and \(Cl^{-}\) reabsorption from collecting ducts, inhibits renin release and reduces aldosterone and ADH, causing natriuresis and diuresis.
Kidneys also synthesise calcitriol, the active form of Vitamin D needed for calcium absorption; its deficiency leads to brittle bones. They also secrete renin and erythropoietin.
Other excretory organs: the skin, through sweat glands that excrete water, \(NaCl\), lactic acid and urea while primarily serving thermoregulation, and sebaceous glands that secrete protective sebum; and the lungs, which excrete \(CO_2\), water vapour and volatile substances from food.
Disorders shown by urine: albuminuria indicates injury to the endothelial-capsular membrane; excess ketone bodies point to diabetes mellitus, starvation or too little carbohydrate; leucocytes indicate infection of the urinary organs.
Kidney stones or renal calculi may be calcium (oxalate or phosphate), struvite, uric acid or cystine stones, with symptoms of intermittent pain, discoloured urine, frequent urge to urinate and painful micturition.
Uremia is a rise of blood urea above 0.05%, against a normal 0.01 to 0.03%. Nephritis is inflammation of the kidney with proteinuria, which lowers blood colloidal osmotic pressure and causes edema.
Renal failure is a decrease or cessation of glomerular filtration. Acute renal failure is sudden, most often after severe bleeding, with oliguria of less than 400 ml/day or less than 0.5 ml/kg/h in children, and raised serum creatinine. Chronic kidney disease is a progressive, generally irreversible decline in GFR, detected by reduced kidney size and possible anaemia.
When renal function falls below 5 to 7%, dialysis is needed. In haemodialysis, blood taken from the radial artery passes through a cellophane tube immersed in dialysate that is isosmotic to normal plasma, so only wastes and excess salts diffuse out; heparin is added and anti-heparin mixed in before the blood is returned. In peritoneal dialysis the peritoneal membrane acts as the dialysing membrane; it is more convenient but less efficient. Neither method can replace the kidney’s secretion of erythropoietin, renin and calcitriol.
A kidney transplant places a healthy kidney into a patient with end stage renal disease. It may be cadaveric or from a living donor, the latter being genetically related or non-related.
Flowchart
FC 15.17 – The chapter in one sequence: metabolism → metabolic wastes → deamination of excess amino acids → ammonia → ammonotelism / ureotelism / uricotelism according to water availability → in humans, blood reaches the kidney → ultrafiltration in the glomerulus → selective reabsorption in the PCT → countercurrent mechanism in the loop of Henle and vasa recta → tubular secretion in the DCT and collecting tubule → concentration of urine under ADH, with \(Na^{+}\) balance under RAAS and ANP → urine through calyces, renal pelvis, ureter, bladder and urethra → micturition
15.9 Points to Ponder
Do organisms differ in the type of metabolic waste they produce? If two animals with very different diets lived in exactly the same habitat, would you expect their nitrogenous waste to be the same?
Do environment and evolution have any effect on the type of waste an organism produces? The chapter says habitat correlates with excretory product while phylogenetic relationship does not — what would this tell you about how easily a mode of excretion can change over evolutionary time?
How do thermoregulation and food habits affect waste production? Endotherms consume more food to meet their energy requirements, and a carnivorous diet contains more protein than a herbivorous one. Follow this through to its consequences for the kidney.
Ammonotelism costs no energy but a great deal of water; uricotelism costs a great deal of energy but almost no water. Under what circumstances might an animal be better off with the more expensive option, and how would you decide?
Like ectothermic and endothermic animals, do organisms differ in the way they maintain salt balance? Think about what an animal gains and what it gives up by being an osmoconformer rather than an osmoregulator.
The chapter states that the composition of blood is determined not by what the mouth ingests but by what the excretory organs retain. How far do you agree with this statement, and can you think of any substance for which it would not hold?
The kidney filters 180 litres a day and then reabsorbs about 99% of it. Would it not be simpler to filter less in the first place? Consider what the body would lose in flexibility if it did.
In which regions of the nephron will the filtrate be isotonic to blood, and why does the answer depend on which region you are asking about rather than on the person’s state of hydration?
A kidney can secrete hormones, sense blood pressure and activate Vitamin D. Should the kidney then be classified as an excretory organ, an endocrine organ, or both? What does your answer suggest about the way we divide the body into organ systems?
Medicines for high blood pressure are often diuretics. Trace the reasoning that connects the volume of urine a person passes with the pressure of blood in their arteries.
How do the kidneys bring about homeostasis, and is there any role of the neuroendocrine system in it? Consider where the disturbance is detected, where the decision is made, and where the correction is carried out.
A dialysis machine can remove wastes but cannot secrete erythropoietin, renin or calcitriol. What does this suggest about the difference between replacing a function and replacing an organ?
If a person had no option but to drink sea water, would the kidney’s response help or harm them in the long run? What does this reveal about the limits of osmoregulation in humans compared with a marine bird?
15.10 Check Your Concepts
Excretion and egestion both remove material from the body, yet only one of them is called excretion. Explain the difference, giving one example of each.
Why is a slight rise in body pH so dangerous that ammonia cannot be allowed to accumulate in the body? Answer in terms of enzymes and the plasma membrane.
A tadpole and the adult frog it grows into have the same genes but different modes of excretion. What does this tell you about the relative influence of phylogeny and habitat on the mode of excretion?
Arrange in the correct sequence: ammonia formed → excess amino acids absorbed → urea released into blood → deamination in liver → ornithine cycle operates → urea excreted by kidney.
Sharks retain urea in their blood instead of excreting all of it. What would happen to a shark’s water balance if it excreted urea as efficiently as a freshwater fish excretes ammonia?
Members of phylum Echinodermata have no specialised excretory organs at all. What features of their habitat and body organisation make this possible?
Why is creatinine a more reliable index of kidney function than blood urea after a heavy protein meal? Base your answer on where each substance comes from.
What happens if the ureters were to enter the urinary bladder perpendicularly instead of obliquely?
Infants below two years of age cannot control micturition. Which single part of the micturition pathway is not yet functional in them, and why does the rest of the pathway still work?
Label the following parts on an outline diagram of the longitudinal section of a kidney: renal cortex, renal medulla, renal pyramid, renal papilla, column of Bertini, minor calyx, major calyx, renal pelvis, ureter, hilus.
Arrange in the correct sequence the path of urine after it leaves the collecting duct: major calyx, ureter, renal pelvis, urethra, minor calyx, urinary bladder, renal papilla.
Why is the diameter of the afferent arteriole greater than that of the efferent arteriole? What would happen to ultrafiltration if the two were of equal diameter?
Explain how the structure of a podocyte suits its function. What would be the first substance to appear abnormally in the urine if the filtration slits became wider than normal?
The PCT is described as highly coiled and lined with a brush border. Explain how each of these two features separately contributes to reabsorption.
What happens if the drug given to a patient blocks \(Na^{+}\) and \(Cl^{-}\) transport out of the thick ascending limb? Trace the effect through the medullary gradient to the final urine volume.
Distinguish between a countercurrent multiplier and a countercurrent exchanger, naming the structure that performs each in the kidney.
Urea is a waste product, yet the kidney deliberately recycles it within the medulla. Explain why this apparent contradiction is useful to the body.
Why is the glomerular filtrate alkaline while the final urine is acidic? Name the step and the ions responsible for the change.
Both diabetes mellitus and diabetes insipidus cause a large volume of urine and severe thirst. Which single test on the urine would distinguish them, and what result would you expect in each?
Arrange in the correct sequence the events of the RAAS pathway: aldosterone released → angiotensinogen converted to Angiotensin I → blood pressure in afferent arteriole falls → Angiotensin II formed → renin released by JG cells → \(Na^{+}\) and water reabsorbed in DCT and collecting ducts.
What happens if ANP secretion fails in a person whose blood volume has risen sharply? Name at least three effects of ANP that would be missing.
Why would a system with only ADH, and no RAAS, be unable to maintain the osmolarity of body fluids?
Sweating is described as primarily a thermoregulatory process rather than an excretory one. Justify this statement using the way the volume of sweat is decided.
A patient’s urine report shows albumin present. Which part of the nephron is most likely damaged, and why does this cause swelling of the tissues?
Explain why a patient on long-term dialysis may still develop anaemia and brittle bones even though the wastes are being removed regularly.
Distinguish between acute renal failure and chronic kidney disease with respect to onset, reversibility and the test used to detect each.
Why is the dialysate made isosmotic to normal blood plasma rather than being plain water? Predict what would happen to the patient’s blood if plain water were used.
Why must heparin be added to the blood during haemodialysis, and why must anti-heparin be added before returning it to the patient?
In which regions of the nephron is the filtrate isotonic, hypertonic and hypotonic with respect to blood? Give the reason for each.
Doctors say Mr. Shaikh is suffering from urolithiasis. How could it be explained in simple words?
Anitaji needs to micturate several times and feels very thirsty. This is an indication of change in permeability of a certain part of the nephron. Which is this part?
Name any one guanotelic organism.
Why are kidneys called ‘retroperitoneal’?
State the role of liver in urea production.
Why do we get bad breath after eating garlic or raw onion?
John has two options as treatment for his renal problem: dialysis or kidney transplant. Which option should he choose? Why?
Amphibian tadpole can afford to be ammonotelic. Justify.
Birds are uricotelic in nature. Give reason.
Nitya has been admitted to hospital after heavy blood loss. Till proper treatment could be given, how must Nitya’s body have tackled the situation?
Complete the flow chart showing return to homeostasis after a fall in arterial pressure in the kidney: decreased arterial pressure in kidney → stimulation of the juxtaglomerular apparatus → renin secretion → the blank steps leading to angiotensin II → increase in blood volume and blood \(Na^{+}\) level → restoration of normal blood pressure. Fill in the blank boxes and write the conceptual details.
Label the given diagram of a nephron with its associated blood vessels and collecting duct, and write the conceptual details regarding it.
Label the given diagram of the renal corpuscle showing the glomerulus, the two walls of Bowman’s capsule and the beginning of the tubule, and write the conceptual details regarding it.
Complete the flow chart in which low water concentration in blood is detected by osmoreceptors in the hypothalamus, ADH is released by the posterior pituitary, facultative water reabsorption occurs, and \(Na^{+}\) transported from the proximal convoluted tubules raises the osmotic pressure of peritubular blood, leading to restoration of normal water concentration in blood. Fill in the blank boxes, including the hormone whose secretion is inhibited, and write the conceptual details.
Label the given diagram of a patient undergoing haemodialysis and write the conceptual details regarding it.
Prove that mammalian urine contains urea.
15.11 Practice With Numericals
In a person, the glomerular hydrostatic pressure is 60 mm Hg, the osmotic pressure of blood is 30 mm Hg and the capsular hydrostatic pressure is 15 mm Hg. Calculate the effective filtration pressure and state whether filtration will occur.
The effective filtration pressure in a patient falls to zero while the osmotic pressure of blood remains 30 mm Hg and the capsular hydrostatic pressure remains 15 mm Hg. Calculate the glomerular hydrostatic pressure at that moment.
Taking the glomerular filtration rate as 125 ml/min, calculate the volume of glomerular filtrate formed in 24 hours. If 99% of it is reabsorbed, calculate the volume of urine formed in that period.
About 600 ml of blood passes through each kidney per minute. Calculate the total volume of blood passing through both kidneys in one hour and in one day.
If one fourth of the cardiac output is supplied to the kidneys and the cardiac output at rest is 5 litres per minute, calculate the volume of blood supplied to the kidneys per minute and per day.
A person excretes 20 g of nitrogenous waste in a day. Calculate the volume of water needed for its elimination as ammonia, taking 300 ml per gram, and as urea, taking 50 ml per gram. By what factor is water saved by being ureotelic?
Uric acid needs about 5 ml of water for the elimination of 1 gm. Calculate the water needed to excrete 8 g of nitrogenous waste as uric acid, and compare it with the water needed to excrete the same mass as ammonia at 400 ml per gram.
The osmolarity of blood is 300 mOsm/L and that of maximally concentrated human urine is 1200 mOsm/L. Calculate how many times more concentrated the urine is than the blood. If a person’s urine measures 600 mOsm/L, what fraction of the maximum concentrating ability is being used?
A child weighing 12 kg passes 60 ml of urine in 10 hours. Calculate the urine output in ml/kg/h and state whether this child meets the criterion for oliguria given in the chapter.
An adult passes 350 ml of urine in a day. State whether this amounts to oliguria as defined in the chapter, and calculate by how much the output falls short of the threshold value.
Blood normally contains 0.01 to 0.03% urea. A patient’s blood urea is found to be 0.06%. Calculate how many times the upper limit of the normal range this value is, and state the condition it indicates.
The urinary bladder has an average capacity of 700 ml, and the desire to urinate begins when it is about half filled. Calculate the volume at which this desire begins. If urine is being formed at 1 ml/min, calculate the time taken from empty to reach that volume.
A vampire bat of body mass 35 g excretes dilute urine amounting to 24% of its body mass while feeding. Calculate the mass of urine excreted, and the resulting body mass of the bat at the end of the feed if it had first taken in blood equal to half its original body mass.
The production of one molecule of urea uses 3 ATP molecules. Calculate the number of ATP molecules used to produce 500 molecules of urea, and state whether a uricotelic animal producing the same mass of nitrogenous waste would use more or less energy.
Effective filtration pressure was calculated to be 20 mm Hg, where glomerular hydrostatic pressure was 70 mm of Hg. Which other pressure is affecting the filtration process? How much is it?
15.12 Objective Questions
Which one of the following organisms would spend maximum energy in production of nitrogenous waste?
- Polar bear
- Flamingo
- Frog
- Shark
In human beings, uric acid is formed due to metabolism of __________.
- amino acids
- fatty acids
- creatinine
- nucleic acids
Visceral layer : Podocytes :: PCT : _______
- Cilliated cells
- Squamous cells
- Columnar cells
- Cells with brush border
Deproteinised plasma is found in __________.
- Bowman’s capsule
- Descending limb
- Glomerular capillaries
- Ascending limb
Specific gravity of urine would _______ if level of ADH increases.
- remain unaffected
- increases
- decreases
- stabilise
What is micturition?
- Urination
- Urine formation
- Uremia
- Urolithiasis
Which one of the following organisms excrete waste through nephridia?
- Cockroach
- Earthworm
- Crab
- Liver Fluke
Person suffering from kidney stone is advised not to have tomatoes as it has _______.
- seeds
- lycopene
- oxalic acid
- sour taste
Tubular secretion does not take place in ________.
- DCT
- PCT
- collecting duct
- Henle’s loop
The minor calyx ____________.
- collects urine
- connects pelvis to ureter
- is present in the cortex
- receives column of Bertini
Which one of the followings is not a part of human kidney?
- Malpighian body
- Malpighian tubule
- Glomerulus
- Loop of Henle
The yellow colour of the urine is due to presence of ___________
- uric acid
- cholesterol
- urochrome
- urea
Hypotonic filtrate is formed in _______
- PCT
- DCT
- LoH
- CT
In reptiles, uric acid is stored in _____
- cloaca
- fat bodies
- liver
- anus
The part of nephron which absorbs glucose and amino acid is______
- collecting tubule
- proximal tubule
- Henle’s loop
- DCT
Bowman’s capsule is located in kidney in the ________
- cortex
- medulla
- pelvis
- pyramids
The snakes living in desert are mainly__________
- aminotelic
- ureotelic
- ammonotelic
- uricotelic
Urea is a product of breakdown of ___________
- fatty acids
- amino acids
- glucose
- fats
Volume of the urine is regulated by__________
- aldosterone
- ADH
- both a and b
- none
Assertion: Animals with plenty of water available generally excrete ammonia. Reason: Ammonia is the least toxic of the three nitrogenous wastes. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Assertion: The renal cortex appears granular while the renal medulla appears striated. Reason: The cortex contains Malpighian bodies and convoluted tubules, while the medulla contains Loops of Henle and collecting ducts. State whether the assertion and reason are true, and whether the reason explains the assertion.
Assertion: The ascending limb of the loop of Henle is impermeable to water. Reason: This allows the medullary tissue fluid to become more concentrated than the tubular fluid. State whether the assertion and reason are true, and whether the reason explains the assertion.
Assertion: Uric acid stones are common in people who drink very little water. Reason: Uric acid is the least soluble of the nitrogenous wastes. State whether the assertion and reason are true, and whether the reason explains the assertion.
Match the following:
| Column A | Column B |
|---|---|
| a. Protonephridia | i. Earthworm |
| b. Metanephridia | ii. Crustaceans |
| c. Malpighian tubules | iii. Platyhelminthes |
| d. Green glands | iv. Insects |
- Match the following:
| Column A | Column B |
|---|---|
| a. ADH | i. Adrenal cortex |
| b. Aldosterone | ii. Atrial wall of heart |
| c. ANP | iii. Neurohypophysis |
| d. Renin | iv. Juxta Glomerular Apparatus |