Chapter 9: The Excretory System
In 1666, in Italy, a physician named Marcello Malpighi was doing something few doctors of his time thought worthwhile. He was looking at organs through a microscope.
The kidney was a puzzle. To the naked eye it is a dark, solid, bean-shaped lump of flesh. Yet everyone knew that urine came out of it. How could a solid lump produce a liquid?
Malpighi tried something you can picture easily. He took an animal’s kidney, found the artery entering it, and pushed a dark ink along that artery until the organ was full. Then he opened the kidney and looked.
What he saw was not solid flesh. Scattered through the outer layer were thousands of tiny dark knots, each one a bunch of blood vessels wound into a ball, each one now stained with his ink. The kidney was not a lump at all. It was built out of countless tiny units, and blood was being carried into every single one of them.
Malpighi could not tell what those knots were for. That answer took almost two hundred more years. In 1842, an English surgeon, William Bowman, described the thin cup that sits around each knot and leads away into a fine, looping tube.
Only then did the shape of the answer appear. The kidney does not make urine out of nothing. It makes urine out of blood.
Diagram
Fig. 9.0-1 — From A Whole Kidney To One Tiny Unit Two-panel line drawing. Panel A: a human kidney seen whole from the front, bean-shaped, with a tube leaving the notch on its inner side; label whole kidney, notch, tube carrying urine away. Panel B: a circular magnified view taken from the outer layer of Panel A, joined to it by two guide lines, showing one knot of blood capillaries sitting inside a thin cup that opens into a coiled, looping tube; label knot of blood capillaries, cup around the knot, coiled tube, incoming blood vessel, outgoing blood vessel. Labels on straight, non-crossing lead lines.
9.1 Excretion And The Steady State Of The Body
Every cell in your body is busy all day. It breaks down glucose for energy, builds new proteins, repairs worn-out parts and stores what it can spare. All these chemical reactions taken together are called metabolism — the whole set of chemical changes going on inside a living body.
Metabolism never runs cleanly. Every reaction leaves something behind, and many of those leftovers are of no further use. A few are worse than useless. If they collect in the blood, they slowly poison the very cells that made them.
So the body has to keep throwing them out, hour after hour, for as long as it is alive. This removal of chemical wastes — mainly the ones containing nitrogen — is called excretion. The word says what happens: ‘ex’ means out, and ‘crete’ means flow.
Keeping The Blood Steady
Wastes do not sit quietly where they are made. They pass into the blood, and blood reaches every tissue in the body. A waste left in circulation is therefore a waste delivered to every cell you have.
This is why excretion cannot be occasional. Your cells work only when the fluid around them stays within narrow limits — of water, of salts, of acid, of dissolved waste. Keeping that internal state steady while conditions outside change is called homeostasis.
Excretion is one of the main ways homeostasis is maintained. The organs concerned with the formation, storage and elimination of urine together make up the excretory system.
9.1.1 Excretion, Secretion And Defaecation
Not everything that leaves the body is a waste. Some cells and glands make substances and release them precisely because the body needs them. Tears keep the surface of the eye moist, saliva begins the digestion of starch, and insulin controls the level of glucose in the blood. Releasing a useful substance in this way is secretion.
The test, then, is usefulness — not the direction of travel. A secreted substance still has a job to do. An excreted substance has no job left, or is harmful if kept.
Faeces are a different case again. The food you cannot digest is never absorbed into your blood at all. It travels the length of the alimentary canal and is passed out through the rectum. Because it was never part of the body’s chemistry, getting rid of it is defaecation, not excretion.
Carbon dioxide is the opposite case. It is genuinely made inside your cells, and it leaves through the lungs in the air you breathe out. The breathing itself is part of respiration, but the gas being removed is a true metabolic waste — which is why the lungs are counted among the organs of excretion later in this chapter. Table 9.1 sets the three processes side by side.
Excretory System Or Urinary System
The excretory system, taken broadly, includes any organ that helps to remove waste. But one set of organs does nothing else: the kidneys, the ureters, the urinary bladder and the urethra exist only to make urine, store it and pass it out.
For the removal of nitrogenous wastes in humans, therefore, urinary system is the more exact name, and it is the name used for these organs through the rest of the chapter.
9.2 Substances To Be Eliminated
Wastes reach your blood by two different routes. Some are manufactured inside your own cells as metabolism goes on. Others arrive in your food and drink in larger amounts than the body has any use for.
Both kinds have to go. A substance that is useful in small amounts can be harmful in large ones, and a substance that is harmful at any amount must be removed as fast as it appears. The main substances your body must eliminate are these:
- Carbon dioxide and water, the products of respiration
- Nitrogenous wastes formed from the breakdown of proteins
- Excess salts and excess water-soluble vitamins
- Excess water
- Bile pigments
9.2.1 Carbon Dioxide And Water
Every living cell releases energy by oxidising glucose. The reaction uses oxygen and produces carbon dioxide and water, and it goes on in every cell of your body at every moment.
In words, glucose plus oxygen gives carbon dioxide plus water plus energy. Written as a balanced equation:
\[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}\]
Carbon dioxide leaves through the lungs, dissolved in the blood on the way there and breathed out as a gas. Water made in this way adds to the water the body already holds, and the surplus is passed out in sweat and in urine.
9.2.2 Nitrogenous Wastes
Proteins are different from carbohydrates and fats in one important way: they contain nitrogen. When the body breaks a protein down, that nitrogen has to go somewhere, and it ends up in the wastes we call nitrogenous wastes — the waste products containing nitrogen, chiefly ammonia, urea and uric acid.
Where do they come from? Worn-out tissue proteins are carried to the liver and broken apart there. So are amino acids left over from a meal — and there are usually some left over, because amino acids, unlike glucose and fat, cannot be stored in the body. Whatever the body cannot use at once must be dismantled the same day.
In the liver, each surplus amino acid is split into two parts. The part that carries no nitrogen is turned into glucose, which the body can burn for energy or store. The nitrogen-carrying part becomes ammonia — a small, highly poisonous nitrogenous waste that no organism can allow to build up.
The liver deals with the danger at once by converting ammonia into urea, which is far less toxic and can be carried safely in the blood to the kidneys. Urea is the chief nitrogenous waste of the human body. It is not harmless, though: if the kidneys stop clearing it and urea collects in the blood, the poisoning that follows can kill. A third waste, uric acid, is produced in much smaller amounts and is also removed by the kidneys. FC 9.1 follows one surplus amino acid from the plate to the urine.
Flowchart
FC 9.1 — What Happens To A Surplus Amino Acid Protein in food → digested to amino acids → absorbed into blood → carried to liver → amino acid split into two parts → branch 1: nitrogen-free part → glucose → used for energy or stored; branch 2: nitrogen-containing part → ammonia (highly poisonous) → converted to urea → carried in blood to kidneys → passed out in urine
9.2.3 Excess Salts And Water-Soluble Vitamins
Salts arrive with almost everything you eat, and common salt, sodium chloride (\(NaCl\)), arrives in the largest quantity. The body needs a certain concentration of salt in its blood and no more, so whatever is taken in beyond that has to be removed. Salts are given out mainly through the kidneys.
The same logic applies to the vitamins that dissolve in water — the B group and vitamin C. The body cannot store them, so an excess simply passes into the urine and is lost.
9.2.4 Excess Water
Water is taken in with food and drink in large quantities, far more than the body needs to hold. The surplus must be removed if the blood is to keep its normal concentration.
Removing it does a second job at the same time. Wastes such as urea and salts are solids, and a solid cannot be swept out of the body on its own. Dissolved in the water leaving the kidneys, they are carried out with it. The water the body must lose is also the vehicle by which everything else is lost.
9.2.5 Bile Pigments
Red blood cells do not last for ever. Each one works for about 120 days, after which it is broken down and its haemoglobin taken apart. The breakdown of haemoglobin leaves coloured substances behind, the bile pigments, of which the chief one is the yellow pigment bilirubin.
The liver cells pick bilirubin out of the circulating blood and pass it into the bile, which is poured into the duodenum through the common bile duct. In the intestine the pigment is changed into other coloured substances, and these give faeces their yellowish-brown colour. A small share of the pigment is not lost this way but is excreted in the urine. FC 9.2 traces the route.
Flowchart
FC 9.2 — The Path Of A Bile Pigment Worn-out red blood cell → haemoglobin broken down → bilirubin released into blood → taken up by liver cells → passed into bile → bile enters duodenum through common bile duct → pigment changed in the intestine → branch 1: passed out in faeces, giving them their brown colour; branch 2: small amount excreted in urine
9.3 The Excretory Organs
No single organ removes all the wastes listed in the previous section. Different wastes have different properties — a gas cannot leave the same way a dissolved solid does — so the body uses several organs, each suited to what it removes.
These organs fall into two groups. One group does nothing but excrete, and the wastes it handles are the dangerous nitrogenous ones. The other group has some other main job in the body and removes waste alongside it. FC 9.3 sets out the grouping.
Flowchart
FC 9.3 — The Excretory Organs Of The Human Body Excretory organs → branch A: main excretory organs → kidneys → ureters → urinary bladder → urethra; branch B: accessory excretory organs → skin → lungs → liver
9.3.1 The Main Excretory Organs
The main excretory organs are the organs of the urinary system, and they exist for excretion alone. There are four of them, and each has one task in the making and passing out of urine.
- The kidneys — a pair of organs that filter the blood and form urine from what they take out of it.
- The ureters — the two tubes that carry urine from the kidneys down to the bladder.
- The urinary bladder — the muscular bag in which urine collects and is stored until it is passed out.
- The urethra — the tube through which urine finally leaves the body.
Of these four, only the kidneys actually remove anything from the blood. They are the primary excretory organs, taking nitrogenous wastes out of the circulation and throwing them out in the form of urine. The chief waste they remove is urea, together with uric acid and creatinine, a nitrogenous waste formed steadily from the breakdown of a substance stored in muscle.
The other three organs handle urine after it has been made. Not one of them changes what is in it.
9.3.2 The Accessory Excretory Organs
The accessory excretory organs are organs whose main work in the body is something else, but which remove some waste as they go about it. Three organs come under this heading.
The Skin
Sweat is produced by the sweat glands of the skin, and it is mostly water and sodium chloride, with small amounts of urea and lactic acid dissolved in it. Some genuine nitrogenous waste therefore leaves through the skin.
But sweating is not controlled by how much waste is in your blood. Sweat glands pour out sweat when the body needs cooling and hold back when it does not, so cooling — not excretion — is what they are truly for. Any waste that leaves in sweat leaves incidentally.
The Lungs
The lungs remove carbon dioxide, which every cell in the body produces and which leaves in the air you breathe out. Water vapour goes out with it. This is the only route by which carbon dioxide can leave the body in any quantity.
The Liver
You have already seen the liver’s most important excretory work: it converts ammonia into urea, which the kidneys then remove. The liver also collects the bile pigments from worn-out red blood cells and passes them into the bile.
Beyond this, the liver breaks down substances that would harm the body if they stayed in the blood — surplus cholesterol, alcohol, nicotine and many medicines. Substances that reach the blood but have no place in the body’s chemistry are dealt with here first. Table 9.2 brings the substances and their organs together.
9.4 The Urinary System
You have already met the four organs of the urinary system by name. Now look at where they sit in the body and how each one is built, because in every case the shape explains the job. The whole system is shown in Fig. 9.4-1.
Diagram
Fig. 9.4-1 — The Human Urinary System Front view line drawing of the human urinary system in position; show both kidneys on either side of the backbone with an adrenal gland capping each, the aorta and the inferior vena cava running vertically between them, a renal artery and a renal vein connecting each kidney to these vessels, a ureter leaving the notch of each kidney and running down to the urinary bladder, and the urethra leaving the bladder below. Label kidney, adrenal gland, aorta, inferior vena cava, renal artery, renal vein, ureter, urinary bladder, sphincter, urethra. Labels on straight, non-crossing lead lines.
9.4.1 Position And External Features Of The Kidneys
The two kidneys are bean-shaped organs, each about 10 cm long and 6 cm wide. They lie against the back wall of the abdomen, one on either side of the backbone, high enough to be covered by the last two pairs of ribs.
That covering matters. A kidney is a soft organ carrying a very large blood supply, and the ribs shield it from blows that would otherwise tear it.
The two kidneys are not quite level. The right one sits slightly lower than the left, because the liver takes up a good deal of room on the right side of the abdomen and pushes it down.
The Notch And The Blood Vessels
The inner border of each kidney is not a smooth curve. It is dented inwards at a notch called the hilum, and everything that enters or leaves the kidney passes through this one gap.
Three structures use it. The renal artery brings blood into the kidney from the aorta, the body’s main artery. The renal vein carries the blood away again into the inferior vena cava, the great vein that returns blood from the lower body to the heart. And the ureter leaves the hilum carrying away the urine that has been made.
A kidney cannot work on blood that does not reach it, and it receives an enormous share of the blood the heart pumps. Everything the kidney removes, it removes from blood delivered through the renal artery.
9.4.2 The Ureters, Urinary Bladder And Urethra
Inside the kidney, the upper end of the ureter is expanded into a wide, funnel-shaped space called the pelvis — the Latin word for a basin. Urine formed anywhere in the kidney drains into this basin first, and the ureter carries it away from there.
The two ureters run down behind the abdomen and open into the urinary bladder in its lower part. Urine trickles into the bladder steadily, all day and all night, because the kidneys never stop working.
The bladder solves the problem this creates. Urine is produced continuously but passed out only from time to time, so something must hold it in the meantime.
Two Ways Out Are Guarded
When the bladder wall contracts to empty itself, the urine inside is under pressure, and pressure pushes fluid in every available direction — including back up the ureters towards the kidneys.
That does not happen, because of the way the ureters enter. Each one opens into the bladder through a slightly projecting mouth that is pressed shut when the bladder squeezes. The openings act as valves, and urine can pass into the bladder but not back out of it.
The other exit is guarded differently. Where the bladder opens into the urethra there is a sphincter, a ring of muscle that stays contracted and keeps the opening closed. It relaxes only when an impulse from the brain allows it to, which is why passing urine is something you can control and delay.
The urethra itself is the final tube to the outside. It is longer in males, in whom it runs through the penis, and shorter in females, in whom it opens directly to the exterior.
9.5 Internal Structure Of The Kidney
From the outside a kidney tells you almost nothing. To see how it is built you have to cut one lengthwise, from the outer curve through to the notch, giving a longitudinal section. Fig. 9.5-1 shows what such a cut reveals.
Diagram
Fig. 9.5-1 — A Kidney In Longitudinal Section Line drawing of a human kidney cut lengthwise from the outer convex border to the hilum, one half shown in section; show the tough outer covering, a broad darker outer zone, an inner zone made of several cone-shaped striped pyramids with their tips pointing inwards, wedges of the outer zone running down between adjacent pyramids, and a wide funnel-shaped space that narrows into the ureter at the notch. Label renal capsule, cortex, medulla, renal pyramid, papilla, renal column, pelvis, hilum, ureter. Labels on straight, non-crossing lead lines.
The whole organ is wrapped in a tough fibrous coat, the renal capsule, which holds its soft contents in shape and protects them.
Under this coat the cut surface is clearly divided into two zones of different colour. The outer zone, just inside the capsule, is darker and looks finely dotted. This is the cortex.
The inner zone is lighter and looks streaked with fine lines running in one direction. This is the medulla. Why one region should look dotted and the other striped will make sense only after you have seen what a single kidney tubule looks like, so hold the question for the next section.
Pyramids, Papillae And Columns
The medulla is not one continuous mass. It is divided into several cone-shaped blocks called renal pyramids, each with its broad base facing the cortex and its tip pointing inwards, towards the notch of the kidney.
The tip of each pyramid is called a papilla, and it projects into the hollow space in the middle of the kidney. Urine formed anywhere in that pyramid is delivered here, and drips from the papilla into the space beyond.
Between one pyramid and the next, the tissue is not medulla at all. Strips of cortex reach down into the gaps and separate adjacent pyramids from each other. These inward extensions are the renal columns.
The hollow space that the papillae project into is the pelvis, the basin you met in the previous section. It collects urine from every pyramid and narrows at the hilum into the ureter.
What The Kidney Is Made Of
Cortex and medulla are regions, not tissues. Look at either of them under a microscope and you find, as Malpighi found, that the kidney is not a solid lump of flesh. It is packed with enormous numbers of microscopic tubes.
Each of these tubes is a nephron — the structural and functional unit of the kidney, also called a uriniferous tubule, a renal tubule or simply a kidney tubule. Every drop of urine you have ever passed was made inside one of them.
Calling the nephron the structural and functional unit says two things at once. The kidney is built out of nephrons, and the whole work of the kidney is done by each nephron separately. A kidney is not one large piece of filtering machinery; it is about a million tiny filtering units working side by side, whose output is collected together at the end.
9.6 The Nephron — Unit Of The Kidney
A nephron is not a simple straight tube. It begins as a closed cup, coils tightly, dives down as a long hairpin, comes back up, coils again, and only then joins a wider duct that carries its contents away.
Every one of those bends is there for a reason, and by the end of this chapter you will be able to say what each region does. Fig. 9.6-1 shows the whole tubule laid out, with the boundary between cortex and medulla marked across it.
Diagram
Fig. 9.6-1 — A Single Nephron And Its Parts Diagrammatic line drawing of one complete nephron drawn vertically, with a horizontal dashed line across the figure marking the boundary between cortex above and medulla below; show a cup-shaped capsule enclosing a knot of capillaries at the top, a tightly coiled tube leading from it, a long hairpin loop descending below the dashed line and returning above it, a second coiled tube, and a wider straight duct running downwards through the medulla to end at the tip of a pyramid. Label Bowman’s capsule, glomerulus, proximal convoluted tubule, descending limb of loop of Henle, ascending limb of loop of Henle, distal convoluted tubule, collecting duct, cortex, medulla, papilla. Labels on straight, non-crossing lead lines.
9.6.1 Parts Of A Kidney Tubule
The closed end of the nephron is a cup called Bowman’s capsule. Picture a hollow rubber ball pressed deep on one side with your thumb: the hollow you have made is the cup, and the space inside the ball leads on into the tubule.
The wall of this cup is a single layer of cells and nothing more. That thinness is the whole point, as the next section will show — anything filtering out of the blood has only one cell layer to cross before it is inside the nephron.
Sitting in the hollow of the cup is a knot of blood capillaries wound tightly into a ball. This knot is the glomerulus, and it is the structure Malpighi’s ink filled in 1666.
The capsule and the glomerulus inside it are always described together, because neither works without the other. The two taken as one structure are called the Malpighian capsule, after the man who first saw the glomerulus.
From The Capsule To The Duct
Leading out of the capsule is the proximal convoluted tubule, the first coiled length of the nephron. ‘Proximal’ means nearer, and this is the coiled region nearer to Bowman’s capsule.
The tubule then straightens and runs downwards as a long hairpin, the loop of Henle. It is not coiled at all. It descends into the medulla, turns back on itself, and climbs again into the cortex.
There the tubule coils for a second time as the distal convoluted tubule — ‘distal’ meaning farther, that is, farther from Bowman’s capsule. This is the last part of the nephron itself, and it opens into a wider tube.
That wider tube is the collecting duct, and it does not belong to any one nephron. Many nephrons empty into a single collecting duct, which runs down through the medulla and delivers what it carries, now urine, into the pelvis at the tip of a pyramid.
Why The Cortex Looks Dotted And The Medulla Striped
Now the appearance of the cut kidney explains itself. Bowman’s capsules and both sets of convoluted tubules lie in the cortex, so that region is full of small rounded structures cut across at every angle — which is exactly what a dotted appearance is.
The loops of Henle and the collecting ducts lie in the medulla, and they all run in the same direction, from the cortex towards the tip of a pyramid. Long parallel tubes cut lengthwise give the medulla its striped look.
FC 9.4 follows the fluid through the nephron in order, from the point where it leaves the blood to the point where it leaves the kidney.
Flowchart
FC 9.4 — The Route Of The Fluid Through A Nephron Blood in glomerulus → Bowman’s capsule → proximal convoluted tubule → descending limb of loop of Henle (into medulla) → ascending limb of loop of Henle (back into cortex) → distal convoluted tubule → collecting duct → papilla → pelvis of kidney → ureter
How Many, And How Long
The two kidneys together hold approximately 2 million nephrons. A single one, uncoiled and straightened out, would measure 4 to 5 cm; laid end to end, all of them together would stretch more than 60 km.
That length is not an accident of packing. Everything the kidney takes back from the fluid, it takes back through the wall of these tubules, and a long tube offers far more wall area than a short one. The 60 km is 60 km of surface across which useful substances can be recovered.
9.6.2 Blood Supply To The Kidney Tubules
A kidney can only remove what is brought to it, so the way blood is delivered to each nephron decides how well the nephron works.
The renal artery, entering at the hilum, branches and rebranches inside the kidney until the branches are extremely fine. One such fine branch runs to each Bowman’s capsule, and at that point it is called the afferent arteriole — ‘afferent’ meaning to bring to.
Inside the capsule the afferent arteriole does something unusual. Instead of thinning gradually into capillaries that carry blood onwards, it breaks up into a tight knot of capillaries, the glomerulus, and those capillaries then reunite and leave the capsule as a single vessel again.
The vessel that leaves is the efferent arteriole — ‘efferent’ meaning to carry away. It is distinctly narrower than the afferent arteriole that brought the blood in, and that difference in width matters enormously, as you will see in the next section.
Having left the capsule, the efferent arteriole runs a short distance and breaks up a second time, into the secondary capillary network that wraps closely around the coiled parts of the tubule. The straight vessels of this network that run alongside the loop of Henle deep in the medulla are known as the vasa recta.
Blood therefore passes through capillaries twice in the kidney — once in the glomerulus and once around the tubule — before it is collected into small veins. These unite again and again until they form the renal vein, which leaves the kidney at the hilum and pours the blood into the inferior vena cava. The path is set out in FC 9.5 and drawn in Fig. 9.6-2.
Flowchart
FC 9.5 — The Path Of Blood Through The Kidney Aorta → renal artery → repeated branching inside the kidney → afferent arteriole → glomerulus (first capillary network) → efferent arteriole → secondary capillary network around the tubule, including the vasa recta → small veins → renal vein → inferior vena cava
Diagram
Fig. 9.6-2 — A Nephron With Its Blood Supply Diagrammatic line drawing of one nephron with its blood vessels; show a wide arteriole entering the cup-shaped capsule and breaking into a knot of capillaries, a distinctly narrower arteriole leaving the capsule and breaking up again into a network of capillaries that wraps around the coiled tubules and runs alongside the hairpin loop, and this network draining into a vein. Use arrows to show the direction of blood flow throughout and separate arrows inside the tubule to show the direction of flow of the fluid. Label afferent arteriole (wide), glomerulus, Bowman’s capsule, efferent arteriole (narrow), proximal convoluted tubule, loop of Henle, distal convoluted tubule, secondary capillary network, collecting duct, to renal vein. Labels on straight, non-crossing lead lines.
The two arterioles look alike in a drawing but differ in ways that decide how the nephron works. Table 9.3 sets them against each other.
9.7 Production Of Urine
Urine is not drawn out of the blood ready-made. It is produced in three steps, one after another, and each step happens in a different part of the nephron.
First the liquid part of the blood is pushed out into the tubule. Then most of what was pushed out is taken back. Finally a few substances are added to what is left. What remains at the end of those three steps is urine.
9.7.1 Ultrafiltration
Blood in the glomerulus is under far greater pressure than blood in capillaries anywhere else in the body. The reason lies in the two arterioles you met in the last section.
Blood enters the knot through the wide afferent arteriole and can leave only through the narrow efferent arteriole. Think of a broad road narrowing suddenly to a lane: traffic backs up because it cannot get out as fast as it comes in. Blood in the glomerulus does the same, and the pressure it exerts outwards on the capillary walls is called hydrostatic pressure — the pressure a fluid exerts because it is being pushed.
Under that pressure the liquid part of the blood is forced out through the capillary walls, and through the single-cell wall of Bowman’s capsule, into the cup. Filtration driven by so great a force is called ultrafiltration.
What Gets Through, And What Does Not
Almost the whole liquid part of the blood passes into the capsule — water, and with it most of the substances dissolved in it, including urea, glucose, amino acids and salts. The fluid that collects in the capsule and moves on into the tubule is called the glomerular filtrate.
Two things stay behind, and they stay behind for the same reason: they are too large to pass. The blood corpuscles — red cells, white cells and platelets — remain in the capillaries, and so do the plasma proteins.
Nothing here is being chosen. A sieve does not decide what to keep; it keeps whatever will not fit through. Glucose and amino acids are as valuable to the body as proteins are, and they are filtered out all the same, simply because they are small.
The blood that leaves through the efferent arteriole has therefore lost much of its water while keeping all its cells and proteins. It is noticeably thicker than the blood that arrived.
Diagram
Fig. 9.7-1 — A Malpighian Capsule During Ultrafiltration Enlarged diagrammatic line drawing of one Malpighian capsule; show a wide afferent arteriole entering, breaking into the knot of glomerular capillaries held inside the cup, and a distinctly narrower efferent arteriole leaving; show the double-walled cup with its cavity, and a broad arrow leading from the cavity into the tubule below to show the direction of flow of the glomerular filtrate. Label afferent arteriole, efferent arteriole, glomerulus, Bowman’s capsule, cavity of the capsule, direction of flow of glomerular filtrate, proximal convoluted tubule. Labels on straight, non-crossing lead lines.
9.7.2 Reabsorption
The glomerular filtrate is not urine, and the difference is not a small one. It is an extremely dilute solution, and it still contains nearly everything useful that the blood was carrying: glucose, amino acids, sodium salts and enormous quantities of water.
If that fluid went straight out of the body, you would lose your entire supply of water and dissolved food within an hour. So as the filtrate travels along the tubule, the useful substances are passed back through the tubule wall into the secondary capillary network wrapped around it.
This taking back is called selective reabsorption, and the word selective is the important one. Substances are returned only to the extent that the blood’s normal composition requires. Glucose is returned completely; water is returned in the amount the body needs to keep that day; a substance already present in the blood in excess is not returned at all.
Where Each Substance Is Recovered
Most of the work happens early. The proximal convoluted tubule reabsorbs about two-thirds of the water, together with much of the glucose and the sodium and chloride ions.
The loop of Henle takes back some further water and sodium ions as the filtrate descends and returns. The distal convoluted tubule then reabsorbs the remaining chlorides and a little more water.
Reabsorption is not a matter of substances drifting back on their own. Tubule cells move them back against the natural direction of flow, which costs energy, and energy costs oxygen. Weight for weight, the kidneys demand six to seven times as much oxygen as muscle does.
9.7.3 Tubular Secretion
Ultrafiltration removes only what the pressure at the glomerulus happens to push out. Some substances need to be got rid of more thoroughly than that, and a few of them are in the blood that has already passed the glomerulus.
For these, the tubule works the other way round. The cells of the tubule wall take substances out of the blood in the surrounding capillaries and pass them into the fluid inside the tubule. This adding of substances to the fluid is called tubular secretion, and it takes place mainly in the distal convoluted tubule.
Two kinds of substance are handled this way. Potassium ions are secreted in the normal course of things, as part of keeping the blood’s salt balance right. Foreign chemicals are secreted too, including medicines such as penicillin — substances the body has no use for and did not evolve to carry.
What is left in the tubule after reabsorption has taken substances out and tubular secretion has put substances in is urine.
Table 9.4 sets out what each part of the nephron does, in the order the fluid meets them.
9.7.4 Urine Excretion And Micturition
Urine leaving the collecting ducts drips from the papillae into the pelvis, and passes from there into the ureter. It does not simply fall down the ureter. The muscular wall of the ureter contracts in waves that travel downwards, squeezing the urine ahead of them — peristalsis, the same kind of travelling wave that moves food along the alimentary canal. Gravity helps, but the waves do the work, which is why urine reaches the bladder just as well when you are lying down.
Urine collects in the bladder until the stretching of its wall is felt as the urge to pass it. When the sphincter guarding the outlet is allowed to relax, the bladder wall contracts and urine is driven out through the urethra. This expulsion of urine, under an impulse from the nervous system, is called micturition.
Comparing the fluid at three points along that journey shows what the nephron has actually done. Table 9.5 places them side by side.
9.8 Properties And Constituents Of Urine
Urine is the finished product of everything described in the last section, and its properties are a record of what the kidneys did. A doctor who knows what is in a sample of urine knows a good deal about what is happening inside the body that produced it.
9.8.1 Physical Properties Of Urine
- Colour: clear yellow, from a pigment called urochrome that is formed when haemoglobin from worn-out red blood cells is broken down. The exact shade varies with the diet.
- Volume: about 1 to 1.5 litres a day in an adult, though it varies a great deal with how much is drunk and how much is lost as sweat.
- pH: between 5 and 8, and usually slightly acidic at about 6.
- Odour: faint when fresh. On standing it grows strong and ammonia-like, because bacteria act on the urea in it and release ammonia.
- Specific gravity: 1.003 to 1.035.
Two of those entries need reading before they mean anything.
The pH scale runs from acidic through neutral at 7 to alkaline. Urine sits a little below 7, and what you eat shifts it: a diet heavy in protein makes it more acidic, while a diet of vegetables pushes it towards the alkaline side.
Specific gravity compares the density of a liquid with the density of pure water, which is taken as 1.000. Urine is always a little denser than water because it has solids dissolved in it, so its value is always above 1.000. The more concentrated the urine, the higher the figure climbs — which is why the range is as wide as 1.003 to 1.035 rather than a single number.
9.8.2 Normal Constituents Of Urine
Normal human urine is about 95 per cent water. The remaining 5 per cent is solid waste dissolved in that water, and the exact proportion shifts with what has been eaten and how long ago.
Those dissolved solids fall into two groups. The organic ones contain carbon and come from the breakdown of the body’s own substances; the inorganic ones are the salts. Table 9.6 gives the usual amounts in grams per litre of urine.
One figure in that table dwarfs the rest. Urea can reach 23 grams in a litre, while no other single substance passes 9 grams even at the top of its range. Urea is not merely one waste among many; it is the waste that urine exists to remove.
Besides these regular constituents, urine carries out whatever else the blood happens to contain and the body does not need — certain hormones, medicines such as antibiotics, and the excess of any vitamin that dissolves in water.
9.8.3 Abnormal Constituents Of Urine
Some substances have no business being in urine at all. When one of them appears, it is a sign that something upstream is not working as it should — and because the finding is so useful, each has its own name.
Blood And Glucose
Blood in the urine is called haematuria. Red blood cells are far too large to pass the glomerular filter, so if they appear in urine they must have entered the urinary passage somewhere further along. An infection of the urinary tract, a kidney stone or a tumour can each let them in.
Glucose in the urine is called glycosuria. Normally the tubules reabsorb every molecule of glucose that is filtered. If the blood carries far more glucose than usual, the tubules cannot take it all back, and the surplus stays in the urine and is passed out. The common cause is diabetes mellitus, a disorder in which the level of glucose in the blood is not properly controlled.
Albumin And Bile Pigments
Albumin is the chief protein of blood plasma, and like all plasma proteins it is too large to be filtered at the glomerulus. Its presence in urine, called albuminuria, therefore means that the filter itself is letting through what it should hold back.
This can happen when blood pressure is very high, or when a bacterial infection makes the membrane of Bowman’s capsule more permeable than it should be.
Bile pigments in the urine point away from the kidney altogether. They appear when the liver is not clearing them normally or when red blood cells are being broken down faster than usual, as in anaemia, hepatitis or cirrhosis of the liver.
9.9 Regulation Of Urine Output
The amount of urine you pass is not fixed. On a day when you drink a great deal it may be large and pale; on a day when you sweat heavily it may be small and dark. Something must be adjusting how much water the tubules take back, hour by hour.
That something is a hormone. The posterior lobe of the pituitary gland, a small gland attached to the underside of the brain, releases antidiuretic hormone, usually written ADH. It travels in the blood to the kidneys and acts on the walls of the distal convoluted tubules and the collecting ducts.
ADH makes those walls more permeable to water. The more ADH reaches them, the more water passes back into the blood as the fluid goes by, and the smaller and more concentrated the urine that finally leaves. FC 9.6 shows the loop in which this happens.
Flowchart
FC 9.6 — How ADH Adjusts The Water In The Blood Body loses water through sweating or drinks too little → blood becomes more concentrated → posterior lobe of pituitary releases more ADH → ADH carried in blood to kidney tubules → walls of distal convoluted tubule and collecting duct reabsorb more water → urine becomes smaller in volume and more concentrated → water saved in the blood → blood concentration returns to normal → less ADH is released, and the loop returns to its starting condition
Diuresis And Diuretics
Now run the loop the other way. If ADH secretion falls, less water is reabsorbed, and a larger volume of dilute urine is produced. This increased production of urine is called diuresis.
Substances that increase the formation of urine are called diuretics. Tea, coffee, alcohol and a diet consisting largely of liquids all act as diuretics, which is why an afternoon of drinking tea sends you to pass urine more often than an afternoon of doing nothing.
When ADH Is Lacking
If the pituitary secretes too little ADH, water is not reabsorbed as it should be, and very large quantities of extremely dilute urine are passed. The condition is called diabetes insipidus, and a person with it must drink constantly to replace what is lost.
Its name is confusing, because it has nothing to do with the diabetes mellitus of the previous section. Two different hormones are involved and two different substances are abnormal. Table 9.7 separates them.
9.10 Osmoregulation
Removing urea is only half of what the kidneys do. Every time they decide how much water and how much salt to return to the blood, they are deciding how concentrated your blood will be — and your cells can only work if that concentration stays within narrow limits.
Cells sit in fluid that comes from the blood, and water moves across their membranes according to the concentrations on either side. Blood that grows too concentrated pulls water out of cells; blood that grows too dilute pushes water into them. Either way the cell is in trouble, so the concentration of the blood is not allowed to wander. This regulation is called osmoregulation.
The Body’s Water Budget
An adult body holds nearly 40 litres of water, which is about 60 per cent of body weight. Water is entering and leaving that store all day, and the two must balance. Table 9.8 shows a day’s account for an adult at rest in a temperate climate.
Read the two columns against each other. The totals are equal, which is what balance means, but the individual entries are not under equal control. Water lost in the breath and in faeces goes whether you can spare it or not, and sweating is governed by temperature, not by your water supply.
That leaves the kidneys. They are the only route by which the body can vary its water loss deliberately, and they must make up the difference for all the others. Notice also the third entry on the gain side: 300 mL of water arrives each day from nowhere but respiration inside your own cells, which is where the water in the equation of Section 9.2.1 ends up.
Summer And Winter
In a tropical climate people drink far more water in summer than in winter. Yet in summer they pass urine less often, and what is passed is darker and more concentrated.
There is no contradiction. In summer a great deal of the water taken in leaves through the skin as sweat, so less of it remains for the kidneys to dispose of. Meanwhile the wastes have not decreased at all — the same quantity of urea must still be removed. The kidneys reabsorb more water from the glomerular filtrate, and the same load of waste goes out dissolved in a smaller volume.
When Water Is Lost Faster Than It Can Be Replaced
Cholera shows what happens when this system is pushed past what it can manage. The disease is caused by the bacterium Vibrio cholerae, and in a person who has it the intestine loses the ability to absorb water into the blood. Water is lost through vomiting and watery stools instead, in large amounts and quickly.
The kidneys respond as they are built to respond: they save water. They reabsorb almost all of the water from the glomerular filtrate — and once the filtrate has been concentrated that far, urea is reabsorbed along with it.
That is the trap. The very reflex that protects the body against drying out now returns waste to the blood. Urea builds up in the body, a poisoned condition called uraemia, which can be fatal. The person is not dying of the loss of water alone, but of the consequences of the body trying to hold on to it.
The treatment follows directly from the cause. Water and salts must be put back into the blood, either through a glucose-saline drip into a vein or, far more simply, by mouth.
9.11 The Artificial Kidney
You have two kidneys, and you need only one. If one is damaged or has to be removed, the other takes over the whole load and the person lives an entirely normal life. This is why a healthy person can donate a kidney at all.
Failure of both kidneys is a different matter. Nothing else in the body removes urea, so it accumulates in the blood, and untreated the condition is fatal. What is needed is a way of taking urea out of the blood from outside the body.
Cleaning Blood By Diffusion
A machine cannot filter blood the way a glomerulus does, and it does not try to. It works instead by diffusion — the movement of a dissolved substance from where it is more concentrated to where it is less so.
Blood is led out of the body and made to flow along one side of a thin membrane. On the other side flows a specially made liquid, the dialysing fluid. This fluid contains glucose and salts in the same concentrations as normal blood, but it contains no urea at all.
Now follow each substance separately. Urea is concentrated in the blood and absent from the fluid, so it diffuses steadily out of the blood and is carried away. Glucose and the necessary salts are at the same concentration on both sides, so there is nothing to make them move, and they stay in the blood. Salts present in excess are more concentrated in the blood than in the fluid, so those leave too.
Nothing has to be selected, because the composition of the dialysing fluid does the selecting. This use of a membrane to separate substances by diffusion is called dialysis, and the machine that does it is a dialysis machine, or artificial kidney.
How A Session Works
Blood is taken from the radial artery in the arm, since arterial blood is already under pressure and flows readily. It passes through tubing into the machine, where a substance is added to stop it clotting on the way, and travels through the dialyser against the flow of the dialysing fluid. The cleaned blood is then returned to a vein in the same arm. FC 9.7 traces the circuit, and Fig. 9.11-1 shows it in use.
Flowchart
FC 9.7 — The Circuit Of Blood Through A Dialysis Machine Radial artery in the arm → tubing → dialyser, where blood flows along one side of the membrane → urea and excess salts diffuse into the dialysing fluid on the other side → used dialysing fluid carried to waste → cleaned blood leaves the dialyser → tubing → vein in the same arm → back into the circulation
Diagram
Fig. 9.11-1 — Dialysis With An Artificial Kidney Simplified line drawing of a person seated beside a dialysis machine with one arm extended; show a tube leading from the artery of that arm into the machine, an enlarged inset of the dialyser showing blood flowing along one side of a membrane and dialysing fluid flowing in the opposite direction along the other side with small arrows crossing the membrane from blood to fluid, and a second tube returning blood to a vein in the same arm. Label radial artery, blood to the machine, dialyser, membrane, dialysing fluid inlet, used dialysing fluid outlet, cleaned blood returning, vein. Labels on straight, non-crossing lead lines.
Where the damage to the kidneys is permanent, this has to be repeated for the rest of the person’s life or until a transplant is possible — usually about four hours at a time, three times a week. Between sessions the wastes build up again, because the machine removes but does not regulate. It cannot reabsorb what the body needs, and it cannot adjust minute by minute the way a nephron does. That is why a person on dialysis must also keep careful control of what they eat and how much they drink.
9.12 Summary
- Metabolism — the whole set of chemical reactions in the body — constantly produces wastes, and some of them are harmful if they remain in the blood.
- Excretion is the removal of harmful and unwanted nitrogenous waste products formed by the body’s own metabolism. It is one of the main ways homeostasis, the steady internal state of the body, is maintained.
- Secretion is the release of a useful substance by a cell or gland; defaecation is the removal of food that was never digested or absorbed. Neither is excretion.
- The substances that must be eliminated are carbon dioxide and water, nitrogenous wastes, excess salts and water-soluble vitamins, excess water, and bile pigments.
- Surplus amino acids cannot be stored. In the liver they are split; the nitrogen-free part becomes glucose, and the nitrogen-containing part becomes ammonia, which is highly poisonous.
- The liver at once converts ammonia into urea, which is far less toxic. Urea is the chief nitrogenous waste of the human body and is removed by the kidneys, along with uric acid and creatinine.
- Bile pigments, chiefly bilirubin, come from the haemoglobin of worn-out red blood cells. The liver passes them into the bile; most leave in the faeces and a small amount in the urine.
- The main excretory organs are the kidneys, ureters, urinary bladder and urethra, together called the urinary system. The accessory excretory organs are the skin, the lungs and the liver.
- Sweat carries out water, sodium chloride and small amounts of urea, but sweat glands work for cooling, not for excretion. The lungs remove carbon dioxide. The liver makes urea, passes out bile pigments, and breaks down alcohol, nicotine, surplus cholesterol and medicines.
- The kidneys are bean-shaped organs about 10 cm long and 6 cm wide, lying on either side of the backbone under the last two pairs of ribs, the right one slightly lower than the left. The renal artery, the renal vein and the ureter pass through a notch called the hilum.
- Urine leaves the kidney by the ureter, is stored in the urinary bladder, and passes out through the urethra. The ureters open into the bladder through valve-like mouths that prevent backflow, and a sphincter guards the bladder’s outlet.
- A kidney cut lengthwise shows a tough renal capsule, an outer dotted cortex and an inner striped medulla. The medulla is divided into renal pyramids, each ending in a papilla; strips of cortex between the pyramids form the renal columns; and the papillae project into the funnel-shaped pelvis.
- The kidney is built of about 2 million microscopic tubules called nephrons, the structural and functional units of the kidney. Each is 4 to 5 cm long, and together they exceed 60 km, providing an enormous surface for reabsorption.
- A nephron consists of Bowman’s capsule enclosing the glomerulus — the two together being the Malpighian capsule — followed by the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and a collecting duct shared by many nephrons.
- Bowman’s capsules and both convoluted tubules lie in the cortex, which is why it appears dotted. The loops of Henle and the collecting ducts lie in the medulla, which is why it appears striped.
- Blood reaches each nephron through a wide afferent arteriole, forms the glomerulus, and leaves through a narrower efferent arteriole, which breaks up again into the secondary capillary network around the tubule before draining into the renal vein and the inferior vena cava.
- Urine is produced in three steps: ultrafiltration, selective reabsorption and tubular secretion.
- In ultrafiltration, the narrow efferent arteriole keeps the blood in the glomerulus under high hydrostatic pressure, forcing the liquid part of the blood into Bowman’s capsule. Blood corpuscles and plasma proteins are too large to pass and stay behind. The fluid that enters the tubule is the glomerular filtrate.
- In selective reabsorption, useful substances are returned to the blood, but only so far as the normal concentration of the blood is not disturbed. About 160 litres of filtrate are formed in a day and only about 1.2 litres leave as urine, so over 99 per cent is taken back.
- The proximal convoluted tubule reabsorbs about two-thirds of the water with much of the glucose, sodium and chloride; the loop of Henle reabsorbs further water and sodium; the distal convoluted tubule reabsorbs the remaining chlorides and some water. This work costs energy, and the kidneys have a very high oxygen demand.
- In tubular secretion, the cells of the distal convoluted tubule pass potassium ions and foreign chemicals such as penicillin from the blood into the fluid. What remains after reabsorption and tubular secretion is urine.
- Urine passes from the collecting ducts into the pelvis, down the ureter by peristalsis, and into the bladder. Its expulsion through the urethra, when the sphincter relaxes under an impulse from the nervous system, is micturition.
- Normal urine is about 95 per cent water and 5 per cent dissolved solids, of which urea is by far the largest. It is clear yellow from urochrome, measures 1 to 1.5 litres a day, has a pH of 5 to 8 and a specific gravity of 1.003 to 1.035.
- Abnormal constituents of urine are blood (haematuria), glucose (glycosuria, usually from diabetes mellitus), albumin (albuminuria) and bile pigments.
- Antidiuretic hormone, secreted by the posterior lobe of the pituitary gland, increases the reabsorption of water in the distal convoluted tubules and collecting ducts. Reduced ADH causes diuresis, and a lasting shortage of it causes diabetes insipidus. Substances that increase urine formation, such as tea, coffee and alcohol, are diuretics.
- Osmoregulation is the regulation of the water and salt content of the blood, and so of its osmotic pressure, by the kidneys. Daily water gain and loss each amount to about 2500 mL, and the kidneys are the only route the body can vary at will.
- In summer more water is lost as sweat, so the kidneys reabsorb more water and the same wastes leave in a smaller, more concentrated volume of urine.
- In cholera, water is lost through vomiting and watery stools; the kidneys reabsorb almost all the water of the filtrate and urea with it, and the resulting accumulation of urea in the blood is uraemia.
- One kidney is sufficient for the body’s excretory needs, but failure of both is fatal without treatment. An artificial kidney, or dialysis machine, removes urea and excess salts by diffusion across a selectively permeable membrane into a dialysing fluid that contains glucose and salts at their normal blood concentrations.
Flowchart
FC 9.7 — The Chapter In One Line Metabolism produces wastes → wastes carried in the blood → renal artery → afferent arteriole → glomerulus → ultrafiltration under pressure → glomerular filtrate in Bowman’s capsule → proximal convoluted tubule, loop of Henle and distal convoluted tubule (selective reabsorption returns water and useful substances to the blood; tubular secretion adds potassium and foreign chemicals) → urine → collecting duct → pelvis → ureter → urinary bladder → urethra → out of the body, leaving the blood at a steady composition
9.13 Key Terms
- Accessory excretory organs — organs whose main work in the body is something else, but which also remove some waste: the skin, the lungs and the liver.
- Afferent arteriole — the wide arteriole that brings blood into Bowman’s capsule and breaks up to form the glomerulus.
- Albuminuria — the presence of albumin, a plasma protein, in the urine.
- Ammonia — a small, highly poisonous nitrogenous waste formed in the liver, which is at once converted into urea.
- Antidiuretic hormone (ADH) — the hormone from the posterior lobe of the pituitary gland that increases the reabsorption of water from the kidney tubules.
- Bile pigments — coloured substances left when the haemoglobin of worn-out red blood cells is broken down.
- Bilirubin — the chief bile pigment, yellow in colour.
- Bowman’s capsule — the thin-walled cup at the closed end of a nephron, one cell thick, that receives the glomerular filtrate.
- Collecting duct — the wider tube that receives urine from many nephrons and carries it to the pelvis of the kidney.
- Cortex — the outer, darker, dotted region of the kidney.
- Creatinine — a nitrogenous waste formed steadily from the breakdown of a substance stored in muscle.
- Defaecation — the removal of undigested food as faeces, which were never absorbed into the body.
- Diabetes insipidus — a condition in which too little ADH is secreted, so that large amounts of very dilute urine are passed.
- Diabetes mellitus — a disorder in which blood glucose is not properly controlled, so that glucose appears in the urine.
- Dialysing fluid — the solution used in an artificial kidney, containing glucose and salts at normal blood concentrations and no urea.
- Dialysis — the removal of urea and excess salts from blood by diffusion across a selectively permeable membrane in an artificial kidney.
- Distal convoluted tubule — the second coiled part of a nephron, farther from Bowman’s capsule, which opens into a collecting duct.
- Diuresis — an increased production of urine.
- Diuretics — substances that increase the formation of urine, such as tea, coffee and alcohol.
- Efferent arteriole — the narrower arteriole that carries blood away from Bowman’s capsule and then forms the secondary capillary network.
- Excretion — the removal from the body of harmful and unwanted nitrogenous waste products formed by its own metabolism.
- Excretory system — the organs concerned with the formation, storage and elimination of urine.
- Glomerular filtrate — the fluid forced out of the glomerulus into Bowman’s capsule during ultrafiltration.
- Glomerulus — the knot of blood capillaries held inside Bowman’s capsule.
- Glycosuria — the presence of glucose in the urine.
- Haematuria — the presence of blood in the urine.
- Hilum — the notch on the inner border of the kidney through which the renal artery, the renal vein and the ureter pass.
- Homeostasis — the maintenance of a steady internal state in the body while conditions outside it change.
- Hydrostatic pressure — the pressure a fluid exerts outwards because it is being pushed.
- Kidneys — the pair of bean-shaped organs that filter the blood and form urine.
- Loop of Henle — the hairpin-shaped middle part of a nephron, which descends into the medulla and returns to the cortex.
- Main excretory organs — the four organs of the urinary system, which exist for excretion alone: kidneys, ureters, urinary bladder and urethra.
- Malpighian capsule — Bowman’s capsule together with the glomerulus inside it, taken as one structure.
- Medulla — the inner, lighter, striped region of the kidney.
- Metabolism — the whole set of chemical reactions going on inside a living body.
- Micturition — the expulsion of urine from the bladder through the urethra, under an impulse from the nervous system.
- Nephron — the microscopic tubule that is the structural and functional unit of the kidney.
- Nitrogenous wastes — waste products containing nitrogen, chiefly ammonia, urea and uric acid.
- Oral rehydration solution (ORS) — a measured mixture of glucose and salts in clean water, taken by mouth to replace fluid lost from the body.
- Osmoregulation — the regulation of the water and salt content of the blood, and so of its osmotic pressure, by the kidneys.
- Papilla — the inward-pointing tip of a renal pyramid, from which urine passes into the pelvis.
- Pelvis — the funnel-shaped space inside the kidney that collects urine and narrows into the ureter.
- Peristalsis — waves of contraction travelling along a muscular tube that push its contents forward.
- Proximal convoluted tubule — the first coiled part of a nephron, nearer to Bowman’s capsule, where most reabsorption occurs.
- Renal artery — the vessel that brings blood into the kidney from the aorta.
- Renal capsule — the tough fibrous coat covering the whole kidney.
- Renal columns — strips of cortex reaching down between adjacent renal pyramids.
- Renal pyramids — the cone-shaped blocks of medulla, each with its base towards the cortex and its tip towards the pelvis.
- Renal vein — the vessel that carries blood away from the kidney into the inferior vena cava.
- Secondary capillary network — the capillaries formed by the efferent arteriole, which wrap around the kidney tubule.
- Secretion — the release by a cell or gland of a substance that the body has a use for.
- Selective reabsorption — the return of useful substances from the glomerular filtrate to the blood, in only such amounts as keep the blood’s normal concentration undisturbed.
- Sphincter — a ring of muscle that keeps an opening closed until it is allowed to relax.
- Tubular secretion — the passing of substances such as potassium and foreign chemicals out of the blood into the fluid in the kidney tubule.
- Ultrafiltration — the filtering out of the liquid part of the blood from the glomerulus into Bowman’s capsule under great pressure.
- Uraemia — the poisoned condition caused by urea accumulating in the blood.
- Urea — the chief nitrogenous waste of the human body, made in the liver from ammonia and removed by the kidneys.
- Ureters — the two tubes carrying urine from the kidneys to the urinary bladder.
- Urethra — the tube through which urine leaves the body from the bladder.
- Uric acid — a nitrogenous waste produced in small amounts and removed by the kidneys.
- Urinary bladder — the muscular bag in which urine is stored until it is passed out.
- Urinary system — the kidneys, ureters, urinary bladder and urethra, taken together.
- Urochrome — the pigment that gives urine its yellow colour, formed when haemoglobin is broken down.
9.14 Points To Ponder
- Ultrafiltration removes glucose, amino acids and salts from the blood indiscriminately, and the tubules then spend energy putting almost all of them back. A filter that let only the wastes through in the first place would seem far cheaper. What might the body gain from the arrangement it actually has?
- Urea is made safe in the liver but thrown out by the kidneys. What does the body gain by separating the organ that converts a poison from the organ that disposes of it — and what does it risk?
- Suppose the membrane of Bowman’s capsule in one person became more permeable than normal. Trace the consequences forward, first for what appears in the urine and then for what happens to the blood over weeks and months.
- Sweat carries out water, salt and a small amount of urea, and yet the skin is described as only an accessory excretory organ. What test would you apply to decide whether an organ is truly an organ of excretion, and would the lungs pass it?
- Mammals that live in dry places tend to have longer loops of Henle than those living where water is plentiful. What connection would you expect between the length of the loop and the concentration of the urine an animal can produce?
- A dialysis machine keeps a person alive, but it works for twelve hours twice a week, while a kidney works unnoticed all day and night. Which of the kidney’s abilities would be hardest for an engineer to build into a machine, and why?
9.15 Objective Questions
Multiple Choice
- The blood vessel that carries blood away from Bowman’s capsule is the: (1 mark)
- renal artery
- afferent arteriole
- efferent arteriole
- renal vein
- The part of the nephron in which the greatest amount of water is reabsorbed is the: (1 mark)
- Bowman’s capsule
- proximal convoluted tubule
- loop of Henle
- collecting duct
- The region of the kidney that appears striped in a longitudinal section is the: (1 mark)
- renal capsule
- cortex
- medulla
- pelvis
- Reabsorption of water from the kidney tubules is increased by: (1 mark)
- insulin
- urochrome
- bilirubin
- antidiuretic hormone
- A substance normally absent from the urine of a healthy person is: (1 mark)
- urea
- creatinine
- glucose
- sodium
- The condition in which blood is passed out along with the urine is called: (1 mark)
- glycosuria
- haematuria
- albuminuria
- uraemia
Assertion And Reason
For each of the following, choose the answer from the codes given below. (1) A is true and R is false (2) A is false and R is true (3) Both A and R are true (4) Both A and R are false
Assertion (A): The efferent arteriole is narrower than the afferent arteriole. Reason (R): This difference in width keeps the blood in the glomerulus under high pressure. (1 mark)
Assertion (A): Glucose is absent from the urine of a healthy person. Reason (R): Glucose molecules are too large to be filtered out at the glomerulus. (1 mark)
Assertion (A): The urine passed on a hot summer day is more concentrated than that passed in winter. Reason (R): More water is lost as sweat, so the tubules reabsorb a greater share of the water in the filtrate. (1 mark)
Match The Following
- Match the items in Column I with those in Column II and write down the matching pairs. (4 marks)
| Column I | Column II |
|---|---|
| a. Loop of Henle | i. Formed by the reuniting capillaries of the glomerulus |
| b. Malpighian capsule | ii. Receives urine from many nephrons |
| c. Collecting duct | iii. Descends into the medulla and returns to the cortex |
| d. Efferent arteriole | iv. Bowman’s capsule together with the glomerulus |
Fill In The Blanks
- Complete the following: (4 marks)
- The structural and functional unit of the kidney is the ____________.
- The fluid that collects in Bowman’s capsule after ultrafiltration is called ____________.
- Potassium ions and drugs such as penicillin are added to the forming urine by the process of ____________.
- The yellow colour of normal urine is due to the pigment ____________.
True Or False
- State whether each statement is true or false. (4 marks)
- Plasma proteins are present in the glomerular filtrate.
- The right kidney lies at a slightly lower level than the left kidney.
- Sweat glands are controlled by the amount of nitrogenous waste in the blood.
- A single collecting duct receives the contents of many nephrons.
Name The Following
- Name the following: (5 marks)
- The notch through which the ureter leaves the kidney.
- The tough fibrous coat that covers the whole kidney.
- The knot of blood capillaries held inside Bowman’s capsule.
- The nitrogenous waste made in the liver from ammonia.
- The hormone whose shortage results in diabetes insipidus.
Odd One Out
- Choose the odd one out in each set and give a reason for your choice. (3 marks)
- Cortex, medulla, pelvis, ureter
- Urea, uric acid, creatinine, sodium chloride
- Bowman’s capsule, glomerulus, loop of Henle, distal convoluted tubule
9.16 Check Your Concepts
Define excretion. (2 marks)
State the exact location of the kidneys in the human body. (2 marks)
Give a reason why the cortex of the kidney shows a dotted appearance. (2 marks)
Name the three steps in the formation of urine, in the order in which they occur. (2 marks)
Differentiate between the afferent arteriole and the efferent arteriole, stating three bases of difference. (3 marks)
Give a reason for each of the following: (3 marks)
- Glucose is absent from the urine of a healthy person.
- Urine that has been standing for some hours smells strongly of ammonia.
- The oxygen demand of the kidneys is very high for their size.
Explain why the blood leaving the glomerulus is thicker than the blood that entered it. (3 marks)
Differentiate between diabetes mellitus and diabetes insipidus, stating in each case the hormone involved and the abnormality found in the urine. (4 marks)
What would happen to the formation of urine if the efferent arteriole were wider than the afferent arteriole? Explain your answer. (4 marks)
Explain what is meant by selective reabsorption, naming the parts of the nephron in which it takes place and the substances recovered in each. (4 marks)
Explain, giving reasons, why a person passes a smaller quantity of more concentrated urine on a hot day than on a cold one. (4 marks)
Trace the path of a molecule of urea from the blood in the renal artery until it leaves the body, naming in order every structure through which it passes. (5 marks)
Explain how an artificial kidney removes urea from the blood while glucose and salts are retained, and state two ways in which it falls short of a working kidney. (5 marks)
Explain how the kidneys carry out osmoregulation, and describe what happens to this process in a person suffering from cholera. (5 marks)
9.17 Figure-Based Questions
Study Fig. 9.5-1 and answer the following: (4 marks)
- Name the region of the kidney that appears dotted, and state why.
- Name the cone-shaped blocks that make up the inner region.
- Name the space into which the tips of these blocks project.
- Name the structure that leaves the kidney from this space.
Redraw the nephron shown in Fig. 9.6-1 and label the glomerulus, Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct. Keep all lead lines straight and do not let them cross. (5 marks)
Study Fig. 9.7-1 and answer the following: (4 marks)
- Name the process represented by the broad arrow leaving the capsule.
- Name the two arterioles shown, and state which of them is wider.
- Name two substances present in the fluid entering the tubule.
- Name two constituents of blood that are not present in that fluid, and state why.
Study Fig. 9.6-2 and answer the following: (3 marks)
- Name the network of capillaries that surrounds the kidney tubule.
- Name the vessel from which this network is formed.
- State one function of this network.
Study Fig. 9.11-1 and answer the following: (3 marks)
- Name the fluid that flows through the chamber around the membrane.
- Name one substance that leaves the blood here and one that does not.
- Explain why this fluid is prepared with glucose and salts at the concentrations found in normal blood.
Study Fig. 9.4-1 and answer the following: (4 marks)
- Name the organ in which urine is stored.
- Name the tubes that carry urine to it from the kidneys.
- Name the vessel that brings blood to the kidney.
- Name the gland shown capping each kidney, and state whether it forms part of the urinary system.