Excretion

• Made by a cell → excreted. Passed through a tube → egested. The liver makes urea; the kidney only removes it.
• If you only fix one thing before the exam, fix the direction of the ureters and the fact that the liver makes urea. Those two errors account for more lost marks in Topic 13 than everything else combined.
• Ureter goes to the bladder. Urethra takes urine through to the outside. Two ureters, one urethra.
• Blood loses the big things at filtration. The tubule gives back the useful things at reabsorption. Whatever survives both steps is urine.
• Liver makes it · blood carries it · kidney removes it · ureter takes it down · bladder holds it · urethra lets it out.
• Three visits, spaced a day, a week and a month apart, beat six visits in one evening. The forgetting between visits is what makes them work.

• State the definition of excretion in full, including all three of its parts: toxic materials, waste products of metabolism, and substances in excess of requirements.
• State the definition of egestion and explain, in terms of where the material came from, why faeces is egested rather than excreted.
• Distinguish excretion, egestion and secretion, and classify an unfamiliar example correctly.
• Identify carbon dioxide and urea as the waste products of metabolism required by the syllabus, and name the organ that excretes each.
• Describe the route carbon dioxide takes from the cell that produced it to the expired air.
• State that the lungs excrete carbon dioxide, the liver forms urea, and the kidneys remove urea and excess water and ions.
• Describe the assimilation of amino acids, including their use by cells to make proteins.
• Explain that excess amino acids cannot be stored as amino acids, and say what the liver does with them instead.
• Define deamination as the removal of the nitrogen-containing part of amino acids in the liver, and state that urea is formed.
• Explain why urea must be excreted rather than allowed to accumulate.
• Identify the kidneys, ureters, bladder and urethra on a diagram and state the function of each.
• Trace the complete route of urine from the collecting duct to the outside of the body, in the correct order, without confusing the ureter with the urethra.
• Identify on a nephron diagram the glomerulus, Bowman's capsule, the nephron tubule, the loop of Henle, the collecting duct and the associated blood vessels.
• Explain filtration at the glomerulus in terms of pressure and molecular size, naming what is filtered and what stays in the blood.
• Explain selective reabsorption, stating that all the glucose, some of the ions and most of the water are returned to the blood.
• Explain how the fluid left after reabsorption becomes urine, and state what normal urine contains.
• Compare the composition of blood entering the glomerulus, glomerular filtrate and urine substance by substance.
• Calculate a urine production rate and a percentage reabsorbed from supplied data, with correct units and sensible significant figures.
• Identify an anomalous result in a table of kidney data and say what should be done about it.
• Evaluate a simplified physical model of the kidney, saying what it represents well and where it breaks down.

Where you meet this outside biology. A hospital measures the urea concentration in blood to judge how well someone's kidneys are working, precisely because urea is produced steadily by the liver and removed only by the kidney. If the kidney slows down, urea rises in the blood. That is a measurement of kidney function, not a diagnosis on its own, and this chapter never asks you to make one.

Egestion

The removal through the anus of undigested and unabsorbed food that has passed along the alimentary canal. The material removed by egestion was never inside a body cell and was never produced by a chemical reaction of the body, so faeces is not a waste product of metabolism and egestion is not a form of excretion. The contrast with excretion, which removes substances that cells made or that the body has in excess, is a frequently examined distinction in Cambridge O Level Biology.

Urea

A soluble, nitrogen-containing waste product formed in the liver from the nitrogen-containing part of excess amino acids. Urea is toxic if it accumulates to a high concentration, so it is not stored: it dissolves in the blood plasma, is carried to the kidneys, is removed from the blood there, and leaves the body dissolved in urine. Urea is produced by the liver and removed by the kidneys, and the two organs must not be interchanged; urea is also distinct from urine, which is the fluid containing urea together with excess water and excess ions.

Deamination

The removal of the nitrogen-containing part of amino acids, which takes place in the liver and results in the formation of urea. Deamination is the body's response to an excess of amino acids, which cannot be stored as amino acids: the nitrogen-containing part is removed and converted into urea, a soluble nitrogen-containing waste that passes into the blood plasma and is later removed by the kidneys, while the remaining carbon-containing part is used in other reactions of the body. Deamination happens in the liver, never in the kidney.

Kidney

One of a pair of organs that remove urea, excess water and excess ions from the blood and form urine. Blood reaches each kidney through the renal artery and leaves through the renal vein. Inside the kidney, about a million microscopic nephrons filter the blood under pressure and then selectively reabsorb the substances the body needs, so that the fluid leaving the kidney contains urea together with excess water and excess ions. The kidney removes urea from the blood but does not produce it: urea is formed in the liver.

Urine

The fluid produced by the kidneys, consisting of urea together with excess water and excess ions dissolved in it. Urine is what remains in the nephron tubule after filtration has removed small molecules from the blood and selective reabsorption has returned all of the glucose, some of the ions and most of the water to the blood. Normal urine contains no glucose, no blood cells and no large plasma proteins. Urine passes from the collecting ducts out of the kidney, along a ureter to the bladder, where it is stored, and leaves the body through the urethra.

Filtration

In the kidney, the movement of small molecules out of the blood in the glomerulus and into the Bowman's capsule, caused by the high pressure of the blood in the glomerulus. Water, glucose, urea and ions are small enough to pass through the capillary wall and the wall of the capsule and enter the capsule as filtrate, while blood cells and large plasma proteins are too large to pass and remain in the blood. Filtration is not selective: it separates by size alone, so useful substances such as glucose are filtered out along with waste, and are recovered afterwards by selective reabsorption.

Glomerular Filtrate

The fluid formed inside the Bowman's capsule after filtration, containing water, glucose, urea and ions but no blood cells and no large plasma proteins. Filtrate has the same composition as blood plasma except that the large plasma proteins have been left behind, because filtration separates purely by molecular size. Filtrate is not urine: it still contains all the glucose that was in the blood, and it becomes urine only after selective reabsorption has returned the useful substances to the blood.

Nephron

The microscopic functional unit of the kidney, of which each human kidney contains about a million. A nephron consists of a Bowman's capsule containing a glomerulus, a nephron tubule that includes the loop of Henle, and a collecting duct, with blood vessels running alongside it throughout. Blood is filtered under pressure from the glomerulus into the Bowman's capsule, useful substances are then selectively reabsorbed from the tubule back into the blood in the surrounding capillaries, and the fluid remaining passes into the collecting duct as urine. A nephron is made of many cells; it is not a single cell.

Selective Reabsorption

The return of useful substances from the nephron tubule to the blood in the surrounding capillaries, after filtration has taken them out of the blood. All of the glucose is reabsorbed, some of the ions are reabsorbed and most of the water is reabsorbed, while urea is not reabsorbed and stays in the tubule. The process is described as selective because only certain substances, and only certain quantities of them, are returned to the blood; what remains in the tubule after reabsorption becomes urine.

Waste Product of Metabolism

A substance produced by the chemical reactions taking place inside the cells of an organism for which that organism has no further use, and which must be removed before it accumulates to a harmful concentration. In Cambridge O Level Biology the two required examples are carbon dioxide, produced during aerobic respiration in every living cell and excreted by the lungs in expired air, and urea, produced in the liver from excess amino acids and removed from the blood by the kidneys in urine.

• “Excretion means removing faeces from the body.” Why wrong Faeces is undigested and unabsorbed food. It was never inside a cell and no reaction of the body produced it, so it is not a waste product of metabolism. Say instead Excretion is the removal of toxic materials, the waste products of metabolism and substances in excess of requirements. Removing faeces is egestion.
• “Egestion removes metabolic waste.” Why wrong This is the previous error running the other way. Egestion removes material that stayed in the alimentary canal from beginning to end. Say instead Egestion removes undigested and unabsorbed food through the anus.
• “The kidneys make urea.” Why wrong It puts manufacture and removal in the same organ, which destroys the whole liver–kidney structure of the topic. Say instead The liver makes urea by deamination; the kidneys remove it from the blood.
• “The liver stores excess amino acids until they are needed.” Why wrong There is no store for amino acids anywhere in the body. If there were, no urea would need to be made. Say instead Excess amino acids cannot be stored, so the liver deaminates them and urea is formed.
• “Deamination happens in the kidneys.” Why wrong Deamination is a chemical process carried out by the liver. The kidney filters and reabsorbs; it does not break down amino acids. Say instead Deamination is the removal of the nitrogen-containing part of amino acids, in the liver.
• “Deamination is how protein is digested.” Why wrong Digestion happens in the alimentary canal and produces amino acids. Deamination happens in the liver and disposes of the ones there is no use for. Different place, different molecule, opposite purpose. Say instead Protein is digested to amino acids; excess amino acids are later deaminated in the liver.
• “The ureters carry urine out of the body.” Why wrong The ureters end at the bladder. Nothing leaves the body through them. Say instead The two ureters carry urine from the kidneys to the bladder; the single urethra carries it from the bladder to the outside.
• “Urine travels kidney → urethra → bladder.” Why wrong It puts the exit tube before the storage bag, which is physically impossible. Say instead Collecting duct → ureter → bladder → urethra → outside the body.
• “Filtration happens because molecules diffuse into the Bowman's capsule.” Why wrong Diffusion needs a concentration gradient and no pressure. Filtration is a physical push driven by blood pressure, and it moves everything small enough regardless of concentration. Say instead Blood in the glomerulus is under high pressure, which forces small molecules through the capillary and capsule walls.
• “Whole blood enters the nephron.” Why wrong Blood is cells plus plasma. The cells stay behind, and so do the large plasma proteins, so what enters the nephron is neither blood nor complete plasma. Say instead Only small molecules dissolved in the plasma pass into the capsule; the blood itself remains in the capillary.
• “Blood cells are normally present in the filtrate.” Why wrong They are far too large to cross either wall. Say instead Blood cells are too large to be filtered and remain in the blood.
• “Large plasma proteins are filtered and then reabsorbed.” Why wrong They never leave the blood in the first place, so there is nothing to reabsorb. This answer invents an extra step. Say instead Large plasma proteins are too large to be filtered and stay in the blood throughout.
• “Filtrate and urine are the same thing.” Why wrong Filtrate contains all the glucose that was filtered, plus far more water and ions. Urine contains none of that glucose. Say instead Filtrate becomes urine only after selective reabsorption has returned the useful substances to the blood.
• “All the filtered water is reabsorbed.” Why wrong If it were, there would be no urine, because urea and excess ions leave the body dissolved in water. Say instead Most of the filtered water is reabsorbed; the excess remains in the tubule and leaves in the urine.
• “All the ions are excreted.” Why wrong Ions are essential. Excreting all of them would be fatal, and it is not what the tubule does. Say instead Some ions are reabsorbed according to the body's needs; the excess is excreted.
• “Glucose is normally present in urine.” Why wrong All of the filtered glucose is reabsorbed, so none reaches the collecting duct. Say instead Glucose is filtered but all of it is reabsorbed, so normal urine contains no glucose.
• “Selective reabsorption removes useful substances from the blood.” Why wrong The direction is backwards. Reabsorption puts substances back into the blood, having previously lost them at filtration. Say instead Selective reabsorption returns useful substances from the tubule to the blood in the surrounding capillaries.
• “The collecting duct carries blood”, or “carbon dioxide is excreted by the kidneys.” Why wrong The collecting duct carries urine, not blood; blood travels in the capillaries alongside it. And carbon dioxide is excreted by the lungs, in expired air. Say instead The collecting duct carries urine toward the ureter. Carbon dioxide is excreted by the lungs; the kidneys excrete urea, excess water and excess ions.

• Examiner's eye. When a question says explain, it is asking for a mechanism, not a list. Compare these two answers to “Explain how filtrate is produced at the glomerulus (3 marks)”: Answer A: “The blood is filtered at the glomerulus and small molecules go into the Bowman's capsule.” — one mark. It says what happens twice and never says why. Answer B: “Blood in the glomerulus is under high pressure, so small molecules such as water, glucose, urea and ions are forced out through the capillary wall into the Bowman's capsule; blood cells and large plasma proteins are too large to pass through, so they stay in the blood.” — three marks.
• A one-mark question worth rehearsing. “Name two waste products of metabolism and, for each, name the organ that excretes it.” The full answer is: carbon dioxide, excreted by the lungs; urea, excreted by the kidneys. Do not write “urea, excreted by the liver” — the liver produces it. Do not offer faeces as a third example.
• Two words that must not be swapped. Deamination happens to amino acids that are already inside the body, in the liver. Digestion happens to protein inside the alimentary canal, and produces amino acids in the first place. Digestion supplies the raw material; deamination disposes of the surplus. A candidate who writes “deamination digests the protein” has merged two processes that happen in different places, to different molecules, for opposite reasons.
• Four words that lose marks here. “The blood is filtered into the capsule” — whole blood does not enter the nephron, only part of the plasma does. “The waste is filtered out” — glucose is filtered too, and glucose is not waste. “The proteins are filtered and then reabsorbed” — large plasma proteins never leave the blood in the first place. “Urine collects in the capsule” — it is filtrate until reabsorption has happened.
• Two absences, two different reasons. Glucose is absent from urine because it was reabsorbed. Protein is absent because it was never filtered. A question that asks you to explain both is testing whether you know that the two absences arise at completely different stages, and a single blanket answer will only score half of it.
• Never diagnose. Data questions in this topic sometimes give the composition of a urine sample and ask what it suggests. The safe answer describes kidney function — “glucose is present, so not all of the filtered glucose has been reabsorbed” — and stops there. Naming a disease is not what is being asked and is not creditworthy at this level.
• A structure worth memorising for any “explain” question in this topic. Write your answer as a chain in which every clause is caused by the one before it, and check that the words so, because or therefore appear between them. If you can delete any clause without breaking the chain, that clause was not earning a mark.

• Excretion is a compact topic that examiners like because it can be tested at three very different levels of demand from the same diagram. Knowing which of the three you are being asked for is most of the skill.
• Label a structure on a urinary system or nephron diagram, or name the organ that excretes a named substance. One mark, no explanation wanted. Spell ureter and urethra so they cannot be confused.
• Say what a named structure does, in one clause. “The ureter carries urine from the kidney to the bladder.” Include both ends of the journey — a bare “carries urine” is usually not enough.
• Give a linked causal sequence for filtration, reabsorption or the formation of urea. Marks are awarded for the links between steps, not for the vocabulary, so every “so that” and “because” is doing work.
• Chapter 13 items are nearly always one of four kinds: pick the correct definition of excretion; pick the correct order of the urine route; pick the row of a table that shows the correct composition of blood, filtrate and urine; or pick the substance that is present in the filtrate but absent from normal urine. Every one of these is decided by the three-column comparison in section M — learn that table and the multiple choice looks after itself.
• The commonest structured question in this topic supplies a nephron diagram with letters instead of labels, asks you to identify two or three of them, then asks you to explain what happens at one of them. The explanation is where the marks are concentrated. A typical four-mark filtration answer needs four separate points, and students routinely write the same point four times in different words.