Excretion in humans

• Made by a cell → excreted. Passed through a tube → egested. Lungs take out carbon dioxide; kidneys take out urea, excess water and excess ions — three things, not one.
• If you are a Core candidate and fix only one thing before the exam, fix the direction of the ureters and the fact that the kidneys remove three things. If you are an Extended candidate, add one more: the liver forms urea and the kidney removes it.
• Ureter goes to the bladder. Urethra takes urine through to the outside. Two ureters, one urethra.
• Six answers, and between them they contain every word Core 13.1.1, 13.1.2 and 13.1.3 ask for: lungs, carbon dioxide, kidneys, urea, excess water, excess ions, urine, ureter, bladder, urethra. If all ten are in your head in the right relationships, the Core half of this topic is finished.
• Cortex is the covering; medulla is the middle. The cortex is plain, the medulla is striped.
• Blood loses the big things at role (a). The nephron gives back the useful things at role (b). Whatever survives both is urine — role (c), and the same three substances Core 13.1.2 names.
• Six answers, and they are the whole of the Core topic. If all six came out cleanly, a Core candidate is finished with Topic 13.
• Eight answers, nine statements: four about where urea comes from and why it matters, and four about what the kidney does with it. Together with the Core six, that is the whole of Topic 13.

• Core 13.1.1 State that carbon dioxide is excreted through the lungs, and describe the route it takes from the cell that made it to the air you breathe out.
• Core 13.1.2 State that the kidneys excrete urea and excess water and ions — all three — and name the fluid they produce as urine.
• Core 13.1.3 Identify the kidneys, the ureters, the bladder and the urethra in diagrams and in images, and trace urine through them in the correct order without confusing the ureter with the urethra.
• Supplement 13.1.9 Explain the importance of excretion, limited to the toxicity of urea — that is, why urea must be removed rather than left to build up.
• Supplement 13.1.6 Describe the role of the liver in the assimilation of amino acids by converting them to proteins, and distinguish assimilation from absorption.
• Supplement 13.1.7 State that urea is formed in the liver from excess amino acids.
• Supplement 13.1.8 Describe deamination as the removal of the nitrogen-containing part of amino acids to form urea.
• Supplement 13.1.4 Identify the cortex and the medulla in diagrams and images of a kidney cut in half.
• Supplement 13.1.5(a) Outline the role of the glomerulus in the filtration from the blood of water, glucose, urea and ions.
• Supplement 13.1.5(b) Outline the role of the nephron in the reabsorption of all of the glucose, some of the ions and most of the water back into the blood.
• Supplement 13.1.5(c) Outline the formation of urine containing urea, excess water and excess ions.
• Plan and evaluate a simple investigation in which urine volume is measured against time, naming the independent variable, the dependent variable and the variables that must be controlled.
• 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, quote it, suggest a measurement cause and say what should be done about it.
• Evaluate a simplified physical model of the kidney, saying both 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 kidneys. If the kidneys slow down, urea rises in the blood — which is 13.1.9 read backwards. That is a measurement of kidney function, not a diagnosis on its own, and no question at this level asks you to make one.

• Core 1.1.1(f) “Excretion means removing faeces from the body.” Why wrong Faeces is undigested food. It was never absorbed, 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 the waste products of metabolism and substances in excess of requirements. Removing faeces is egestion.
• Core 7.2.2(e) “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.
• Supplement 13.1.7 “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 Urea is formed in the liver from excess amino acids (13.1.7); the kidneys excrete it from the blood (Core 13.1.2).
• Supplement 13.1.7 “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.
• Supplement 13.1.7 “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 to form urea (13.1.8), and urea is formed in the liver (13.1.7). 0610 keeps those as two separate statements, so a complete answer needs both.
• Supplement 13.1.8 “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.
• Core 13.1.3 “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.
• Core 13.1.3 “Urine travels kidney → urethra → bladder.” Why wrong It puts the exit tube before the storage bag, which is physically impossible. Say instead Inside the kidney → ureter → bladder → urethra → outside the body.
• Supplement 13.1.5(a) “Filtration happens because molecules diffuse out of the blood.” 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 water, glucose, urea and ions out of the blood and into the nephron.
• Supplement 13.1.5(a) “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 water, glucose, urea and ions pass out of the blood into the nephron; the blood itself remains in the capillary.
• Supplement 13.1.5(a) “Blood cells are normally present in the fluid inside the nephron.” 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.
• Supplement 13.1.5(a) “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.
• Supplement 13.1.5(c) “The fluid just after filtration and urine are the same thing.” Why wrong The fluid just after filtration contains all the glucose that was filtered, plus far more water and ions. Urine contains none of that glucose. Say instead It becomes urine only after reabsorption has returned all the glucose, some ions and most of the water back into the blood.
• Supplement 13.1.5(b) “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 nephron and leaves in the urine.
• Supplement 13.1.5(b) “All the ions are excreted.” Why wrong Ions are essential. Excreting all of them would be fatal, and it is not what the nephron does. Say instead Some of the ions are reabsorbed according to the body's needs; the excess is excreted.
• Supplement 13.1.5(b) “Glucose is normally present in urine.” Why wrong All of the filtered glucose is reabsorbed, so none is left by the time urine leaves the kidney. Say instead Glucose is filtered, but all of it is reabsorbed, so normal urine contains no glucose.
• Supplement 13.1.5(b) “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 nephron back into the blood in the capillaries around it.
• Core 13.1.1 “Carbon dioxide is excreted by the kidneys.” Why wrong It swaps the two organs in the topic. Carbon dioxide leaves through the lungs, in the air you breathe out. Say instead Carbon dioxide is excreted through the lungs; the kidneys excrete urea, excess water and excess ions.
• Core 13.1.2 “The kidneys excrete urea.” — given as a complete answer. Why wrong It is true, but it is a third of the statement. 13.1.2 names three things, and a three-mark question is telling you so. Say instead The kidneys excrete urea, excess water and excess ions.

• Supplement 13.1.5(a) Outline means state the role completely. Compare two answers to “Outline the role of the glomerulus”: Answer A: “The blood is filtered at the glomerulus and small molecules go into the nephron.” This says the same thing twice and names nothing. Set it against the statement and most of 13.1.5(a) is missing. Answer B: “Blood in the glomerulus is under high pressure, so water, glucose, urea and ions are filtered out of the blood into the nephron; blood cells and large plasma proteins are too large to pass through, so they stay in the blood.” The difference is the four named substances. 13.1.5(a) names them — water, glucose, urea and ions — and an answer that lists them is answering the statement rather than describing a picture.
• A one-mark question worth rehearsing. “Name two substances excreted by humans and, for each, name the organ that excretes it.” The full answer is: carbon dioxide, excreted through the lungs; urea, excreted by the kidneys. Add excess water and excess ions if the question gives you room, because 13.1.2 names them too. Do not offer faeces as an example — it is egested, not excreted. Supplement 13.1.7 Extended candidates should also never write “urea, excreted by the liver”. The liver forms urea; it does not excrete it. At Core the question does not arise, because the liver is not part of the Core answer at all.
• The complete Core answer, in one sentence. “The kidneys excrete urea, excess water and excess ions from the blood, in urine.” Four nouns and one verb. Learn it as a sentence rather than as a list, and the third item stops going missing.
• 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 phrasings that lose marks here. “The blood is filtered into the nephron” — whole blood does not enter the nephron, only four named substances do. “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 is produced at the glomerulus” — it is not urine until role (c).
• 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 has only accounted for one of them.
• The pattern to carry into any planning question. Independent variable, dependent variable and how it is measured, control variables and how they are controlled, range and intervals, repeats, hazard and precaution, table, graph, conclusion, limitation with a matched improvement. The word matched is the one that does the work: an improvement is only an improvement if it fixes the limitation you have just named.
• Never diagnose. A practical or data question in this topic may give you 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 it is not creditworthy at this level.
• A closing reminder on units. Every one of these calculations is lost more often on the unit than on the arithmetic. A rate has a unit (cm3/h); a percentage has none; and two quantities can only be compared once they are in the same unit and cover the same period. Check that before you divide, not after.
• 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 doing any work.

• Excretion is a compact topic, and its nine statements use only three command words: identify, state or outline, and explain. Each one asks for a different kind of answer, and the tier line runs straight between the first and the third. Working out which of the three a question wants is most of the skill, so the three levels below are read off the syllabus statements themselves, not off any prediction about what a paper will contain.
• Core 13.1.3 Supplement 13.1.4 Pick out a structure on a diagram or a photograph. One mark, no explanation wanted. Both of Topic 13's identify statements live here, and between them they are a third of the whole topic.
• Core 13.1.1 Core 13.1.2 Supplement 13.1.5 Say what happens, in the syllabus's own words. “The kidneys excrete urea, excess water and excess ions.” The marks are in the completeness of the list, not in the explanation.
• Supplement 13.1.9 Give a linked causal sequence. Topic 13 has exactly one statement with the verb explain — why urea must be excreted — and it is Supplement. Marks are awarded for the links between steps, so every “so” and “because” is doing work.
• Core candidates take Paper 1; Extended candidates take Paper 2. A multiple-choice item has to be decidable from one fact, so the things Topic 13 gives it are its shortest statements: which organ excretes a named substance (13.1.1, 13.1.2), the order of the urine route (13.1.3), what the kidneys remove (13.1.2), and — Extended only — which substance is in the fluid inside the nephron but not in urine (13.1.5). The first three are decided by sections B, C and D; the fourth by the three-column comparison in section M.
• Core candidates take Paper 3; Extended candidates take Paper 4. Because 13.1.3 is an identify in diagrams and images statement, a structured question on this topic has a natural shape: a urinary-system diagram with letters instead of labels, identification of two or three of them, and then what the kidneys remove. For an Extended candidate the same diagram can continue into the nephron — what is filtered out of the blood, what is put back, and what is left — because 13.1.5 is written in exactly that order. Practise both shapes; the identification part is also what tells you which statement the rest of the question is testing.