Hi Tara. One honest note before you start: Cambridge numbers this whole topic as a single section, 13.1. The five sub-sections below are our split, not theirs, made so that each one is a sitting you can actually finish — the syllabus content underneath is exactly what 13.1 asks for and nothing more.
Here is the shape of it. 13.1 is the definition and the two organs: carbon dioxide leaves through the lungs, urea and excess water and ions leave through the kidneys. It also settles the confusion that costs more marks in this topic than anything else — excretion versus egestion. 13.2 is plumbing: kidneys, ureters, bladder, urethra, and a kidney cut in half showing a cortex and a medulla. 13.3 is the nephron, and it is three sentences long: what is filtered out, what is reabsorbed, what is left. 13.4 is the liver — where urea actually comes from, which is not the kidney. Finish with 13.5, the checklist and the vocabulary.
Two warnings worth several marks each. First: faeces are not excreted. Undigested food has never been inside a cell and is not a product of metabolism, so removing it is egestion. Second: the kidney does not decide what is bad. It filters a batch of small molecules out indiscriminately — useful ones included — and then takes back the useful ones. That two-step design is the answer to half the questions in the topic.
The Definition, Word for Word
You met this in Topic 1 as one of the seven characteristics of living organisms, and you probably wrote it down and moved on. It is worth going back to, because the exact wording does all the work.
Excretion is the removal from organisms of the waste products of metabolism and substances in excess of requirements.
Read it as two separate halves, because things qualify by either route:
— waste products of metabolism: made by chemical reactions inside cells. Carbon dioxide from respiration. Urea from breaking down excess amino acids.
— substances in excess of requirements: perfectly useful things you happen to have too much of. Water. Ions. They are not poisons; there is simply more of them than the body needs, and the surplus has to go.
The Single Most Expensive Confusion in This Topic
Egestion is the removal of undigested food from the body as faeces. It is not excretion, and calling it excretion will lose you the mark every time.
The test is simple and it never fails: did the substance ever get inside a cell? Carbon dioxide was made inside your cells. Urea was made inside liver cells. Excess water and ions were absorbed into your blood and carried to your cells. All excreted. Undigested fibre travelled from your mouth to your anus through the gut — a tube that is, in a sense, still the outside world. It was never absorbed, never took part in a single reaction, and was never in you. Removing it is egestion.
The diagram below is worth thirty seconds of your attention because it lays out the whole of 13.1 in one picture: two exits that are excretion, one that is not.
The Two Organs the Syllabus Names
| Organ | What it excretes | Where that substance came from |
|---|---|---|
| Lungs | Carbon dioxide | Aerobic respiration in every cell of the body. It diffuses into the blood, is carried to the lungs, and diffuses across the alveolus wall into the air you breathe out. |
| Kidneys | Urea, plus excess water and excess ions | Urea is made in the liver from excess amino acids. Water and ions come from what you have eaten and drunk, and from respiration. |
The kidney excretes urea. It does not make it. Urea is made in the liver and travels to the kidney dissolved in the blood plasma. “The kidney produces urea” is a favourite wrong answer and it is refused every time.
By the same logic, water vapour is lost from the lungs and salts are lost in sweat, but the two organs the 0610 syllabus names for excretion are the lungs and the kidneys. Answer with those unless a question steers you elsewhere.
Why Bother? The Toxicity of Urea
Cambridge asks you to explain the importance of excretion, and it limits that explanation to one idea: urea is toxic.
1. Excess amino acids cannot be stored, so they are deaminated in the liver and the nitrogen-containing part becomes urea.
2. Urea is toxic. If it is not removed it builds up in the blood.
3. A high concentration of urea in the blood damages cells and interferes with the enzyme-controlled reactions of metabolism.
4. So urea must be removed continuously by the kidneys. That is the importance of excretion — not tidiness, but preventing a poison from accumulating.
Urea, not “urine”, when you mean the chemical. Urea is a dissolved substance; urine is the liquid that contains it. Writing “the liver makes urine” turns a correct idea into a wrong one.
Excess, not “extra” or “too much”. The syllabus phrase is substances in excess of requirements, and it is worth using because it makes clear that water and ions are not waste — only the surplus is removed.
Four Organs and Two Blood Vessels
This is a labelling section, and labelling sections are the cheapest marks in the paper provided you get the spellings right. There are four organs, and Cambridge names all four: kidneys, ureters, bladder, urethra. Two blood vessels serve each kidney and you already met them in Topic 9: the renal artery in, the renal vein out.
Ureter: kidney → bladder. There are two, one from each kidney.
Urethra: bladder → outside. There is one, and it is the last tube in the system.
If you can never remember which is which, use the position on the diagram instead: the two tubes that come down from the kidneys are ureters; the single tube below the bladder is the urethra. Position is easier to read off a diagram than a spelling is to recall.
Urine is formed in the kidney. By the time it reaches the ureter it is already finished; nothing is added to it or taken from it afterwards. The bladder is a muscular bag that holds it until it is convenient to release it.
“The bladder makes urine” is the mirror image of “the kidney makes urea”, and both come from the same habit — assuming that the organ where you notice a substance is the organ that produced it.
What the Blood Looks Like Going In and Coming Out
A very common data question gives you the composition of blood in the renal artery and the renal vein and asks you to explain the differences. Every difference in that table comes from something you already know.
| Substance | Renal artery (entering) | Renal vein (leaving) | Why |
|---|---|---|---|
| Urea | Higher | Much lower | Most of the urea that is filtered out is not taken back, so it leaves in the urine. This is the biggest difference in the table and the one to quote. |
| Oxygen | Higher | Lower | Nothing to do with excretion — the kidney is an organ made of living cells, and they respire. This one catches people out because they expect every row to be about urine. |
| Carbon dioxide | Lower | Higher | Same reason: respiration in the kidney cells. |
| Glucose | Same | Same | All the glucose that is filtered out is reabsorbed, so none is lost. A tiny fall from respiration is possible, but the mark scheme wants “little or no change”. |
| Protein | Same | Same | Protein molecules are too large to be filtered out at all, so they never leave the blood. |
| Water and ions | Variable | Variable | Depends entirely on how much the person has drunk and eaten — the kidney removes the excess, whatever that happens to be that day. |
Cutting a Kidney in Half
Cambridge limits the internal structure of the kidney to two regions: the cortex on the outside and the medulla inside it. That is genuinely all you need — there is no third region to learn and no part of a nephron to locate on this diagram.
Cortex is the outer region. The word comes from the Latin for bark — and bark is on the outside of a tree. Medulla is the inner one, from the Latin for marrow, which is on the inside of a bone.
You will meet both words again in other subjects and in later biology, always with the same meaning: cortex outside, medulla inside. Learn the pair once and you never have to guess.
Three Sentences, and Then You Are Done
A nephron is one of about a million microscopic tubes in each kidney, and it is where urine is actually formed. Cambridge asks you to outline its structure and function, and then says something unusually generous in the next line: details of these processes are not required. That is a promise, and it is worth taking seriously. You do not need to know how the filter works, or what drives the reabsorption, or what happens in each separate stretch of tubing. You need three sentences.
1. Filtration at the glomerulus. Small molecules are filtered out of the blood into the nephron: water, glucose, urea and ions. Blood cells and proteins are too large and stay in the blood.
2. Reabsorption along the nephron. Useful substances are taken back into the blood: all of the glucose, some of the ions, and most of the water.
3. What is left is urine: urea, excess water and excess ions.
Learn those three lines in that order and every question in this section is a rearrangement of them.
ALL of the glucose. SOME of the ions. MOST of the water.
Mark schemes award those words specifically, because they are what tells the examiner you understand the design. “All” explains why healthy urine contains no glucose. “Some” explains why urine still contains ions. “Most” explains why urine is a small volume of liquid and not the 180 litres that are filtered every day.
Writing “glucose, ions and water are reabsorbed” with no quantities usually earns one mark out of three.
The Nephron and Its Blood Supply
Everything on the diagram below is on the syllabus, and nothing that is not on the syllabus is on the diagram. Trace it once with your finger: blood in, filter, tube, take back, what is left.
Two Reasons for the Same Empty Column
Healthy urine contains no glucose and no protein. Those two facts look identical and have completely different causes, and telling them apart is the most reliable way to score well in this section.
Protein molecules are too large to be filtered. They never leave the blood in the first place, so they are absent from the filtrate as well as from the urine.
Glucose is a small molecule and is filtered out, so it is present in the filtrate. It is absent from urine only because all of it is reabsorbed further along the nephron.
This is why the filtrate column of a table is such a good question: it is the one place where glucose and protein behave differently, and a candidate who has learned “neither appears in urine” without the reason gets it wrong.
Reading a Concentration Table
The classic question gives you numbers rather than words. Here is a typical set, in grams per 100 cm³. The units do not matter — what matters is which column each row empties in.
| Substance | Blood plasma | Filtrate | Urine |
|---|---|---|---|
| Water | 90.0 | 90.0 | 95.0 |
| Protein | 8.0 | 0.0 | 0.0 |
| Glucose | 0.10 | 0.10 | 0.0 |
| Urea | 0.03 | 0.03 | 2.00 |
| Ions | 0.72 | 0.72 | 1.50 |
It does, and there is a good reason. A filter that separated by size is simple to build; a filter that separated by usefulness would have to recognise every molecule individually. So the kidney does the easy thing first — push everything small out — and then reclaims the valuable items with a second, selective step.
If a question asks why glucose is filtered out at all when the body needs it, that is the answer: the filter cannot tell the difference, so the nephron corrects for it afterwards.
Where Urea Actually Comes From
Everything so far has been about getting urea out. This section is about where it comes from, and the answer is the liver — an organ that has already appeared twice in your course, in Topic 7 for bile and in Topic 9 for the hepatic portal vein. It is the same organ doing a third job here.
Start from a fact that sounds small and is not: the body cannot store amino acids. Carbohydrate can be stored as glycogen. Fat can be stored as fat. Amino acids cannot be stockpiled anywhere. So if you absorb more amino acids than you need for building proteins, the surplus has to be dealt with immediately — and dealing with it is what produces urea.
Assimilation — Building Up
Assimilation is what happens after absorption: the absorbed molecules are taken into cells and made part of the body, or used.
For amino acids specifically, the syllabus wants: the liver assimilates amino acids by converting them into proteins. The liver makes several of the proteins found dissolved in blood plasma — fibrinogen, the clotting protein you met in Topic 9, is one of them.
Absorption is crossing the wall of the small intestine. Assimilation is being built into something. They are different words for different events and both appear in mark schemes.
Deamination — Breaking Down
Deamination is the removal of the nitrogen-containing part of amino acids to form urea.
Three things to keep straight, all of them commonly lost:
— it happens in the liver, not the kidney;
— it happens to excess amino acids, not to all of them;
— the part removed is the nitrogen-containing part. Amino acids are the only one of the three food groups that contain nitrogen, which is exactly why they alone need this treatment. Carbohydrates and fats contain only carbon, hydrogen and oxygen, and can be broken down to carbon dioxide and water with nothing awkward left over.
What about the rest of the molecule? Once the nitrogen-containing part has been removed, what is left is essentially a carbohydrate, and the body treats it as one: it is respired to release energy, or stored as glycogen. That is why a very high-protein diet does not simply pass through you — the carbon in the surplus protein still ends up as energy or as a store.
Topic 7: it produces bile, which emulsifies fats and neutralises stomach acid.
Topic 9: it receives the hepatic portal vein, the one vein that carries blood from one organ to another rather than back to the heart — which is exactly how absorbed amino acids reach it first.
Topic 13: it assimilates amino acids into proteins and deaminates the excess to form urea.
Three topics, one organ. If a question mentions the hepatic portal vein in this topic, it is inviting you to say that every absorbed amino acid passes through the liver before it reaches the rest of the body.
Ten Words That Decide the Marks
This topic has very little mechanism in it, so examiners test it through precision. Almost every mark lost on Topic 13 is lost to one of the ten words below being used loosely.
| Say this | Not this | Because |
|---|---|---|
| excretion | “getting rid of waste” | Faeces are waste and are not excreted. The definition is about products of metabolism and substances in excess. |
| egestion | “excreting faeces” | Undigested food never entered a cell. Getting this wrong signals to an examiner that the whole concept is missing. |
| urea | “urine” (when you mean the chemical) | Urea is a dissolved substance; urine is the liquid containing it. “The liver makes urine” is a wrong answer built on a right idea. |
| ureter | “urethra” | Kidney to bladder, two of them. Read the position on the diagram if the spelling deserts you. |
| urethra | “ureter” | Bladder to outside, one of them. |
| filtered | “absorbed” / “taken out” | Filtration is the first step and it happens at the glomerulus, once. |
| reabsorbed | “absorbed” | The re matters: the substance is going back into the blood it came from. Absorption is what happens in the small intestine. |
| all / some / most | “the useful things are reabsorbed” | All the glucose, some of the ions, most of the water. Three separate marks live in those three words. |
| deamination | “breaking down protein” | Breaking protein into amino acids is digestion. Deamination removes the nitrogen-containing part of an amino acid. |
| assimilation | “absorption” | Absorption is crossing into the blood; assimilation is being built into the body as protein. |
How to Attack a Topic 13 Data Question
The numbers in this topic are always concentrations, and they always come as a table of three columns or as a comparison between two blood vessels. Work through them in the same order every time.
1. Read the column headings. Plasma, filtrate and urine behave completely differently, and a value means nothing until you know which one you are looking at.
2. Find the row that empties, and note where it empties. A zero in the filtrate column means protein. A zero in the urine column only means glucose.
3. For every rise in concentration, ask whether water was removed rather than whether solute was added. In this topic it is nearly always the water.
4. Quote figures. “Urea rises from 0.03 to 2.00, about 67 times” is very often a mark on its own and costs five seconds.
5. Separate the organ’s job from its respiration. When comparing blood in and blood out, urea is about excretion but oxygen and carbon dioxide are about the kidney cells respiring.
6. Do not claim more than the data allow. If a patient has glucose in the urine, the data show that reabsorption has failed — they do not tell you why, and a suggest question expects you to say so.
State — a fact, no reason needed. “State what is reabsorbed in the nephron” wants the list with its three quantity words, and nothing else.
Describe — say what happens, in order. Deamination described properly has three parts: excess amino acids, removal of the nitrogen-containing part, formation of urea.
Explain — every mark needs a because. “Explain why there is no protein in urine” is not answered by “there is no protein in urine”.
Suggest — you are being asked to apply the topic to something you have not been taught. There is usually more than one acceptable answer, and the marks are for the reasoning being consistent with the data.
Three Scenarios to Test Yourself On
Urea and water are handled differently by the nephron. Urea is not reabsorbed, so essentially all of what is filtered leaves the body — and how much is filtered depends on how much the liver made, which depends on the protein each person ate, not on how much they drank.
Water is reabsorbed, and the kidney removes only the amount that is in excess of requirements. Person A has a large excess, so little water is reabsorbed and a large volume of urine is produced. Person B has lost water in sweat and has none to spare, so almost all the filtered water is reabsorbed and the volume is small.
The same mass of urea dissolved in 2400 cm³ is dilute and pale; dissolved in 300 cm³ it is concentrated and dark. Concentration changed; amount did not. This distinction is the single most useful idea in the whole topic and it comes up in almost every data question.
The filtrate figure for protein is 0.0, which means protein never leaves the blood. The plasma figure of 8.0 is unchanged all the way through. Nothing has been removed — the molecules are simply too large to pass the filter.
Plasma proteins are useful and are made deliberately by the liver. Calling them waste inverts the biology. Only urea in this table is a waste product of metabolism.
The plasma and filtrate figures are identical at 0.03, which shows that urea passes freely through the filter. The jump to 2.00 in the urine is not extra urea being added but water being reabsorbed from around it. A correct answer quotes both of those observations.
“Urea is filtered out of the blood at the glomerulus, as shown by the identical plasma and filtrate values, and is not reabsorbed, so it leaves in the urine. Its concentration rises to 2.00 because most of the water around it is reabsorbed. Protein is not filtered at all because its molecules are too large, which is why the filtrate value is zero.”
Urea is being made normally but is not being removed, so the fault is with the kidneys. The liver has no way of slowing down; it deaminates whatever excess arrives, so a rise means the exit is blocked.
If the concentration is low and the kidneys are removing urea normally, then less is being made. Urea is made in the liver, so the fault lies there. Notice how the same measurement points at two opposite organs depending on which way it moves.
The urea figure is normal, so filtration and removal are working. Glucose in the urine means the second stage has failed: either reabsorption is not working, or there was so much glucose in the blood that the nephron could not reabsorb all of it. A suggest command means both answers are creditable provided the reasoning is stated.
One number, three diagnoses. Before writing anything, ask which stage of the story the observation belongs to — making the urea, filtering it out, or reabsorbing what should be kept.
The Night-Before Checklist
The definition of excretion, both halves. The definition of egestion and the test that tells them apart. Carbon dioxide is excreted through the lungs; the kidneys excrete urea, excess water and excess ions. The importance of excretion, limited to the toxicity of urea, in three linked steps. Kidneys, ureters, bladder, urethra — which is which and how many of each. Renal artery in, renal vein out, and what differs between them and why, including the two rows that are about respiration and not excretion. Cortex outside, medulla inside. What is filtered out at the glomerulus — four substances. What is too large to be filtered — two. What is reabsorbed, with all three quantity words. What urine is made of — three components, two of them with the word excess. Why there is no glucose in urine and why there is no protein in urine, and why those are different reasons. Where urea is made. What deamination is, in the syllabus wording. What happens to the remainder of the amino acid. What assimilation means. Why kidney failure raises blood urea while liver failure lowers it.
That list is the entire topic. If you can produce it out loud in four minutes, you are ready — and if a revision guide is telling you about anything else, it is telling you about a different syllabus.