Cambridge IGCSE Biology 0610: Excretion in humans – Study Guide

This study guide teaches topic 13, Excretion in humans, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. It covers syllabus section 13.1 and every learning outcome in it. Outcomes 1–3 are Core, so every candidate needs them. Outcomes 4–9 are Supplement and are labelled Extended only: they are examined on Papers 2 and 4, which Extended candidates sit, but not on the Core Papers 1 and 3.

Other pages for this topic: revision notes for the final weeks, and a practice set with fully worked answers. For the whole course, see the Cambridge IGCSE Biology hub and the printable syllabus checklist. To find your gaps first, take the free Core diagnostic or Extended diagnostic.

What this topic covers

Syllabus ref	What you must be able to do	Tier
13.1.1	State that carbon dioxide is excreted through the lungs	Core
13.1.2	State that the kidneys excrete urea and excess water and ions	Core
13.1.3	Identify the kidneys, ureters, bladder and urethra in diagrams and images	Core
13.1.4	Identify the cortex and medulla of the kidney	Extended only
13.1.5	Outline the structure and function of a nephron and its blood vessels: filtration at the glomerulus, reabsorption, formation of urine	Extended only
13.1.6	Describe the role of the liver in assimilating amino acids into proteins	Extended only
13.1.7	State that urea is formed in the liver from excess amino acids	Extended only
13.1.8	Describe deamination	Extended only
13.1.9	Explain the importance of excretion, limited to the toxicity of urea	Extended only

What excretion means

Topic 1 of the syllabus defines excretion as the removal of the waste products of metabolism and substances in excess of requirements. Two parts of that definition matter.

• Waste products of metabolism are made by chemical reactions inside cells. Carbon dioxide from respiration and urea from the breakdown of amino acids are both examples.
• Substances in excess of requirements are things the body has more of than it needs, such as extra water and extra ions.

Do not confuse excretion with egestion, which the syllabus defines as the removal of undigested food from the body as faeces. Undigested food has never been inside cells, so it is not a product of metabolism. If a question asks for an excretory product, “faeces” scores nothing.

Excretory organs (Core)

The lungs

Carbon dioxide is excreted through the lungs. Carbon dioxide is produced by aerobic respiration in every living cell. It diffuses into the blood, is carried to the lungs, diffuses into the alveoli and is breathed out. The detail of gas exchange belongs to topic 11; for topic 13 you only need to state that the lungs are the organ that excretes carbon dioxide.

The kidneys

The kidneys excrete urea and excess water and ions. Together these make urine. The kidneys do not remove all the water and ions from the blood; they remove only the amounts that are in excess. That is why the volume and concentration of your urine change from day to day.

The urinary system

You must identify four structures on a diagram or photograph:

Structure	Where it is	What it does
Kidneys	Two bean-shaped organs at the back of the abdomen, one each side of the spine	Filter the blood and make urine
Ureters	One tube from each kidney down to the bladder	Carry urine from the kidney to the bladder
Bladder	Muscular bag low in the abdomen	Stores urine
Urethra	Single tube from the bladder to the outside	Carries urine out of the body

Ureter or urethra? There are two ureters (one per kidney) and one urethra. The ureters come first on the way out: kidney → ureter → bladder → urethra. A reliable memory aid: “ureter” has two e’s and there are two of them.

Structure of the kidney (Extended only)

If you cut a kidney lengthways you see two main regions:

• the cortex, the darker outer layer just under the surface
• the medulla, the inner region, arranged in cone-shaped sections

The ureter leaves from the concave (inner) side of the kidney. On a diagram, label the cortex by touching the outer band with your label line and the medulla by touching one of the inner cone-shaped regions. A label line that ends on a boundary between regions is ambiguous and may not be credited.

The nephron (Extended only)

Each kidney contains a very large number of tiny tubes called nephrons. Each nephron is the unit that filters blood and makes urine. The syllabus says the details of the processes are not required, so you need the outline below, not a molecule-by-molecule account.

Structure and associated blood vessels

A nephron starts with a cup-shaped capsule. Inside the cup sits the glomerulus, a knot of blood capillaries. Blood arrives at the glomerulus in a small blood vessel and leaves in another. After the capsule, the nephron continues as a long, coiled tubule, which is surrounded by more capillaries. The tubule finally drains into a collecting duct, which carries urine towards the ureter.

What happens along a nephron

1. Filtration at the glomerulus. Blood in the glomerulus is under high pressure. Water, glucose, urea and ions are filtered out of the blood into the capsule. The liquid formed is called the filtrate. Blood cells and large protein molecules stay in the blood because they are too large to be filtered.
2. Reabsorption along the nephron. As the filtrate passes along the tubule, useful substances are taken back into the blood in the surrounding capillaries. The nephron reabsorbs all of the glucose, some of the ions and most of the water.
3. Formation of urine. What is left in the nephron is urine. It contains urea, excess water and excess ions. It flows to the ureter and then to the bladder.

Learn the three reabsorption quantities exactly: all the glucose, some of the ions, most of the water. Mixing these up is the commonest way to lose a mark on this outcome. Healthy urine contains no glucose, because it has all been reabsorbed.

Worked example 1: how much water is reabsorbed?

In one person, the kidneys produce filtrate at a rate of 120 cm³ per minute. Urine is produced at 1.5 cm³ per minute. Calculate the percentage of the filtered water that is reabsorbed. Assume the filtrate and the urine are both almost entirely water.

Volume reabsorbed = volume filtered − volume of urine
                  = 120 − 1.5
                  = 118.5 cm³ per minute

Percentage reabsorbed = (118.5 ÷ 120) × 100
                      = 98.75
                      = 98.8% (3 significant figures)

This fits the syllabus statement that most of the water is reabsorbed. Only about 1.25% of the filtered volume leaves as urine.

Worked example 2: reading a data table

The table shows the concentration of three substances in the blood plasma, in the filtrate in the capsule, and in the urine.

Substance	Plasma / g per dm³	Filtrate / g per dm³	Urine / g per dm³
Protein	75	0	0
Glucose	1.0	1.0	0
Urea	0.30	0.30	18

(a) Explain why protein is absent from the filtrate. Protein molecules are too large to be filtered out of the blood at the glomerulus, so they stay in the blood.

(b) Explain why glucose is in the filtrate but not in the urine. Glucose is small enough to be filtered at the glomerulus, so it enters the filtrate at the same concentration as in plasma. All of the glucose is then reabsorbed into the blood along the nephron, so none is left in the urine.

(c) Calculate how many times more concentrated urea is in urine than in plasma.

18 ÷ 0.30 = 60 times

(d) Explain why urea is much more concentrated in urine. Urea is filtered but not reabsorbed in the way glucose is, while most of the water is reabsorbed. The same urea is therefore dissolved in a much smaller volume of water.

The liver and amino acids (Extended only)

Assimilation

After digestion, amino acids are absorbed into the blood and carried to the liver. The syllabus defines assimilation as the uptake and use of nutrients by cells. The liver assimilates amino acids by converting them to proteins. One example is fibrinogen, the plasma protein needed for blood clotting (topic 9).

Deamination and the formation of urea

The body cannot store excess amino acids. The liver deals with them by deamination: the removal of the nitrogen-containing part of amino acids to form urea.

Learn the two linked statements:

• Urea is formed in the liver from excess amino acids.
• Deamination is the removal of the nitrogen-containing part of amino acids, which is used to form urea.

Urea then travels in the blood plasma from the liver to the kidneys, where it is filtered out and excreted in urine. Trace the route in one sentence: excess amino acids → deamination in the liver → urea in the blood plasma → filtered at the kidney → excreted in urine.

Why excretion matters (Extended only)

The syllabus limits this outcome to the toxicity of urea. Urea is toxic. If the kidneys stopped removing it, urea would build up in the blood and in tissue fluid and would damage cells. Excretion keeps the concentration of urea in the blood low so that it does not reach harmful levels. When a question says “explain the importance of excretion”, your answer must name urea, say that it is toxic, and link its removal to preventing harm to cells.

Worked example 3: joining the ideas

Question: A person eats a meal containing much more protein than the body needs. Describe what happens to the excess amino acids and explain why the kidneys must then excrete the product.

Model answer:

• The amino acids are absorbed and carried in the blood to the liver.
• Some are assimilated: the liver converts them into proteins.
• The excess amino acids cannot be stored, so they are deaminated in the liver.
• Deamination removes the nitrogen-containing part of each amino acid, which forms urea.
• Urea is carried in the blood plasma to the kidneys.
• At the glomerulus urea is filtered from the blood; it is not reabsorbed like glucose, so it leaves in the urine.
• Urea is toxic, so it must be excreted before it builds up and damages cells.

Each bullet is one separate point, which is how structured-question mark schemes usually credit a description.

Common errors

• Calling faeces an excretory product. Faeces are egested, not excreted.
• Mixing up ureter and urethra. Two ureters take urine from kidneys to bladder; one urethra takes it out of the body.
• Saying the kidneys make urea. Urea is made in the liver. The kidneys excrete it.
• Saying glucose is not filtered. Glucose is filtered at the glomerulus; it is then all reabsorbed.
• Saying “some” of the water is reabsorbed. The syllabus says most of the water; it is “some” of the ions.
• Describing deamination as “breaking down protein”. Deamination acts on amino acids and removes the nitrogen-containing part.
• Explaining the importance of excretion without naming urea or toxicity. “To get rid of waste” is too vague for credit.
• Using the word “urine” for the filtrate. Filtrate becomes urine only after reabsorption.

Where to go next

• Condense everything above with the excretion revision notes.
• Test yourself with the excretion practice questions.
• The liver also controls blood glucose concentration; see the coordination and response study guide (section 14.4).
• Check your whole-course coverage with the 0610 checklist.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), published by Cambridge University Press & Assessment (Cambridge International Education). Topic 13, section 13.1 Excretion in humans.