Cambridge IGCSE Biology 0610: Excretion in humans – Revision Notes

These revision notes cover topic 13, Excretion in humans, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028: section 13.1, outcomes 1–9. Outcomes 1–3 are Core and are tested on every paper. Outcomes 4–9 are Supplement, marked Extended only, and appear only on the Extended Papers 2 and 4. For full explanations and worked examples, use the excretion study guide.

Also useful: the excretion practice set, the Cambridge IGCSE Biology hub, the printable 0610 checklist, and the free Core diagnostic and Extended diagnostic.

Key definitions

Term	What to write
Excretion	The removal of the waste products of metabolism and substances in excess of requirements
Egestion	The removal of undigested food from the body as faeces (not excretion)
Urea	Nitrogen-containing waste made in the liver from excess amino acids; toxic
Deamination (Extended only)	The removal of the nitrogen-containing part of amino acids to form urea
Assimilation (Extended only)	The uptake and use of nutrients by cells; the liver converts amino acids to proteins
Glomerulus (Extended only)	Knot of capillaries at the start of a nephron where the blood is filtered
Filtrate (Extended only)	Liquid filtered out of the blood into the nephron; it becomes urine after reabsorption

Core: 13.1.1–13.1.3

What is excreted, and where

Excretory organ	What it excretes
Lungs	Carbon dioxide
Kidneys	Urea, excess water and excess ions (as urine)

The urinary system, in order

kidney  →  ureter  →  bladder  →  urethra  →  outside
(makes     (one per    (stores     (one only)
 urine)     kidney)     urine)

• Two ureters, one urethra.
• The kidneys lie at the back of the abdomen, one each side of the spine.
• The bladder sits low in the abdomen, in front.

Extended only: 13.1.4–13.1.5

Kidney structure

• Cortex: outer, darker layer.
• Medulla: inner region, made of cone-shaped sections.
• The ureter leaves the kidney from its inner, concave side.

Nephron: method in three steps

Step 1 – Filter. At the glomerulus, water, glucose, urea and ions are filtered from the blood into the nephron. Blood cells and proteins are too large and stay in the blood.

Step 2 – Reabsorb. Along the nephron, all of the glucose, some of the ions and most of the water go back into the blood.

Step 3 – Urine. What remains is urine: urea, excess water and excess ions. It drains to the ureter.

The syllabus says details of these processes are not required. Learn the three steps and the words all / some / most exactly.

Present or absent? A quick grid

Substance	Blood plasma	Filtrate	Healthy urine
Protein	Yes	No	No
Glucose	Yes	Yes	No
Urea	Yes (low)	Yes (low)	Yes (high)
Water	Yes	Yes	Yes (small volume)
Ions	Yes	Yes	Yes (the excess)

Worked reminder: percentage reabsorbed

% reabsorbed = (volume filtered − volume of urine) ÷ volume filtered × 100

Example: filtrate 120 cm³ per minute, urine 1.5 cm³ per minute → (118.5 ÷ 120) × 100 = 98.8%.

Worked reminder: concentration factor

times more concentrated = concentration in urine ÷ concentration in plasma

Urea is concentrated in urine because most of the water is reabsorbed while urea is not reabsorbed like glucose.

Extended only: 13.1.6–13.1.9

The liver and amino acids

amino acids absorbed → blood → liver
        ├── needed: assimilated → converted to proteins
        └── excess: deaminated → nitrogen-containing part removed → urea
urea → blood plasma → kidneys → filtered → excreted in urine

• Urea is formed in the liver, not in the kidneys.
• Excess amino acids cannot be stored, so they are deaminated.

Why excretion matters

The syllabus limits this to the toxicity of urea. Urea is toxic; if it were not excreted it would build up in the blood and damage cells. An answer must contain both ideas: urea and toxic / harmful to cells.

Links to other 0610 topics

Excretion questions often borrow ideas from other parts of the syllabus. Be ready to connect them.

• Topic 7, human nutrition: proteins are digested to amino acids, which are absorbed in the small intestine and carried to the liver. This is where the story of urea begins.
• Topic 9, transport in animals: plasma transports urea (and carbon dioxide) around the body. If a question asks how urea reaches the kidneys, the answer is “dissolved in the blood plasma”.
• Topic 11, gas exchange: carbon dioxide diffuses from the blood into the alveoli and is breathed out. For topic 13 you only need to state that the lungs excrete it.
• Topic 12, respiration: carbon dioxide is a waste product of aerobic respiration, which makes it a product of metabolism.
• Topic 14, homeostasis: the liver also helps control blood glucose concentration, see the coordination and response revision notes.

Answer frames for common question types

“State where urea is made and where it is excreted.” One organ each: made in the liver; excreted by the kidneys. Do not add the lungs; they excrete carbon dioxide only.

“Outline the role of the nephron.” Three sentences in order: filtration at the glomerulus (name water, glucose, urea, ions) → reabsorption (all glucose, some ions, most water) → urine (urea, excess water, excess ions).

“Explain why substance X is (or is not) in the urine.” Decide two things. Was it filtered at the glomerulus? Was it reabsorbed? Protein: not filtered. Glucose: filtered, all reabsorbed. Urea: filtered, not reabsorbed like glucose, so it stays in the urine.

“Describe deamination.” Use the syllabus words: removal of the nitrogen-containing part of amino acids to form urea, and say it happens in the liver.

“Explain the importance of excretion.” Name urea, say it is toxic, say it would build up and damage cells if not removed.

“Name the structures urine passes through.” Always start at the kidney and finish at the urethra. If the question gives a diagram, check whether it shows one kidney or both, and remember that each kidney has its own ureter.

Must-know distinctions

Don’t confuse	With	The difference
Excretion	Egestion	Excretion removes products of metabolism; egestion removes undigested food
Ureter	Urethra	Ureters (two) carry urine kidney → bladder; urethra (one) carries it bladder → outside
Filtrate	Urine	Filtrate still contains glucose and far more water; urine is what is left after reabsorption
Filtration	Reabsorption	Filtration takes substances out of the blood at the glomerulus; reabsorption returns useful substances to the blood
Liver	Kidney	Liver makes urea; kidneys excrete it
Deamination	Digestion	Digestion breaks proteins into amino acids; deamination removes the nitrogen-containing part from amino acids

Quick self-test

1. Name the organ that excretes carbon dioxide.
2. List the three things the kidneys excrete.
3. Put these in the order urine passes through them: bladder, urethra, kidney, ureter.
4. How many ureters does a person have? (Core)
5. (Extended) Name the two regions of the kidney seen in a lengthways section.
6. (Extended) Name the part of the nephron where blood is filtered.
7. (Extended) Which substance is completely reabsorbed along the nephron?
8. (Extended) A kidney filters 150 dm³ of fluid in a day and produces 1.2 dm³ of urine. Calculate the percentage of the filtered volume that is reabsorbed.
9. (Extended) Urea is 0.25 g per dm³ in plasma and 20 g per dm³ in urine. How many times more concentrated is urea in the urine?
10. (Extended) Name the organ where urea is formed, and the process that forms it.
11. (Extended) Define deamination.
12. (Extended) Explain why the body must excrete urea.

Answers

1. The lungs.
2. Urea, excess water and excess ions.
3. Kidney → ureter → bladder → urethra.
4. Two (one from each kidney).
5. Cortex (outer) and medulla (inner).
6. The glomerulus.
7. Glucose.
8. (150 − 1.2) ÷ 150 × 100 = 148.8 ÷ 150 × 100 = 99.2%.
9. 20 ÷ 0.25 = 80 times.
10. The liver; deamination.
11. The removal of the nitrogen-containing part of amino acids to form urea.
12. Urea is toxic, so if it built up in the blood it would damage cells.

Where marks are usually lost

• Writing “faeces” or “undigested food” as an excretory product.
• Swapping ureter and urethra, or drawing the ureter leaving the bladder.
• Saying the kidneys produce urea. They excrete it; the liver makes it.
• Saying glucose is not filtered. It is filtered, then all reabsorbed.
• Writing “some water” or “all the ions” for reabsorption. It is most of the water and some of the ions.
• Giving detailed mechanisms (for example active transport in the tubule) instead of the three-step outline the question needs; extra detail uses time but earns nothing extra.
• Defining deamination as “breaking down proteins” or “removing amino acids”. It removes the nitrogen-containing part of amino acids.
• Explaining the importance of excretion as “removing waste” without saying urea is toxic.
• Dropping units or rounding too early in a percentage calculation; give the final answer to a sensible number of significant figures.
• Labelling the cortex with a line that stops on the boundary between cortex and medulla.

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.