Cambridge O Level Biology 5090: Excretion (5090) – Revision Notes

For full explanations and worked examples, use the study guide for this topic. These notes condense Topic 13, Excretion, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4): every learning outcome in sections 13.1 Excretion (13.1.1 to 13.1.3) and 13.2 Urinary system (13.2.1 to 13.2.6). The syllabus is not tiered, so all of it applies to every candidate.

When you are ready, move on to the practice questions. The course hub and the printable checklist cover the rest of the course, and the free 10-minute diagnostics show where to focus. Absorption, assimilation and the hepatic portal vein are revised in Human nutrition; carbon dioxide crossing the alveoli is in Human gas exchange.

Key definitions

Term	Meaning
Excretion	Removal of toxic materials and the waste products of metabolism from organisms
Metabolism	All the chemical reactions in cells (for example respiration, deamination)
Egestion	Removal of undigested food as faeces (not excretion)
Assimilation	Uptake and use by cells of nutrients from the blood
Deamination	Removal of the nitrogen-containing part of amino acids, in the liver, forming urea
Urea	Toxic waste product made in the liver from excess amino acids
Urine	Solution of urea, excess water and excess ions made by the kidneys
Nephron	One of the many tiny tubes in a kidney that makes urine
Filtrate	Fluid filtered from the blood into the Bowman’s capsule

13.1 The excretory products

Product	Where it is made	How it is made	Excreted by
Carbon dioxide	Every living cell	Respiration	Lungs
Urea	Liver	Deamination of excess amino acids	Kidneys (in urine)
Excess salts and water	(taken in with food and drink)	—	Kidneys (in urine)

Carbon dioxide route: cell → plasma → lungs → diffuses into alveoli → breathed out.

13.2.5 and 13.2.6 The liver and amino acids

amino acids absorbed in small intestine
        │ hepatic portal vein
        ▼
      LIVER
        ├─ needed  → ASSIMILATION: built into proteins (e.g. plasma proteins)
        └─ excess  → DEAMINATION: nitrogen-containing part removed
                          │
                          ▼
                        UREA → blood plasma → kidneys → urine

• Excess amino acids cannot be stored, so they are deaminated.
• Urea is made in the liver, not the kidneys.
• The rest of a deaminated amino acid can be used, for example in respiration.

13.2.2 Why urea must be excreted

• Urea is toxic.
• The liver keeps making it, so without excretion its concentration in the blood would keep rising and damage cells.
• That is the only reason the syllabus asks for here: keep the answer on toxicity of urea.

13.2.1 The urinary system

Organ	Number	Function
Kidney	2	Removes urea and excess salts and water from the blood as urine
Ureter	2	Carries urine from kidney to bladder
Bladder	1	Stores urine
Urethra	1	Carries urine from bladder out of the body

Memory hook: ureter comes two-by-two from the kidneys; urethra goes to the outside.

Blood supply: renal artery in (more urea), renal vein out (less urea).

13.2.3 Nephron: parts in order

Fluid route:

Bowman's capsule → tubule (first coiled part) → loop of Henle → tubule (second coiled part) → collecting duct → ureter

Blood route:

renal artery branch → arteriole → glomerulus → arteriole → capillaries around the tubules → renal vein

Part	Recognise it as
Glomerulus	Knot of capillaries inside the capsule
Bowman’s capsule	Cup around the glomerulus
Tubules	Coiled tubes before and after the loop
Loop of Henle	U-shaped section
Collecting duct	Shared tube draining many nephrons

13.2.4 Nephron: what happens (outline only)

Method in three steps

1. Filtration at the glomerulus: water, glucose, urea and ions leave the blood and enter the Bowman’s capsule. Blood cells and large proteins stay in the blood.
2. Reabsorption along the nephron: all the glucose, some of the ions and most of the water go back into the blood.
3. Urine forms: urea, excess water and excess ions flow down the collecting duct.

Details of how filtration and reabsorption happen are not required.

What is where

Substance	Plasma	Filtrate	Urine (healthy)
Blood cells, large proteins	Yes	No	No
Glucose	Yes	Yes	No
Urea	Yes	Yes	Yes, much more concentrated
Ions	Yes	Yes	Yes (the excess)
Water	Yes	Yes	Yes (a small fraction of what was filtered)

Small worked reminders

• Urea in plasma 0.25 g/dm³, in urine 15 g/dm³: 15 ÷ 0.25 = 60 times more concentrated, because water is reabsorbed but urea is not.
• Filtrate 150 dm³ per day, urine 1.5 dm³ per day: (150 − 1.5) ÷ 150 × 100 = 99% of the water reabsorbed.

Answer frames to learn

Short model answers you can adapt. Each line is one separate point.

“Describe deamination.”

• Happens in the liver.
• The nitrogen-containing part of an excess amino acid is removed.
• It is converted into urea, which enters the blood.

“Describe how carbon dioxide is excreted.”

• Made in respiration in body cells.
• Diffuses into the blood and is carried in the plasma to the lungs.
• Diffuses into the alveoli and is breathed out.

“Explain why the urine of a healthy person contains no glucose.”

• Glucose is filtered at the glomerulus into the Bowman’s capsule.
• All of it is reabsorbed into the blood along the nephron.

“Explain why urea is more concentrated in urine than in plasma.”

• Urea is filtered but not reabsorbed.
• Most of the water is reabsorbed.
• So the urea is left in a much smaller volume of water.

“Explain why blood in the renal vein has less urea than blood in the renal artery.”

• Urea is filtered out of the blood in the glomerulus.
• It is not reabsorbed, so it leaves in urine instead of returning to the blood.

Links to other topics

• Human nutrition (8.3): amino acids are absorbed in the small intestine and reach the liver in the hepatic portal vein. Assimilation is the uptake and use of nutrients by cells.
• Human gas exchange (9.1): carbon dioxide diffuses from the blood into the alveoli.
• Blood (11.4): plasma transports carbon dioxide, urea and plasma proteins.
• Blood vessels (11.3): hepatic vein carries blood (and urea) away from the liver; renal artery and renal vein serve the kidneys.

Must-know distinctions

• Excretion vs egestion: excretion removes waste made by metabolism (carbon dioxide, urea); egestion removes undigested food (faeces).
• Assimilation vs deamination: both happen to amino acids in the liver. Assimilation builds them into proteins; deamination breaks down the excess.
• Urea vs urine: urea is one compound; urine is the whole solution.
• Ureter vs urethra: two ureters (kidney to bladder); one urethra (bladder to outside).
• Filtration vs reabsorption: filtration moves substances out of the blood into the nephron; reabsorption moves useful ones back into the blood.
• Glomerulus vs Bowman’s capsule: the glomerulus is blood capillaries; the capsule is part of the nephron.
• “All” vs “most” vs “some”: all the glucose, most of the water, some of the ions.

Quick self-test

1. Define excretion.
2. Name the waste product of respiration and the organ that excretes it.
3. In which organ is urea made?
4. Define deamination.
5. What does the liver do with amino acids that the body needs?
6. State the function of the bladder.
7. Which tube carries urine out of the body?
8. Name the knot of capillaries inside the Bowman’s capsule.
9. List the four substances the syllabus says are filtered at the glomerulus.
10. Which filtered substance is completely reabsorbed?
11. Why must urea be excreted?
12. Filtrate is formed at 180 dm³ per day and urine at 1.8 dm³ per day. What percentage of the water is reabsorbed? Urea is 0.4 g/dm³ in plasma and 24 g/dm³ in urine: how many times more concentrated is it in urine?

Answers

1. Removal of toxic materials and the waste products of metabolism from organisms.
2. Carbon dioxide; the lungs.
3. The liver.
4. Removal of the nitrogen-containing part of amino acids, forming urea.
5. Converts them to proteins (assimilation).
6. Stores urine.
7. The urethra.
8. The glomerulus.
9. Water, glucose, urea and ions.
10. Glucose.
11. Urea is toxic; if it built up in the blood it would damage cells.
12. (180 − 1.8) ÷ 180 × 100 = 99%; 24 ÷ 0.4 = 60 times.

Where marks are usually lost

• Writing “the kidneys produce urea” instead of “the liver produces urea; the kidneys excrete it”.
• Defining excretion as “getting rid of waste” without saying the waste comes from metabolism.
• Including faeces as an excretory product.
• Swapping ureter and urethra on a labelled diagram.
• Saying glucose “is not filtered” when it is filtered and then fully reabsorbed.
• Writing “all the water is reabsorbed”: urine would then contain no water.
• Describing deamination as “breaking down protein”: it removes the nitrogen-containing part of excess amino acids.
• Placing the glomerulus in the tubule or the loop of Henle; it sits inside the Bowman’s capsule.
• Explaining the need for excretion with vague words like “harmful waste” without naming urea as toxic.
• Giving a fold-change as a subtraction (15 − 0.25) instead of a division (15 ÷ 0.25).

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 13: Excretion (13.1 Excretion; 13.2 Urinary system), published by Cambridge Assessment International Education.