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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Excretion is the removal of toxic waste products of metabolism and substances in excess of requirements from the body. The kidneys excrete urea, excess water and mineral ions in urine. The lungs excrete carbon dioxide. The liver produces urea from excess amino acids.
The most common error is confusing excretion with egestion. Excretion removes waste products of metabolism (chemical reactions inside cells). Egestion removes undigested food that was never absorbed: it was never part of the body's metabolism. This distinction is tested directly.
Excretion versus egestion
Excretion: the removal of toxic waste products of metabolism and substances in excess of requirements. Examples: carbon dioxide from respiration, urea from deamination of excess amino acids.
Egestion: the removal of undigested food (faeces) from the body through the anus. This material passed through the alimentary canal but was never absorbed into the blood, so it is not a metabolic waste product.
Faeces are not excreted: they are egested. Using the wrong term in an answer costs marks.
Excretory organs and their waste products
Organ
Waste product excreted
Source
Kidneys
Urea, excess water, excess mineral ions
Urea from deamination of amino acids in the liver
Lungs
Carbon dioxide (and some water)
Aerobic respiration in all cells
Skin
Small amounts of urea and salts in sweat
Metabolic waste, though sweating is primarily for thermoregulation
Why excretion is necessary
Metabolic waste products are toxic if allowed to accumulate. Carbon dioxide dissolves in blood to form carbonic acid, which would lower blood pH dangerously. Urea at high concentrations is toxic to cells. Excess mineral ions would disrupt osmotic balance.
The body must continuously remove these substances to maintain a stable internal environment. This links excretion to homeostasis: the maintenance of a constant internal environment.
Try a focused question (3 marks)
Define excretion and explain why it is necessary.
Show the answer and marking guidance
Excretion is the removal of toxic waste products of metabolism and substances in excess of requirements.
These waste products, such as urea and carbon dioxide, are toxic if they accumulate.
Removal maintains a stable internal environment and prevents damage to cells.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Confusing excretion with egestion: faeces are egested, not excreted.
Saying excretion means removing all waste, including undigested food.
Forgetting that the lungs are excretory organs (they excrete carbon dioxide).
Describing excretion as just urination: it includes CO₂ excretion by the lungs too.