Explain why the kidneys are described as organs of excretion.

Pearson Edexcel International GCSE Biology (4BI1) — 2.13 Excretion: humans · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Excretion is the removal from the body of toxic materials and the waste products of metabolism. Urea is produced in the liver when excess amino acids are broken down. It is then transported in the blood to the kidneys.

Model answer (4 marks)

1. Excretion is the removal of waste products of metabolism and toxic materials from the body.
2. The kidneys filter the blood and remove urea, a waste product produced in the liver when excess amino acids are broken down.
3. By excreting urea, the kidneys prevent its accumulation and toxicity.
4. They also eliminate excess water, ions and salts in urine, maintaining body balance.

Examiner tips

  • Use the word ‘filter’ or ‘remove’ to show kidney action; mention urea explicitly; include water/ions to hit all points; keep each point short and clear.

Common mistakes

  • Confusing the liver with the kidneys as the excretory organ; omitting the removal of water/ions; writing vague statements like ‘kidneys keep the body clean’ without specific waste products.

Mark scheme (4 marks)

  1. Excretion is the removal of waste products of metabolism (from the body) / toxic materials
  2. The kidneys remove / filter urea from the blood
  3. Urea is a waste product / toxic product formed when excess amino acids are broken down (in the liver)
  4. The kidneys also excrete excess water / ions / salts in urine

Key terms in this question

excretion · kidneys

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