A person has not drunk any water for several hours and their blood water concentration has fallen below the normal level. Explain how the kidneys respond to return blood water concentration to normal.

Eduqas GCSE Biology — 4.3 Homeostasis in humans · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The pituitary gland releases more ADH.
ADH travels in the blood to the kidneys.
The kidneys reabsorb more water back into the blood.
A smaller volume of more concentrated urine is produced.

Examiner tips

  • Use the exact sequence of events; show cause and effect. Mention ADH and its effect on water reabsorption. Keep answer short – one sentence per point. Use UK spelling (e.g. ‘reabsorb’).

Common mistakes

  • Forgetting to mention ADH or its release from the pituitary. Describing the kidneys producing more urine instead of less. Using vague terms like ‘kidneys respond’ without specifying water reabsorption.

Mark scheme (4 marks)

  1. The pituitary gland releases more ADH (antidiuretic hormone)
  2. ADH travels in the blood to the kidneys
  3. The kidneys reabsorb more water (back into the blood)
  4. A smaller volume of more concentrated urine is produced

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