Explain how the kidneys help to maintain water balance in the human body when a person becomes dehydrated.

Eduqas GCSE Biology — 2.5.3 Homeostasis · Explain · 4 marks · View as Markdown

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

A person spends a long time exercising on a hot day and loses a large amount of water through sweating. They do not drink any water during this time.

Model answer (4 marks)

The blood water potential falls, signalling dehydration. The hypothalamus senses the change and releases more antidiuretic hormone (ADH) from the posterior pituitary. ADH acts on the kidneys, increasing water reabsorption in the collecting ducts. Consequently less water is excreted and the urine becomes more concentrated, helping to restore water balance.

Examiner tips

  • Use the exact terms: blood water potential, hypothalamus, ADH, posterior pituitary, reabsorption, concentrating urine
  • Show the causal chain: dehydration → low water potential → hypothalamus → ADH → kidney action → less urine
  • Keep the answer concise, 4 points, no extra detail

Common mistakes

  • Mixing up ADH with oxytocin or missing the role of the hypothalamus
  • Failing to mention that urine becomes more concentrated
  • Using vague phrases like ‘body tries to fix it’ instead of specific mechanisms

Mark scheme (4 marks)

  1. The blood water potential/concentration is detected as having fallen (becomes too low / too concentrated)
  2. The hypothalamus / brain detects the change and more ADH is released from the pituitary gland
  3. The kidneys reabsorb more water back into the blood
  4. Less urine is produced / urine is more concentrated

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