A person is dehydrated after spending several hours in hot conditions without drinking water. Explain how the body detects and responds to this change to restore normal water balance.

Pearson Edexcel A-Level Biology A (9BI0) — Topic 6 Response to Change · Explain · 5 marks · View as Markdown

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

Water makes up a large proportion of the human body. When water levels fall below normal, the body must act quickly to restore balance and maintain a stable internal environment.

Model answer (5 marks)

The loss of water is sensed by osmoreceptors in the hypothalamus, which detect an increase in plasma osmolality. The hypothalamus signals the posterior pituitary to release more ADH into the bloodstream. ADH binds to receptors on the collecting ducts of the kidneys, increasing their permeability to water via aquaporin channels. Consequently, more water is re‑absorbed from the tubules back into the blood, producing a small volume of highly concentrated urine. This restores the body’s water balance and normalises plasma osmolality.

Examiner tips

  • Mention the hypothalamic osmoreceptors first, then the posterior pituitary ADH release, kidney action and urine concentration.
  • Use the exact terms: hypothalamus, ADH, posterior pituitary, collecting ducts, aquaporins, re‑absorption, concentrated urine.
  • Show the sequence clearly – detection → hormone release → kidney response → outcome.

Common mistakes

  • Failing to specify the hypothalamus as the detector or the posterior pituitary as the source of ADH.
  • Describing ADH action as affecting the glomerulus instead of the collecting ducts.
  • Not mentioning that the urine becomes concentrated or that water balance is restored.

Mark scheme (5 marks)

  1. The change in water content / loss of water is detected by the hypothalamus (receptor in the brain)
  2. The pituitary gland releases more ADH (anti-diuretic hormone) into the blood
  3. ADH acts on the kidneys / kidney tubules
  4. More water is reabsorbed from the kidney tubules back into the blood
  5. A small volume of concentrated urine is produced, restoring water balance / returning water levels to normal

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