A person has not drunk any water for several hours and their blood water concentration has dropped below the normal level. Explain how the body responds to restore normal blood water concentration.

OCR GCSE Biology A: Gateway Science (J247) — B3.3 Maintaining internal environments · Explain · 4 marks · View as Markdown

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

The body constantly monitors and adjusts its internal conditions. Water balance is one of the key variables that must be kept within narrow limits.

Model answer (4 marks)

The pituitary gland releases more ADH into the bloodstream.
ADH acts on the kidney, increasing water reabsorption by making the tubules more permeable to water.
This reduces water loss in the urine, so the urine becomes more concentrated.
The rise in blood water concentration restores it to normal, completing a negative‑feedback loop.

Examiner tips

  • Use the exact terms ADH, pituitary, kidney tubules, water reabsorption, negative feedback.
  • Show the sequence: pituitary → ADH → kidney → urine concentration → blood water concentration restored.
  • Mention that the loop stops once normal levels are reached.

Common mistakes

  • Confusing ADH with antidiuretic hormone or using the wrong gland (e.g., hypothalamus instead of pituitary).
  • Failing to state that the urine becomes more concentrated or that the loop is negative feedback.

Mark scheme (4 marks)

  1. The pituitary gland releases (more) ADH into the bloodstream
  2. ADH increases water reabsorption in the kidney / makes kidney tubules more permeable to water
  3. This raises/restores the blood water concentration back to normal / less water is lost in the urine (urine becomes more concentrated)
  4. This is an example of negative feedback / once water concentration is restored, less ADH is released

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