A person drinks a large volume of water on a hot day. Describe how the body responds to maintain water balance.

Eduqas A-Level Biology — 3.7 Homeostasis and the kidney · Describe · 5 marks · View as Markdown

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

After drinking a large volume of water, the concentration of the blood plasma falls below normal levels.

Model answer (5 marks)

The fall in blood plasma concentration is detected by the hypothalamus, which signals the posterior pituitary to release less ADH. Consequently the kidneys reabsorb less water, so more water is excreted in the urine, making it more dilute and increasing urine volume. This restores the blood plasma concentration to normal, maintaining water balance.

Examiner tips

  • Use the exact sequence: detection → ADH ↓ → renal water reabsorption ↓ → urine ↑ and dilute → homeostasis restored.
  • Show the cause–effect chain clearly; include all five points.
  • Use UK spelling (e.g., ‘urine’).

Common mistakes

  • Failing to mention the hypothalamus as the detector.
  • Confusing ADH release with ADH secretion; or omitting the effect on kidneys.
  • Not specifying that urine becomes more dilute and larger in volume.

Mark scheme (5 marks)

  1. The fall in blood plasma concentration / lower water concentration in blood is detected by the hypothalamus
  2. Less ADH is released / secreted from the pituitary gland
  3. The kidneys reabsorb less water back into the blood
  4. More water passes out in the urine / urine becomes more dilute / larger volume of urine is produced
  5. This returns blood plasma concentration back to normal / restores water balance (homeostasis)

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