A person has been exercising vigorously in hot weather and has not drunk any water for several hours. Explain what happens in the body to reduce the amount of water lost in urine.

WJEC A-Level Biology (Wales) — 3.7 Homeostasis and the kidney · Explain · 5 marks · View as Markdown

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

When the body loses more water than it takes in, the concentration of the blood plasma rises above normal levels.

Model answer (5 marks)

The increased plasma osmolality is sensed by osmoreceptors in the hypothalamus, which signals the posterior pituitary to release more ADH into the bloodstream. ADH acts on the collecting ducts of the kidney, increasing their permeability to water by inserting aquaporin channels. Consequently less water is excreted and a smaller volume of more concentrated urine is produced.

Examiner tips

  • Use the exact sequence of events: osmoreceptor → pituitary → ADH → collecting duct → water reabsorption
  • Mention the role of aquaporin channels to show understanding of mechanism
  • Keep answer concise and use correct terminology

Common mistakes

  • Confusing ADH with vasopressin or omitting the collecting duct step
  • Failing to explain that ADH increases water permeability, not solute transport
  • Using vague terms like "body responds" without detailing the hormonal pathway

Mark scheme (5 marks)

  1. The rise in blood plasma concentration / increased solute concentration is detected by osmoreceptors in the hypothalamus
  2. The pituitary gland releases more ADH (anti-diuretic hormone)
  3. ADH travels in the blood to the kidneys
  4. The collecting duct / tubules become more permeable to water
  5. A smaller volume of more concentrated urine is produced

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