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

Eduqas A-Level Biology — 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 a person does not drink enough water, the concentration of their blood plasma rises above normal levels.

Model answer (5 marks)

The hypothalamus senses the rise in blood plasma osmolarity and signals the posterior pituitary to release more ADH.
The ADH enters the bloodstream and travels to the kidneys.
In the renal tubules ADH binds to receptors on the collecting duct cells.
This binding causes the insertion of aquaporin channels, making the ducts more permeable to water.
Consequently, less water is excreted and a smaller volume of concentrated urine is produced.

Examiner tips

  • Use the exact sequence: hypothalamus → pituitary → ADH → kidney tubules → water reabsorption → concentrated urine.
  • Mention the role of aquaporin channels to show understanding of the mechanism.
  • Keep the answer concise – one sentence per point to fit the 5‑mark structure.

Common mistakes

  • Confusing ADH with vasopressin or omitting the aquaporin step.
  • Writing the process in the wrong order or missing the hypothalamus detection step.
  • Using vague terms like "more water" instead of specifying increased permeability and concentrated urine.

Mark scheme (5 marks)

  1. The hypothalamus detects the increase in blood plasma concentration (water potential falls / blood becomes more concentrated)
  2. The pituitary gland releases more ADH (antidiuretic hormone)
  3. ADH travels in the blood to the kidneys
  4. The kidney tubules become more permeable to water
  5. A smaller volume of more concentrated urine is produced

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