Explain how the body uses ADH to respond when a person becomes dehydrated after exercising on a hot day.

WJEC A-Level Biology (Wales) — 4.5 Homeostasis and the kidney · Explain · 5 marks · View as Markdown

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

During exercise on a hot day, a person loses water through sweating. This causes the water content of the blood to fall below the optimum level.

Model answer (5 marks)

The hypothalamus senses the fall in blood water content and signals the posterior pituitary to release more ADH. ADH enters the bloodstream and travels to the kidney, where it binds to receptors on the collecting ducts. This action increases the permeability of the ducts to water, allowing more water to be re‑absorbed back into the blood. Consequently, a smaller volume of more concentrated urine is produced, restoring water balance and homeostasis.

Examiner tips

Use the word ‘secretes’ for the pituitary, not ‘produces’.
Show the sequence: detection → release → action in kidney → effect on urine.
Mention that ADH acts on the collecting ducts, not the proximal tubule.

Common mistakes

Confusing ADH with vasopressin or omitting the collecting duct.
Saying the hypothalamus releases ADH instead of the pituitary.

Mark scheme (5 marks)

The hypothalamus detects the fall in water content of the blood (acts as a receptor / monitors blood water concentration).
The pituitary gland secretes / releases more ADH into the bloodstream.
ADH travels in the bloodstream to the kidney (tubules / collecting duct).
ADH makes the kidney tubules / collecting duct more permeable to water.
More water is reabsorbed back into the blood, so a smaller volume of more concentrated urine is produced, restoring water balance / homeostasis.

Key terms in this question

ADH