Describe the role of the kidneys and the hormone ADH in maintaining water balance in the body when a person has been exercising on a hot day and has not drunk enough water.

Eduqas A-Level Biology — 4.5 Homeostasis and the kidney · Describe · 5 marks · View as Markdown

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

A person has been exercising on a hot day and has not drunk enough water. As a result, they lose water through sweating and their blood water concentration falls below the normal level.

Model answer (5 marks)

The hypothalamus senses the drop in blood water concentration.
The pituitary gland releases more ADH into the bloodstream.
ADH travels to the kidneys.
ADH increases the permeability of the kidney tubules/collecting duct to water.
More water is reabsorbed into the blood, producing a small volume of concentrated urine.

Examiner tips

Use the exact sequence of events: hypothalamus → pituitary → ADH → kidneys → water reabsorption.
Show the effect of ADH on the collecting duct (increased permeability).
Mention the result: concentrated urine and restored water balance.

Common mistakes

Confusing the hypothalamus with the pituitary as the sensor.
Forgetting to state that ADH acts on the collecting duct.
Describing urine as dilute instead of concentrated.

Mark scheme (5 marks)

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

Key terms in this question

ADH · kidney