A person eats a large meal containing very little water on a hot day and sweats heavily. Explain how the kidneys respond to maintain water balance in the body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sweating causes a significant loss of water from the body. The kidneys play a central role in homeostasis by adjusting the volume and concentration of urine produced.
Model answer (5 marks)
Sweating reduces the amount of water in the blood, so the blood becomes more concentrated.
The hypothalamus senses this drop in water concentration.
It signals the posterior pituitary to release more ADH into the bloodstream.
ADH acts on the kidney tubules, especially the collecting ducts, making them more permeable to water.
Consequently more water is reabsorbed into the blood, producing a smaller volume of more concentrated urine, which restores water balance.
The hypothalamus senses this drop in water concentration.
It signals the posterior pituitary to release more ADH into the bloodstream.
ADH acts on the kidney tubules, especially the collecting ducts, making them more permeable to water.
Consequently more water is reabsorbed into the blood, producing a smaller volume of more concentrated urine, which restores water balance.
Examiner tips
- Use the exact sequence of events: sweat → blood concentration ↑ → hypothalamus → pituitary → ADH ↑ → kidney → water reabsorption ↑ → urine ↓ volume, ↑ concentration.
- Mention the collecting duct as the site of ADH action.
- Show cause–effect links clearly to earn all 5 marks.
Common mistakes
- Confusing ADH release with secretion of water; ADH is released, not water. Forgetting that ADH acts on the collecting duct. Using vague terms like "the kidneys" without specifying the collecting duct or water reabsorption.
Mark scheme (5 marks)
- Sweating causes blood water concentration to fall / blood becomes more concentrated
- The hypothalamus detects the decrease in blood water concentration
- The pituitary gland releases / secretes more ADH into the bloodstream
- ADH makes the kidney tubules / collecting duct more permeable to water
- More water is reabsorbed back into the blood / a smaller volume of more concentrated urine is produced
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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