Explain why the concentration of urine produced by a person is greater during vigorous exercise than when the person is resting.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
During vigorous exercise the body loses a large amount of water through sweat and respiration, so the water content of the blood falls. The kidneys therefore reabsorb more water from the filtrate in the renal tubules to maintain blood volume. As a result, less water remains in the urine and the urine becomes more concentrated.
Examiner tips
- Use the word ‘concentrated’ to show understanding of osmolality; mention reabsorption in the proximal tubule; link sweating to reduced plasma water; keep answer concise and to the point
Common mistakes
- Confusing ‘less water excreted’ with ‘more water excreted’; forgetting to mention reabsorption; using vague terms like ‘more concentrated’ without explaining the mechanism
Mark scheme (4 marks)
- During exercise, more water is lost through sweating / breathing
- Less water is available in the blood / water content of blood decreases
- Less water is reabsorbed back into the blood in the renal tubule / less water remains in the urine after reabsorption
- Less water is excreted in the urine, so urine is more concentrated
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →