A long-distance runner loses a large amount of water and produces very little urine during a race. After the race, the runner drinks several litres of water and begins to produce large amounts of dilute urine. Explain how the kidneys and the process of excretion are involved in removing waste products from the body, and why the concentration of urine changes between during the race and after the race.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The kidneys are organs of excretion. They filter the blood and produce urine, which contains waste products including urea. Urea is produced in the liver from the breakdown of excess amino acids. The concentration of urine produced by the kidneys can change depending on conditions in the body.
Model answer (5 marks)
1. Excretion is the removal of waste products produced by the body’s metabolic reactions.
2. Urea is a waste product formed in the liver from the breakdown of excess amino acids.
3. The kidneys filter the blood and remove urea (and other waste products) to produce urine.
4. During the race the runner loses water through sweat, so less water is available for the kidneys; they reabsorb more water, producing a small, concentrated volume of urine.
5. After the race the runner drinks several litres of water, increasing body water and blood volume; the kidneys reabsorb less water, producing a large volume of dilute urine.
2. Urea is a waste product formed in the liver from the breakdown of excess amino acids.
3. The kidneys filter the blood and remove urea (and other waste products) to produce urine.
4. During the race the runner loses water through sweat, so less water is available for the kidneys; they reabsorb more water, producing a small, concentrated volume of urine.
5. After the race the runner drinks several litres of water, increasing body water and blood volume; the kidneys reabsorb less water, producing a large volume of dilute urine.
Examiner tips
- Use the exact terminology (excretion, urea, kidneys, reabsorb).
- Show the cause–effect link between water loss and urine concentration.
- Include both the production of urea and its removal by the kidneys.
Common mistakes
- Confusing excretion with secretion or failing to mention urea.
- Losing the link between water intake and kidney reabsorption.
Mark scheme (5 marks)
- Excretion is the removal of waste products made by the body's metabolic reactions
- Urea is a waste product that must be excreted / urea is produced from the breakdown of excess amino acids (in the liver)
- The kidneys filter the blood and remove urea (and other waste products) to produce urine
- During the race, the body loses water through sweating, so less water is available, and the kidneys reabsorb more water, producing less / more concentrated urine
- After the race, the runner drinks water so there is more water in the body / blood, and the kidneys reabsorb less water, producing more / dilute urine
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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