A person drinks a large amount of water on a cold day. 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).
After drinking a large volume of water, the water content of the blood rises above the optimum level. The body must respond to restore the correct water balance.
Model answer (5 marks)
The hypothalamus senses that the blood is too dilute and signals the posterior pituitary to release less ADH.
The reduced ADH level means the kidney tubules and collecting ducts become less permeable to water.
Consequently less water is reabsorbed from the filtrate back into the bloodstream and more remains in the tubules.
A larger volume of dilute urine is therefore produced, which removes excess water from the body and restores the blood water content to the optimum level.
The reduced ADH level means the kidney tubules and collecting ducts become less permeable to water.
Consequently less water is reabsorbed from the filtrate back into the bloodstream and more remains in the tubules.
A larger volume of dilute urine is therefore produced, which removes excess water from the body and restores the blood water content to the optimum level.
Examiner tips
- Mention the hypothalamus detecting dilution first. State the decrease in ADH release. Explain the effect on tubule permeability and water reabsorption. Finish with the production of dilute urine and restoration of balance.
Common mistakes
- Forgetting to mention the hypothalamus or ADH. Saying the kidneys *increase* water reabsorption instead of decrease. Using vague terms like "the body drinks water" instead of specific renal actions.
Mark scheme (5 marks)
- The hypothalamus detects the rise in water content of the blood (or detects that the blood is too dilute)
- Less ADH is released / secreted by the pituitary gland
- The kidney tubules / collecting duct become less permeable to water
- Less water is reabsorbed back into the blood / more water remains in the tubules
- A larger volume of dilute urine is produced, reducing the water content of the blood back to the optimum level
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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