# Explain how the kidneys regulate blood osmolarity when a person becomes dehydrated.

> IB DP Biology Standard Level (2023 syllabus) — D3.3 Homeostasis · Explain · 4 marks

> During prolonged exercise in hot conditions, a person loses significant water through sweating, causing blood osmolarity to rise above the normal range.

## Mark scheme (4 marks)

1. Rise in blood osmolarity is detected by osmoreceptors in the hypothalamus
2. The posterior pituitary releases ADH (antidiuretic hormone) into the blood
3. ADH increases the permeability of the collecting duct (and distal convoluted tubule) to water
4. More water is reabsorbed by osmosis into the blood, producing a smaller volume of more concentrated urine, restoring blood osmolarity to normal (negative feedback)

## Key terms

- [osmolarity](https://www.gradenine.co.uk/glossary/osmolarity)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-kidneys-regulate-blood-36c7cd89) · Published by Druglandscape Ltd.