# A person is dehydrated after spending several hours in hot conditions without drinking water. Explain how the body detects and responds to this change to restore normal water balance.

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 6 Response to Change · Explain · 5 marks

> Water makes up a large proportion of the human body. When water levels fall below normal, the body must act quickly to restore balance and maintain a stable internal environment.

## Mark scheme (5 marks)

1. The change in water content / loss of water is detected by the hypothalamus (receptor in the brain)
2. The pituitary gland releases more ADH (anti-diuretic hormone) into the blood
3. ADH acts on the kidneys / kidney tubules
4. More water is reabsorbed from the kidney tubules back into the blood
5. A small volume of concentrated urine is produced, restoring water balance / returning water levels to normal

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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