# Explain how the body maintains blood osmolarity when a person becomes severely dehydrated.

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

> During prolonged exercise in hot conditions, significant water loss through sweating can raise blood osmolarity well above the normal range of approximately 285–295 mOsm kg⁻¹.

## Mark scheme (4 marks)

1. Osmoreceptors in the hypothalamus detect the rise in blood osmolarity / loss of water from osmoreceptor cells by osmosis
2. The hypothalamus signals the posterior pituitary to release ADH (antidiuretic hormone / vasopressin) into the blood
3. ADH increases the permeability of the collecting ducts (and distal convoluted tubules) of the nephron to water by inserting aquaporins into the tubule cell membranes
4. More water is reabsorbed from the filtrate back into the blood by osmosis, producing a smaller volume of more concentrated urine, reducing blood osmolarity back towards the set point (negative feedback)

## Key terms

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

## Related

- [Revision notes for IB DP Biology Higher 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-body-maintains-blood-7e6ad2f4) · Published by Druglandscape Ltd.