# Explain how the renin–angiotensin–aldosterone system (RAAS) maintains blood pressure homeostasis when blood pressure falls.

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

> A person experiences a significant drop in blood pressure following heavy blood loss. Sensors in the kidney detect this change and initiate a hormonal cascade.

## Mark scheme (4 marks)

1. Juxtaglomerular cells in the kidney detect the fall in blood pressure and secrete renin into the blood.
2. Renin catalyses the conversion of angiotensinogen (produced by the liver) to angiotensin I, which is then converted to angiotensin II by ACE in the lungs.
3. Angiotensin II causes vasoconstriction of arterioles, which directly raises blood pressure.
4. Angiotensin II stimulates the adrenal cortex to secrete aldosterone, which increases sodium (and water) reabsorption in the collecting duct/distal tubule, raising blood volume and therefore blood pressure.

## Key terms

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

## 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-renin-angiotensin-aldosterone-system-raas-b5a3e602) · Published by Druglandscape Ltd.