Explain how the renin–angiotensin–aldosterone system (RAAS) maintains blood pressure homeostasis when blood pressure falls.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Juxtaglomerular cells in the kidney sense the fall in blood pressure and secrete renin into the bloodstream.
Renin cleaves angiotensinogen (from the liver) to form angiotensin I, which is then converted to angiotensin II by ACE in the lungs.
Angiotensin II causes vasoconstriction of arterioles, directly raising systemic blood pressure.
Angiotensin II also stimulates the adrenal cortex to release aldosterone, which increases Na⁺ (and water) reabsorption in the distal tubule/collecting duct, raising blood volume and sustaining blood pressure.
Renin cleaves angiotensinogen (from the liver) to form angiotensin I, which is then converted to angiotensin II by ACE in the lungs.
Angiotensin II causes vasoconstriction of arterioles, directly raising systemic blood pressure.
Angiotensin II also stimulates the adrenal cortex to release aldosterone, which increases Na⁺ (and water) reabsorption in the distal tubule/collecting duct, raising blood volume and sustaining blood pressure.
Examiner tips
- Use the exact sequence of events: renin → angiotensin I → angiotensin II → vasoconstriction & aldosterone release.
- Show the two main effects of angiotensin II – immediate vasoconstriction and long‑term volume increase via aldosterone.
Common mistakes
- Confusing the order of renin, angiotensinogen, angiotensin I/II or ACE location.
- Omitting the role of aldosterone or the site of Na⁺ reabsorption.
Mark scheme (4 marks)
- Juxtaglomerular cells in the kidney detect the fall in blood pressure and secrete renin into the blood.
- 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.
- Angiotensin II causes vasoconstriction of arterioles, which directly raises blood pressure.
- 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 in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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