Overview
This lecture explains the renin–angiotensin–aldosterone system (RAAS), highlighting its components, sequence of actions, physiological effects, and clinical relevance to blood pressure regulation.
RAAS Overview and Activation
- RAAS is a hormonal system responsible for long-term regulation of blood pressure.
- Baroreflex acts as a short-term response, while RAAS maintains long-term blood pressure control.
- Juxtaglomerular cells in the kidney's afferent arterioles produce prorenin and release renin when blood pressure drops.
Sequence of Hormonal Events
- Renin converts angiotensinogen (from the liver) into angiotensin I.
- Angiotensin-converting enzyme (ACE), mainly in the lungs/kidneys, transforms angiotensin I into angiotensin II.
Actions of Angiotensin II
- Angiotensin II increases sodium reabsorption in the kidney's proximal tubules.
- It causes vasoconstriction (narrowing) of systemic arterioles, raising blood pressure.
- Triggers aldosterone release from adrenal cortex, which promotes sodium and water retention in kidneys.
- Stimulates thirst via the hypothalamus and water intake.
- Promotes antidiuretic hormone release from the posterior pituitary, increasing kidney water retention.
- Lowers baroreceptor sensitivity to elevated blood pressure, allowing continued RAAS activity.
- All effects together increase blood volume and blood pressure.
Termination and Clinical Relevance
- Angiotensin II has a short half-life (1–2 minutes), degraded into weaker metabolites (angiotensin III, IV).
- Overactive RAAS can lead to hypertension (high blood pressure).
- RAAS inhibitors (ACE inhibitors and angiotensin receptor blockers) are common antihypertensive medications.
Key Terms & Definitions
- RAAS (Renin–Angiotensin–Aldosterone System) — A hormone system controlling long-term blood pressure.
- Juxtaglomerular cells — Specialized kidney cells releasing renin.
- Renin — Enzyme converting angiotensinogen to angiotensin I.
- Angiotensinogen — Liver-produced plasma protein, renin’s substrate.
- Angiotensin I/II — Peptides produced in sequence; II is the active hormone.
- ACE (Angiotensin-Converting Enzyme) — Enzyme converting angiotensin I to angiotensin II.
- Aldosterone — Adrenal hormone promoting kidney sodium/water retention.
- Antidiuretic hormone (ADH) — Hormone increasing kidney water reabsorption.
Action Items / Next Steps
- Review RAAS cascade steps and associated hormones.
- Understand antihypertensive drug mechanisms targeting RAAS.