RAAS System Overview

Overview

This lecture explains the renin–angiotensin–aldosterone system (RAAS), its role in long-term blood pressure regulation, its components, actions, and clinical significance.

RAAS Function and Components

• RAAS is a hormonal system responsible for long-term regulation of blood pressure.
• Baroreflex provides rapid, short-term blood pressure control, while RAAS acts over the long term.
• Juxtaglomerular cells in the kidney's afferent arterioles produce prorenin, which is converted to renin when blood pressure drops.

RAAS Cascade and Hormone Actions

• Renin converts angiotensinogen (from the liver) into angiotensin I (10 amino acids).
• Angiotensin I is converted to angiotensin II (8 amino acids) by angiotensin-converting enzyme (ACE), mostly in the lungs and kidneys.
• Angiotensin II causes sodium reabsorption in the kidney's proximal tubules and vasoconstriction in systemic arterioles.
• Angiotensin II stimulates adrenal cortex to release aldosterone, increasing sodium and water retention.
• In the brain, angiotensin II increases thirst, induces antidiuretic hormone (ADH) release, and reduces baroreceptor sensitivity.

Effects and Clinical Relevance

• Combined RAAS actions increase blood volume and blood pressure.
• Angiotensin II is rapidly degraded to angiotensin III and IV, which have smaller effects.
• Overactive RAAS can cause hypertension.
• ACE inhibitors and angiotensin receptor blockers are used to treat high blood pressure by targeting RAAS.

Key Terms & Definitions

• RAAS — Renin–angiotensin–aldosterone system, a hormonal pathway regulating blood pressure and fluid balance.
• Baroreflex — Rapid, reflexive response to sudden blood pressure changes.
• Juxtaglomerular cells — Specialized kidney cells releasing renin.
• Renin — Enzyme initiating RAAS by converting angiotensinogen to angiotensin I.
• Angiotensinogen — Liver-produced precursor to angiotensin peptides.
• Angiotensin II — Hormone causing vasoconstriction, sodium retention, and stimulation of aldosterone and ADH release.
• Aldosterone — Adrenal hormone promoting sodium and water retention.
• ACE — Angiotensin-converting enzyme, converts angiotensin I to II.

Action Items / Next Steps

• Review the RAAS cascade and associated hormones.
• Study mechanisms of antihypertensive drugs targeting RAAS.