Renin-Angiotensin-Aldosterone System (RAAS)
Hormonal cascade regulating blood pressure, fluid balance, and electrolytes.
Overview
When blood pressure or renal perfusion drops, juxtaglomerular cells secrete renin, which cleaves angiotensinogen (from liver) to angiotensin I. ACE (angiotensin-converting enzyme, primarily in lung endothelium) converts angiotensin I to angiotensin II. Ang II acts on AT1 receptors to cause vasoconstriction, aldosterone release (from adrenal cortex, promoting Na⁺/water retention), ADH release, and sympathetic activation. ACE2 converts Ang II to Ang(1-7), which has opposing (vasodilatory) effects via Mas receptor.
Cellular Location
Kidney → Lung → Adrenal → Systemic vasculature
Clinical Significance
ACE inhibitors and ARBs are first-line hypertension drugs; ACE2 is the SARS-CoV-2 receptor; aldosterone antagonists (spironolactone) in heart failure; RAAS overactivation in diabetic nephropathy.