Renin-Angiotensin-Aldosterone System

The renin-angiotensin-aldosterone system (RAAS) is the principal hormonal cascade for defending extracellular fluid volume and arterial pressure. When the kidney senses reduced perfusion, low distal sodium delivery, or sympathetic activation, it releases the enzyme renin, setting off a chain that generates angiotensin II and stimulates aldosterone. The net effect is to constrict blood vessels and to make the kidney retain sodium and water, restoring volume and pressure.

Definition

The renin-angiotensin-aldosterone system is a coordinated endocrine cascade in which renal renin initiates the production of angiotensin II, which raises blood pressure directly and stimulates adrenal aldosterone to promote renal sodium retention and potassium excretion.

Scope

The topic covers the enzymatic cascade from renin to angiotensin II, the stimuli that trigger renin release, and the renal and vascular actions of angiotensin II and aldosterone, including aldosterone's role in distal sodium reabsorption and potassium secretion. It is a physiological reference; it describes the system's biology and does not provide guidance on RAAS-modifying medications.

Core questions

• What stimuli trigger renin release from the kidney?
• How is angiotensin II generated from angiotensinogen?
• What are the vascular and renal actions of angiotensin II?
• How does aldosterone couple sodium retention to potassium secretion?

Key concepts

• Renin release from juxtaglomerular cells
• Angiotensinogen and angiotensin-converting enzyme (ACE)
• Angiotensin II as vasoconstrictor and growth/inflammatory mediator
• Aldosterone and the mineralocorticoid receptor
• Distal sodium retention and potassium secretion
• Macula densa and baroreceptor control of renin
• Negative feedback by restored volume and pressure

Mechanisms

Renin is secreted by juxtaglomerular cells in response to reduced renal perfusion pressure, low sodium chloride sensed at the macula densa, and beta-adrenergic stimulation. Renin cleaves circulating angiotensinogen to angiotensin I, which angiotensin-converting enzyme converts to angiotensin II. Angiotensin II raises blood pressure by direct vasoconstriction, by enhancing proximal tubular sodium reabsorption, and by stimulating the adrenal cortex to secrete aldosterone. Aldosterone acts on principal cells of the distal nephron through the mineralocorticoid receptor to increase epithelial sodium channel activity, retaining sodium and, through the resulting lumen-negative voltage, promoting potassium secretion. The restored volume and pressure feed back to suppress further renin release. Beyond acute regulation, sustained angiotensin II and aldosterone signalling can drive tissue inflammation and fibrosis.

Clinical relevance

The RAAS is central to the physiology of blood pressure and volume regulation and is one of the most important pharmacological targets in cardiovascular and renal medicine; understanding its normal operation underpins the interpretation of volume status and of renin and aldosterone measurements. This entry describes physiology and is not a guide to the use of any medication.

Evidence & guidelines

The cascade and its actions are summarised from physiology texts and from reviews of the system's role in salt and blood-pressure regulation and in aldosterone-mediated tissue injury. As a basic-science entry it does not adopt clinical practice guidelines.

History

The system traces to Tigerstedt and Bergman's 1898 discovery of renin as a pressor substance in kidney extracts; subsequent work in the mid-twentieth century identified angiotensin and the converting-enzyme step, and aldosterone's role in sodium retention was later integrated into the cascade. The historical sequence is summarised in standard physiology texts rather than verified here from primary sources.

Debates

How much of aldosterone's harm is independent of blood pressure?

Whether aldosterone contributes to cardiovascular and renal inflammation and fibrosis beyond its sodium-retaining, pressure-raising effects has been an active question in the physiology of the system.

Key figures

• Robert Tigerstedt
• Eduardo Braun-Menéndez
• Irvine Page

Seminal works

• he-2003
• palmer-2015-na

Frequently asked questions

What is the difference between RAS and RAAS?

RAS refers to the renin-angiotensin system - the cascade producing angiotensin II; RAAS adds aldosterone, the adrenal steroid that angiotensin II stimulates, emphasising the downstream effect on renal sodium and potassium handling.

How does the system raise blood pressure?

Angiotensin II constricts blood vessels and promotes sodium and water retention directly and through aldosterone, expanding extracellular volume; together these raise arterial pressure.