Renin-Angiotensin System

The renin-angiotensin system is an endocrine cascade that regulates blood pressure and fluid balance over minutes to days. The kidney releases renin in response to low perfusion pressure, reduced sodium delivery, or sympathetic stimulation; renin initiates a cascade producing angiotensin II, a potent vasoconstrictor that also stimulates aldosterone release and promotes sodium and water retention.

Definition

The renin-angiotensin system is a hormonal cascade in which renin cleaves angiotensinogen to angiotensin I, which is converted by angiotensin-converting enzyme to angiotensin II, the principal effector that raises vascular resistance and, via aldosterone, promotes renal sodium and water retention.

Scope

This topic covers the enzymatic cascade from renin to angiotensin II, the stimuli for renin release, the actions of angiotensin II on vessels, the kidney, and the adrenal cortex, and the system's role in longer-term blood-pressure and volume control. It is a physiology reference and does not provide treatment or dosing guidance.

Core questions

• What stimuli trigger renin release from the kidney?
• How is angiotensin II generated and what are its main actions?
• How does the system contribute to longer-term blood-pressure and volume control?
• How do angiotensin II receptor subtypes mediate its effects?

Key concepts

• Renin release from juxtaglomerular cells
• Angiotensinogen and angiotensin I
• Angiotensin-converting enzyme (ACE)
• Angiotensin II and AT1 receptor signalling
• Aldosterone and sodium-water retention
• ACE2 and the counter-regulatory arm

Mechanisms

Juxtaglomerular cells of the kidney secrete renin in response to reduced renal perfusion pressure, decreased sodium chloride delivery to the macula densa, and beta-adrenergic sympathetic stimulation. Renin cleaves circulating angiotensinogen to angiotensin I, which angiotensin-converting enzyme converts to angiotensin II (Forrester, 2018). Angiotensin II acts predominantly through the AT1 receptor to cause vasoconstriction, stimulate aldosterone secretion from the adrenal cortex, enhance renal sodium reabsorption, and facilitate sympathetic transmission, collectively raising arterial pressure and conserving volume (te Riet, 2015). A counter-regulatory arm involving ACE2 and angiotensin-(1-7) opposes some of these effects (Forrester, 2018). Through its renal and vascular actions the system is central to the long-term regulation of arterial pressure (Cowley, 1992).

Clinical relevance

The renin-angiotensin system is the target of several widely used cardiovascular drug classes, and understanding its physiology clarifies why it is central to blood-pressure and fluid regulation. This entry describes mechanisms for reference and is not a basis for individual diagnosis, drug selection, or dosing.

Evidence & guidelines

The mechanisms summarised rest on physiological and pharmacological reviews; clinical use of agents acting on this system is governed by disease-specific guidelines outside this educational scope.

History

Tigerstedt and Bergman described a pressor substance from kidney extracts, renin, in 1898. Over the following century angiotensinogen, angiotensin-converting enzyme, the angiotensin peptides, and their receptors were identified, and more recently the counter-regulatory ACE2-angiotensin-(1-7) axis was characterised, broadening the classical view of the system.

Debates

Balance between the classical and counter-regulatory arms

How the pressor ACE-angiotensin II-AT1 axis and the counter-regulatory ACE2-angiotensin-(1-7) axis are balanced in health and disease remains an active area of research.

Key figures

• Robert Tigerstedt
• Steven J. Forrester
• A. H. Jan Danser
• Allen W. Cowley Jr.

Seminal works

• tigerstedt-bergman-1898
• forrester-2018

Frequently asked questions

What triggers the release of renin?

Renin is released by the kidney when renal perfusion pressure falls, when sodium delivery to the macula densa decreases, or when sympathetic (beta-adrenergic) activity to the kidney increases.

Why is angiotensin II important for blood pressure?

Angiotensin II is a potent vasoconstrictor that also stimulates aldosterone and promotes sodium and water retention, so it raises both vascular resistance and blood volume, supporting longer-term blood-pressure control.