Renin-Angiotensin-Aldosterone System

The renin-angiotensin-aldosterone system (RAAS) plays an important role in regulating blood volume and systemic vascular resistance, which together influence cardiac output and arterial pressure. As the name implies,
there are three important components to this system:   1) renin, 2) angiotensin, and 3) aldosterone

Renin, which is primarily released by the kidneys, stimulates the formation of angiotensin in blood and tissues, which in turn stimulates the release of aldosterone from the adrenal cortex.

Renin is a proteolytic enzyme that is released into the circulation primarily by the kidneys. Its release is stimulated by:

1. sympathetic nerve activation (acting via β₁-adrenoceptors)
2. renal artery hypotension (caused by systemic hypotension or renal artery stenosis)
3. decreased sodium delivery to the distal tubules of the kidney.

Juxtaglomerular (JG) cells associated with the afferent arteriole entering the renal glomerulus are the primary site of renin storage and release in the body. A reduction in afferent arteriole pressure causes the release of renin from the JG cells, whereas increased pressure inhibits renin release. Beta₁-adrenoceptors located on the JG cells respond to sympathetic nerve stimulation by releasing renin. Specialized cells (macula densa) of distal tubules lie adjacent to the JG cells of the afferent arteriole.  The macula densa senses the amount of sodium and chloride ion in the tubular fluid. When NaCl is elevated in the tubular fluid, renin release is inhibited. In contrast, a reduction in tubular NaCl stimulates renin release by the JG cells. There is evidence that prostaglandins (PGE₂ and PGI₂) stimulate renin release in response to reduced NaCl transport across the macula densa. When afferent arteriole pressure is reduced, glomerular filtration decreases, and this reduces NaCl in the distal tubule. This serves as an important mechanism contributing to the release of renin when there is afferent arteriole hypotension. When renin is released into the blood, it acts upon a circulating substrate, angiotensinogen, that undergoes proteolytic cleavage to form the decapeptide angiotensin I. Vascular endothelium, particularly in the lungs, has an enzyme, angiotensin converting enzyme (ACE), that cleaves off two amino acids to form the octapeptide, angiotensin II (AII), although many other tissues in the body (heart, brain, vascular) also can form AII.               AII has several very important functions: 

• Constricts resistance vessels (via AII [AT₁] receptors) thereby increasing systemic vascular resistance and arterial pressure
• Acts on the adrenal cortex to release aldosterone, which in turn acts on the kidneys to increase sodium and fluid retention
• Stimulates the release of vasopressin (antidiuretic hormone, ADH) from the posterior pituitary, which increases fluid retention by the kidneys
• Stimulates thirst centers within the brain
• Facilitates norepinephrine release from sympathetic nerve endings and inhibits norepinephrine re-uptake by nerve endings, thereby enhancing sympathetic adrenergic function
• Stimulates cardiac hypertrophy and vascular hypertrophy

The renin-angiotensin-aldosterone pathway is regulated not only by the mechanisms that stimulate renin release, but it is also modulated by natriuretic peptides (ANP and BNP) released by the heart. These natriuretic peptides acts as an important counter-regulatory system.
Therapeutic manipulation of this pathway is very important in treating hypertension and heart failure. ACE inhibitors, AII receptor blockers and aldosterone receptor blockers, for example, are used to decrease arterial pressure, ventricular afterload, blood volume and hence ventricular preload, as well as inhibit and reverse cardiac and vascular hypertrophy.