The Renin-Angiotensin-Aldosterone System plays a crucial role in the regulation of renal, cardiac, and vascular physiology, and its activation is central to many common pathologic conditions including hypertension, heart failure, and renal disease.

Renin

RENIN: released by the kidneys through sympathetic nerve activation, renal artery hypotension (systemic hypotension or renal artery stenosis) or decreased sodium delivery to the distal tubules of the kidney.

1. Renin is synthesized from its precursor, prorenin, in the juxtaglomerular cells of the kidney and is released from secretory granules.
2. It splits angiotensin off of angiotensinogen. This process is stimulated by both kidney hypoperfusion (seen in hypotension and volume depletion) and increased sympathetic activity.

Angiotensin I and II and ACE

When reaching certain points in the body (primarily the lungs, but also the heart, brain, and vascular tissues),

• Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II.

ANGIOTENSIN II: 

A. Begins hormonal orchestration of fluid dynamics:

• stimulates secretion from the adrenal cortex of aldosterone (which causes the kidneys to retain sodium and fluid),
• stimulates thirst,
• stimulates the release of ADH from the posterior pituitary (increasing fluid retention), 
• stimulates enhanced sympathetic adrenergic function; and

B. Produces arteriolar vasoconstriction, increasing systemic vascular resistance and arterial pressure, to increase blood pressure.

Both of these actions tend to reverse the hypotension or hypovolemia that is usually responsible for the stimulation of renin secretion in the first place.

(The heart releases natriuretic peptides (ANP and BNP) that are counter-regulatory systems to the renin-angiotensin-aldosterone system.)

Aldosterone

ALDOSTERONE: a steroid hormone from the adrenal gland. Angiotensin II is the most powerful stimulus for adrenal aldosterone secretion, but both adrenocorticotropic hormone (ACTH) and potassium also stimulate aldosterone secretion.

Aldosterone increases sodium and water reabsorption and potassium secretion. These renal actions contribute to extracellular fluid volume expansion, increasing blood pressure.

CHEAT SHEET CASCADE:

Prorenin → renin ⇒ angiotensinogen → angiotensin I → (via ACE) angiotensin II ⇒ aldosterone, ADH, sympathetic adrenergics (vasoconstriction)