Hypoperfusion of which organ triggers the renin-angiotensin-aldosterone system (RAAS)?

The kidney is the organ that triggers the renin-angiotensin-aldosterone system (RAAS) in response to hypoperfusion. When blood flow to the kidneys decreases, such as in the case of reduced blood volume or low blood pressure, the renal juxtaglomerular cells sense this decrease and respond by releasing renin.

Renin is an enzyme that catalyzes the conversion of angiotensinogen, produced by the liver, into angiotensin I. This peptide is then converted to angiotensin II, primarily in the lungs, through the action of the angiotensin-converting enzyme (ACE). Angiotensin II has several critical effects, including vasoconstriction, which increases blood pressure, and stimulation of the adrenal cortex to secrete aldosterone. Aldosterone promotes sodium and water reabsorption in the kidneys, further increasing blood volume and, consequently, blood pressure.

The function of RAAS is a vital compensatory mechanism in the body to restore perfusion to vital organs during states of hypovolemia or hypotension. This system highlights the kidneys' crucial role in maintaining homeostasis through the regulation of blood pressure and fluid balance.