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The Renin-Angiotensin-Aldosterone System, often abbreviated as RAAS, is a crucial hormonal system that regulates blood pressure and fluid balance in the body. It is a complex cascade of biochemical reactions involving the liver, kidneys, lungs, and adrenal glands. The system is primarily activated by a drop in blood pressure or blood volume, or by a decrease in sodium levels in the bloodstream.

The process begins in the kidneys, which sense these changes and release an enzyme called renin. Renin then acts on angiotensinogen, a protein produced by the liver, to form angiotensin I. Angiotensin I is then converted into angiotensin II by an enzyme called angiotensin-converting enzyme (ACE), which is found mainly in the lungs.

Angiotensin II is a potent vasoconstrictor, meaning it narrows blood vessels, which directly increases blood pressure. It also stimulates the adrenal glands to release aldosterone. Aldosterone is a hormone that acts on the kidneys, promoting the reabsorption of sodium and water back into the bloodstream. This increase in fluid volume further contributes to raising blood pressure.

In addition to its effects on blood vessels and sodium reabsorption, angiotensin II also plays a role in the sensation of thirst and the release of antidiuretic hormone (ADH), both of which help to conserve water and increase blood volume. The RAAS is a vital mechanism for maintaining homeostasis, but its overactivity can contribute to conditions like hypertension. Medications that target components of the RAAS, such as ACE inhibitors and angiotensin II receptor blockers (ARBs), are widely used to treat high blood pressure and heart failure.