What is the primary role of renin in the body related to blood pressure?

Renin plays a crucial role in the regulation of blood pressure through the renin-angiotensin-aldosterone system (RAAS). When blood pressure decreases or when there is reduced blood flow to the kidneys, renin is secreted by the juxtaglomerular cells in the kidneys. This enzyme initiates a cascade of reactions that leads to the conversion of angiotensinogen, a protein produced by the liver, into angiotensin I.

Once angiotensin I is formed, it is further converted into angiotensin II by the enzyme angiotensin-converting enzyme (ACE), primarily in the lungs. Angiotensin II is a potent vasoconstrictor that raises blood pressure by narrowing blood vessels and stimulating the release of aldosterone from the adrenal glands. Aldosterone promotes sodium and water retention by the kidneys, further increasing blood volume and pressure.

Thus, the primary and initiating action of renin is the conversion of angiotensinogen into angiotensin I, setting off the chain reaction that ultimately raises blood pressure through various mechanisms.