Renin is an enzyme hormone that is secreted by the kidney, specifically by the juxtaglomerular cells of the afferent arterioles. It plays a key role in the renin-angiotensin-aldosterone system (RAAS), which helps regulate blood pressure and fluid balance in the body. Renin converts angiotensinogen, a protein produced in the liver, into angiotensin I. Angiotensin I is then converted to angiotensin II by the enzyme angiotensin converting enzyme (ACE), which is mainly produced by the lungs. Angiotensin II is a potent vasoconstrictor that raises blood pressure by constricting blood vessels and also stimulates the secretion of aldosterone from the adrenal glands, which promotes the retention of sodium and water in the body.