renin

Renin is the kidney’s pressure-and-salt alarm that kicks off a powerful blood-pressure-raising chain reaction. When the kidneys sense that blood pressure or blood flow is too low, they release renin, which sets in motion a cascade ending in salt retention and vessel constriction.

Renin is an enzyme secreted by specialized cells (juxtaglomerular cells) in the kidney’s small arteries. It is sometimes called an enzyme-hormone because, although it is released into the blood like a hormone, it acts by cleaving a target protein. Renin converts angiotensinogen (made by the liver) into angiotensin I, which is then converted by ACE into angiotensin II — a potent vasoconstrictor that also triggers aldosterone release, leading to sodium and water retention. The result is higher blood pressure and restored volume.

Renin release is triggered by low blood pressure, low sodium delivery sensed at the macula densa, and sympathetic nerve activity. Because the whole renin-angiotensin-aldosterone system raises blood pressure, it is a central target of medicine: ACE inhibitors, angiotensin-receptor blockers, and direct renin inhibitors all treat hypertension and heart and kidney disease by damping this cascade.