A proteolytic enzyme (see protease) that is involved in the formation of the hormone angiotensin, which raises blood pressure, in part by stimulating the release of the hormone aldosterone from the adrenal cortex (the renin-angiotensin-aldosterone system; RAAS). Renin is secreted into the blood by juxtaglomerular cells (see juxtaglomerular apparatus) of the kidney under the control of the sympathetic nervous system; its release also occurs in response to a fall in blood-sodium levels and to falling blood pressure. Renin catalyses cleavage of the circulating precursor angiotensinogen to produce angiotensin I, precursor of the active hormone. The net effect is to raise blood pressure, blood volume, and the glomerular filtration rate.