renin A proteolytic enzyme (see protease) that is involved in the formation of the hormone angiotensin, which raises blood pressure. 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.

renin (ree-nin) n. an enzyme released into the blood by the kidney in response to stress. It produces angiotensin, which causes constriction of blood vessels and thus an increase in blood pressure. Excessive production of renin results in renal hypertension.