angiotensin Any of three related peptide hormones, two of which raise blood pressure. Angiotensin I is derived, by the action of the enzyme renin, from a protein (α-globulin) secreted by the liver into the bloodstream. As blood passes through the lungs, another enzyme (angiotensin-converting enzyme; ACE) splits angiotensin I, forming angiotensin II. This causes constriction of blood vessels and stimulates the release of antidiuretic hormone and aldosterone, which increase blood pressure. Angiotensin III, formed by removal of a single amino acid from angiotensin II, also stimulates aldosterone release by the adrenal gland.

angiotensin (an-ji-oh-ten-sin) n. either of two peptides. a. I a peptide derived, by the action of renin, from a protein secreted by the liver into the bloodstream. a. II a peptide, formed from angiotensin I by enzyme action, that causes constriction of blood vessels and stimulates the release of vasopressin and aldosterone, which increase blood pressure. See also ACE inhibitor.