a peptide, one form of which, angiotensin II, causes constriction of blood vessels (blood vessel).

There are three forms of angiotensin. Angiotensin I is produced by the action of renin (an enzyme produced by the kidneys (kidney)) on a protein called angiotensinogen, which is formed by the liver. Angiotensin I is transformed into angiotensin II in the blood by the action of angiotensin-converting enzyme (ACE). Angiotensin II acts directly on blood vessels, causing their constriction and thereby raising blood pressure. This substance also can cause vessel constriction through indirect mechanisms, such as by stimulating the release of the steroid hormone aldosterone and substances called catecholamines (catecholamine) from the adrenal glands (adrenal gland) and by blocking the reuptake of the hormone norepinephrine (epinephrine and norepinephrine) into neurons (neuron). Angiotensin III is a metabolite of angiotensin II and shares similar, though less potent, actions.

Drugs that inhibit ACE, and thus block the conversion of angiotensin I to angiotensin II, are used to lower blood pressure in patients with hypertension. Blood pressure also can be lowered using drugs that are designed to block the receptors (receptor) to which angiotensin II must bind to exert its actions.