Angiotensin is the name of two related proteins in the blood that are involved in regulating blood pressure. The first of these, angiotensin I, is itself inactive and is converted to the second, active form, angiotensin II, by the action of a converting enzyme. Angiotensin II causes constriction of the small blood vessels in tissues, resulting in an increase in blood pressure. It also stimulates the release from the adrenal cortex (the outer part of each adrenal gland) of the hormone aldosterone, which increases blood pressure.

Certain kidney disorders can increase the production of angiotensin II causing high blood pressure (hypertension). Whatever the cause of hypertension it may be treated with drugs known as ACE inhibitors (angiotensin-converting enzyme inhibitors), which work by reducing angiotensin II formation.