The name of 2 related proteins involved in regulating blood pressure. The 1st, angiotensin I, is inactive and is formed when renin, which is produced by the kidneys, acts on the substance angiotensinogen. Angiotensin I is then converted to the second, active, form, angiotensin II, by angiotensin-converting enzyme. Angiotensin II causes narrowing of the small blood vessels in tissues, resulting in increased blood pressure. It also stimulates release (from the adrenal cortex, the outer part of each adrenal gland) of the hormone aldosterone, which also increases blood pressure. Certain kidney disorders can increase the production of angiotensin II, causing hypertension. Whatever the cause of hypertension, it may be treated with drugs called ACE inhibitors, which work by reducing angiotensin II formation.