angiotensin /-ten″sin/ [Gk, angeion + L, tendere, to stretch] , a polypeptide in the blood that causes vasoconstriction, increased blood pressure, and the release of aldosterone from the adrenal cortex. Angiotensin is formed by the action of renin on angiotensinogen, an alpha-2-glycoprotein that is produced in the liver and that constantly circulates in the blood. Renin, stimulated by juxtaglomerular cells in the kidney in response to decreased blood volume and serum sodium levels, acts as an enzyme in the conversion of angiotensinogen to angiotensin I, which is rapidly hydrolyzed to form the active compound, angiotensin II. The vasoconstrictive action of angiotensin II decreases the glomerular filtration rate, and the concomitant action of aldosterone promotes sodium retention, with the result that blood volume and sodium reabsorption increase. Plasma angiotensin II increases during the luteal phase of the menstrual cycle and is probably responsible for an elevated level of aldosterone during that period. Angiotensin is inactivated by peptidases, called angiotensinases, in plasma and tissues.
