Angiotensin II, molecular model. Angiotensin II is a polypeptide hormone that functions in the body to control arterial blood pressure. It is the principle mediator of the renin-angiotensin system (RAS), synthesised from angiotensin I by the angiotensin-converting enzyme ACE. It stimulates vasoconstriction (narrowing of the blood vessels to raise the blood pressure) and the retention of salt and water. The hormone has been found to increase the risk of thrombosis through the adhesion and aggregation of platelets, and has also been put forward as the possible cause of vascular and cardiac muscle hypertrophy (enlargement of the heart). In this model atoms are shown as balls; carbon in yellow, hydrogen in white, nitrogen in blue and oxygen in red.