Blood vessel dilation. Computer artwork of blood vessel dilation (expansion) in response to drugs taken to control arterial blood pressure. The drugs, angiotensin receptor blockers (ARBs), block angiotensin II receptors and are used primarily for the treatment of hypertension (high blood pressure). Angiotensin II is a hormone of the renin-angiotensin system (RAS) that 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).