Irbesartan's prevention of angiotensin II binding causes vascular smooth muscle relaxation and prevents the secretion of aldosterone, lowering blood pressure. Angiotensin II would otherwise bind to the AT1 receptor, inducing vasoconstriction and aldosterone secretion, raising blood pressure. Hydrochlorothiazide is a thiazide diuretic (water pill). It reduces the amount of water in the body by increasing the flow of urine, which helps lower the blood pressure. Hydrochlorothiazide directly inhibits the sodium chloride cotransporter located on the apical membrane of the distal convoluted tubules in the kidney.