Thrombosis is the formation or presence of a blood clot inside a blood vessel or cavity of the heart. Platelets play a central role in hemostasis through their ability to adhere to damaged vessel walls and to aggregate in response to agonists such as thrombin. Platelet aggregates, stabilized by fibrin, rapidly form hemostatic plugs when blood vessels are severed or arterial thrombi at sites of vessel injury, such as ruptured atherosclerotic plaques, or regions where blood flow is disturbed, such as stenoses. These thrombi cause the thromboembolic complications of atherosclerosis: heart attacks, strokes, and peripheral vascular disease.