The main role of blood clotting is to prevent bleeding by sealing breaks in damaged blood vessels. However, blood clots can form on any damaged area on the surface of blood vessels. As layers of clotted blood are deposited on a tiny blood clot in an artery, the clot can grow big enough to block the flow of blood, which causes damage to the tissues that get their blood supply from that artery. The clot can also break off of the blood vessel and travel through the bloodstream and cause damage by blocking blood flow to an organ. For example, most heart attacks occur when a clot forms on the damaged surface of a small artery that supplies blood to the heart. The lining of veins can also suffer damage that leads to the formation of a clot, typically in the veins of the legs.