Over many years, plaque builds up on artery walls. Plaque is a sticky, yellow substance made of fatty substances like cholesterol, as well as calcium and waste products from your cells. It narrows and clogs the arteries, slowing the flow of blood. This condition is called atherosclerosis, which may begin as early as childhood. It can occur anywhere in the body, but it usually affects large and medium-sized arteries.
Sometimes plaque in an artery can rupture. The body's repair system in turn creates a blood clot to heal the wound. The clot, however, can block the artery, leading to either a heart attack or stroke.