A heart attack results when an atheroma becomes unstable and breaks away from the artery wall. Atheroma form when the lining of the blood vessels become damaged (for example, from chemicals in cigarette smoke) and are composed of cholesterol, white blood cells, smooth muscle cells, calcium and platelets. The unstable atheroma forms a clot and occludes arterial supply of oxygen-rich blood to the heart muscle for a long enough time that part of the muscle is permanently damaged.
