formation of a blood clot in the heart or in a blood vessel. Factors that play a role in the formation of clots (thrombi) include injury to a blood vessel and alterations from normal blood flow; changes in the coagulability of the blood may also cause clot formation. Injury to the lining of a blood vessel or of the heart that results, for example, from inflammation or from the formation of fatty plaques (atherosclerosis) may lead to clot formation. Normally the formed elements of the blood—the red and white blood cells and the platelets—move along in the centre of the stream in a blood vessel; if there is turbulence or other alteration from normal flow, the platelets and blood cells may scrape along the blood vessel lining. Such a situation may arise at the site of an aneurysm—an abnormal widening of the vessel. Confinement in bed also may result in more sluggish blood flow in the veins and consequent formation of a clot. Abnormally large numbers of platelets may cause an increased tendency of the blood to coagulate, as may abnormally high levels of fats in the blood.

Effects of thrombosis may be blockage of the blood vessel at the point of clot formation or blockage of a vessel at some other point by a clot that has broken free from its point of origin. Such a clot is called an embolus. See also embolism.