obstruction of the flow of blood by an embolus, a particle or aggregate of substance that is abnormally present in the bloodstream. The substance may be a blood clot that has broken loose from its point of formation (while it is still adherent to the vessel at the point where it was formed, the clot is called a thrombus (thrombosis)); it may be a drop of soluble fat from a crushing injury of fatty tissue; it may be a clump of tumour cells, bacteria, or detached tissue cells; it may be a foreign body such as a bullet, which has penetrated a vessel wall; it may be a drop of amniotic fluid that has entered the maternal circulation during childbirth; or it may be an air bubble (called an air embolism) or a bubble of some other gas—e.g., nitrogen in decompression sickness.

So long as the embolus travels unimpeded through the bloodstream, it is not likely to cause symptoms or damage. However, if the substance blocks a vessel that supplies blood to the brain, a stroke may occur, with effects that include a period of unconsciousness, temporary or lasting paralysis of all or part of one side of the body, inability to use words ( aphasia), impaired memory, and, in severe cases, death. A pulmonary embolism—an obstruction of blood flow to the lungs by an embolus in the pulmonary artery or in one of its branches—results in difficulty in breathing and an unpleasant sensation beneath the breastbone, similar to that experienced in angina pectoris. Embolism in a coronary artery, which supplies blood to the heart muscle, can cause a number of serious effects, including death of a section of the heart muscle (myocardial infarction (heart attack), or heart attack). Treatment varies with the cause and site of the embolus, although anticoagulant drugs are generally administered to help prevent recurrence due to blood clot formation.