A heart attack (myocardial infarct, MI) is caused by a blockage of the arterial blood supply to heart muscle for sufficient time to cause the affected muscle to die and be replaced by fibrous scar tissue. All the blood to the heart muscle passes through three small coronary arteries. If one of these is blocked, one part of the heart muscle cannot obtain sufficient blood and dies. The arteries may be blocked by fatty deposits because the patient is overweight or has high cholesterol levels, by clots or fat globules breaking off from damaged blood vessels elsewhere in the body, or by damage to the artery from high blood pressure. The severity of a heart attack depends on the size of the affected artery and the amount of heart muscle damaged, and its position in the heart. A small amount of damage in a vital area may cause death, while significant damage in a less important area will not be fatal. In angina the blood supply to the heart muscle is reduced but not completely cut off, so no permanent damage occurs, but angina may lead to a heart attack in some cases.