Most cases of heart disease are not "heart" diseases as such. They develop as a result of changes within the blood vessels that supply the myocardium. Skeletal muscles can contract in the absence of oxygen; cardiac muscle cannot. Cardiac muscle (myocardium) extracts about 80% of the oxygen brought to it by its own system of blood vessels, which include the coronaries arteries. Sometimes these vessels become constricted or blocked, which results in ischemia (insufficient blood flow). When they are constricted, the heart works harder than it does normally to push blood through them to supply its own needs. This constriction and the resulting efforts by the heart cause intense pain of angina pectoris (literally "pain of the chest"). Angina pectoris can be relieved temporary by drugs such as nitroglycerin, which dilate the vessels.
When coronary vessels are blocked by clot, the result is coronary thrombosis. There is evidence that fatty substances and cholesterol are culprits in this disease. These substances form deposits which harden on the inner walls of the vessels; platelets rupture against these deposits and trigger clotting. Sometimes anti-clotting agents, such as aspirin, help reduce the number of coronary attacks, as does restricting the fats in one's diet. When a coronary artery blocks, the part of the myocardium that this artery supply dies, giving way to necrosis; then this affected area of the myocardium is replaced by connective tissue and that part of the heart becomes fibrous and dysfunctional. If the necrosis of the myocardium covers a large area, it causes a myocardial infarction or, in other words, a massive heart attack, which in turn can cause the death of the person.
When it is a small branch of the coronary arteries that becomes blocked or constricted, new blood vessels would grow around the diseased area of the heart to replace the blocked artery. Exercise helps stimulate the spreading of these new small branches. This is why many doctors recommend mild to moderate physical exercises for patients whose hearts have not been badly damaged by a previous coronary attack.
One of the chief causes of the heart trouble is hypertension or high blood pressure. The immediate cause of hypertension is constriction of the arteries and arterioles. However, many factors contribute to constriction; one is hardening of the arteries, or atherosclerosis, in which cholesterol coats the arteries and them hardens. Whatever the cause of constriction, the end result is that the heart has to work harder and harder to pump blood through the narrowed vessels which no longer stretch when the blood pressure rises at each heart beat.