The heart works 24 hours a day, pumping oxygen- and nutrient-rich blood to the body. Blood is supplied to the heart through its coronary arteries. In coronary heart disease (CHD), plaques or fatty substances build up inside the walls of the arteries. The fatty buildup or plaque can break open and lead to the formation of a blood clot that seals the break. The clot reduces blood flow. The cycle of fatty buildup, plaque rupture, and blood clot formation causes the coronary arteries to narrow, reducing blood flow. If a blood clot suddenly cuts off most or all blood supply to the heart, a heart attack results. Cells in the heart muscle that do not receive enough oxygen-carrying blood begin to die. The more time that passes without treatment to restore blood flow, the greater the damage to the heart.