The platelets secrete a chemical that stimulates the growth of more smooth muscle cells in the wall of the artery. This repair attempt thickens and stiffens the artery wall. An important side effect of this muscle cell activity is to increase the amount of fibrous, or connective, tissue in the injured area. The entire thickened area is called a plaque. If the plaque remained stable all would probably be well.
Some plaques grow very slowly and gradually cause the artery to become more narrow.
In time, this narrowing may block the artery enough to reduce the blood flow through it. When that happens, heart muscle tissue beyond the narrowing begins to suffer from lack of oxygen. Rupture, or breaking open, of the plaque surface is a serious problem. No one knows what causes it. When rupture occurs, bad things may happen quickly.