Over time, LDL cholesterol can build up on the walls of the coronary arteries and form hard plaques, reducing blood supply to the heart. This stiffening and narrowing of the arteries is called atherosclerosis. Sometimes the surface of a plaque ruptures, releasing substances that make platelets stickier and encouraging clots to form on the surface of the plaque. The clot can block the flow of blood through the already-narrowed artery entirely. Without blood, heart muscle tissue starts to die in what's termed a "myocardial infarction" a heart attack.