This 3D animation from The Human Atlas explains Myocardial Infarction. 

The heart is the primary organ of the cardiovascular system.  It is a beating muscle that continually pumps blood to the rest of the body. The coronary arteries supply the heart itself with the necessary oxygen and nutrients it needs to function effectively.

Red blood cells, white blood cells and other substances flow freely to the heart and other parts of the body. In a healthy person, the walls of the artery are smooth and uniform in thickness. Over time, however, a high level of circulating cholesterol can cause fatty deposits, called plaque, to accumulate.

As plaque is deposited, it can harden and cause the artery to become narrowed and less flexible, a condition called atherosclerosis. If atherosclerosis develops in the coronary arteries, the condition is called coronary artery disease, or CAD. If blood flow is severely interrupted, a myocardial infarction can occur.  A myocardial infarction, or MI, is another term for heart attack.

If blockage of a coronary artery exceeds 90 percent, the risk is increased for experiencing a heart attack; the risk is almost certain when plaque completely blocks a coronary artery.

Another way CAD can increase the risk for an MI is by the development of a blood clot. Oftentimes a crack can develop at the site of plaque buildup. When this happens, blood can coagulate, or clump together, at the site of the crack, or a blood clot, called a thrombus, can grow in size until it completely blocks blood flow.

The extent of damage sustained by the heart during an MI depends on the severity and location of the blockage, and the speed at which medical treatment is received. Fortunately, there are many ways to prevent atherosclerosis, and to lower the risk for developing a heart attack.