Like the rest of the muscles in your body, the heart requires steady supply of oxygenated blood in order to function. But heart muscle cells cannot extract oxygen directly from the blood that passes through the heart’s chambers. Instead, the heart must pump blood to itself via the right coronary artery and left main coronary artery and their branches, which nourish every portion of the heart with oxygen and nutrients.
If blood flow through one of these vital arteries is obstructed by atherosclerotic plaque, the heart muscle cells supplied by that artery may not receive enough blood during periods of exertion, resulting in ischemia (oxygen deprivation to cells). In most cases, rest or medications such as nitrates can promptly restore sufficient blood flow to the area.
On the other hand, if a coronary artery becomes completely blocked by the formation of a blood clot at the site of a plaque, a heart attack occurs (see the illustration above). Unless the blood supply is restored within minutes, heart muscle cells will die from lack of oxygen.
The extent of permanent damage depends on the location of the blockage (the larger the affected artery, the more heart muscle will die), the availability of alternative (collateral) arteries to supply blood to the deprived area, and the promptness of treatment.