During a heart attack, a clot forms inside, or near, one of the four coronary arteries, according to HowStuffWorks. Often, this is a result of a slight abrasion to plaque inside the artery, which causes platelets to rush to the area.
Plaque can tear loose inside the coronary artery at any time and for any reason, but it is most likely to happen during or just after physical exertion, explains HowStuffWorks. When the plaque loosens, the cholesterol beneath it is exposed to the bloodstream. The body is able to sense damage to the inside of arteries and, mistaking the loosening plaque for a tear to the wall of the coronary artery, it dispatches platelets to the region to begin the process of repairing a break.
A rapidly growing clot constricts the coronary artery and reduces blood flow to the heart, as explained by HowStuffWorks. Starved of blood, the heart begins to beat faster, consuming more oxygen and building up more waste products that the blood normally clears away. Eventually, the heart cannot keep up its elevated pace and slips into an abnormal rhythm called "ventricular fibrillation." Without immediate treatment, this leads to asystole and eventually death as the heart muscle dies.