How can physical activity prevent a heart attack?                        

MI's, Myocardial Infarctions, or Heart Attacks while can happen at random, are greatly exaggerated by disorders that are themselves exaggerated by sedentary lifestyles. Arteriosclerosis (hardening of Artery walls) and Atherosclerosis (plaque buildup on artery walls) are two disorders that contribute a lot towards MI's, and these both can be prevented via physical activity.

Artery walls are literally muscles, so with physical activity artery wall hardening can be prevented. Plaque buildup usually results in clots called Thrombi, which can eventually occlude the coronary blood vessels. These plaques consist of cellular debris, platelet and blood cells, and lipids called cholesterol an accumulation of which is called Hyperlipidemia. Exercise can greatly decrease the lipid count in the blood stream (as well as a healthy diet). Additionally by exercising you can decreases blood pressure which contributes to clot formation, when it is excessive (Hypertension) by actually scratching the artery walls (clots don't start all of a sudden).