Aspirin interferes with your blood s clotting action. When you bleed, your blood cells, called platelets, accumulate at the site of your wound and help form a plug to seal the opening in your blood vessel. But this clotting can also happen within the vessels that supply your heart. If your blood vessels are already narrowed from atherosclerosis — the accumulation of fatty deposits in your arteries — a blood clot can quickly form and block the artery causing a hear attact. Aspirin therapy reduces the clumping action of platelets thus reducing the risk of a heart attack.