Atherosclerosis happens when fatty plaque starts collecting inside the walls of arteries, which are the tubes that carry oxygen rich blood through the body. Over many years this buildup makes the artery narrower and stiffer and it becomes harder for blood to move smoothly. When blood flow drops, organs get less oxygen and the artery becomes more sensitive to stress. If the plaque becomes irritated or breaks open, a blood clot can suddenly form and fully block the vessel which can cause a heart a