Atherosclerosis describes the thickening of arterial walls by the deposition of plaque, which can consist of calcium, fatty matter, cholesterol, cellular debris, or fibrin. It generally begins in youth and progresses with age. The danger from these plaques is two-fold. First, the narrowing of the arterial lumen by the thickened wall restricts blood flow to organs sensitive to ischemia, like the heart and brain. Second, if the plaque ruptures, a clot can form at the site of the damaged endothelium and either completely occlude the vessel, causing a thrombotic stroke or myocardial infarction, or break off and cause an embolic stroke.