Atherosclerosis. Light micrograph of a section through part of a human artery with atherosclerosis. Atherosclerosis is a build-up of fatty plaques on the walls of arteries. The fibrolipid atheroma plaque (upper frame) is composed of areas of fatty and fibrous tissue. This narrows the lumen of vessel (not visible), causing reduced or irregular blood flow. Lower frame is the pink-staining muscular wall of the artery. Atherosclerosis also increases the chance of clot formation, which can block a coronary artery resulting in heart attack, or a brain artery causing a stroke.