Atherosclerosis. Light micrograph of a section through part of a human coronary (heart) artery with atherosclerosis and thrombosis (a clot). Atherosclerosis is a build-up of fatty plaques on the walls of arteries. The fibrolipid atheroma plaque (right) is composed of areas of fatty and fibrous tissue. This narrows the artery, causing reduced or irregular blood flow. It also increases the chance of clot formation. A clot has recently formed (left, darker stained) which has blocked the artery. This can result in heart attack, as the oxygen supply to the heart muscle is compromised.