Thrombus. Coloured Scanning Electron Micrograph (SEM) of a thrombus (blood clot) inside a coronary artery of the human heart. The artery has been cross-sectioned, showing its wall (brown) and inner lumen (blue). A blood clot (red) is seen, blocking about 30% of the width of the artery. Two coronary arteries supply blood to the heart muscle. A clot in one of these arteries (called coronary thrombosis) is most commonly caused by the build-up of atheroma (fatty deposits) on the artery walls. Impeded blood flow from a clot starves the heart of oxygen and may lead to heart attack. Magnification: x30 at 6x7cm size. x40 at 4x5"~LANDSCAPE"