Vascular blood clot. Coloured transmission electron micrograph (TEM) of a section through a blood vessel that is blocked by an abnormal blood clot (thrombus). A large aggregate of platelets is visible. Blood should not normally clot in a blood vessel. A thrombus can form due to blood disorders, or excessive time spent in a cramped position. If the thrombus detaches from the blood vessel wall, it may travel to other parts of the body and disrupt the blood flow there. A thrombus can be fatal if it blocks blood flow to a vital organ, such as the heart, lung, or brain. Magnification: x3000 when printed 10 centimetres high.