Illustration of a blood clot (thrombus) narrowing a vein. 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.