Thrombosis is the formation of a blood clot or thrombus inside a blood vessel, obstructing the flow of blood. It occurs when components in the blood known as platelets and fibrin form clots in response to an injury inside the blood vessel. Common types of thrombosis include deep vein thrombosis and pulmonary embolism, which can develop from blood clots that form in the deep veins of the legs and travel to the lungs.