This computer illustration shows a blood clot in a the wall of a vein. When a blood vessel is injured or ruptures, the body uses platelets and fibrin to form a blood clot which forms a solid 'plug' to prevent blood loss. In veins the primary clotting mechanism depends on the thrombin system.
The thrombin system consists of several blood proteins that, when bleeding occurs, become activated. Activated clotting proteins engage in an array of chemical reactions that finally produce a substance called fibrin. Fibrin strands stick to the exposed vessel wall, clumping together and forming a web-like complex of strands. Red blood cells become caught up in the web and form a blood clot.
Deep vein thrombosis usually occurs in a deep leg vein, a larger vein that runs through the muscles of the calf and the thigh and can result when the walls of the vein become damaged. It can cause pain and swelling in the leg and may lead to complications such as pulmonary embolism. This is when a piece of blood clot breaks off into the bloodstream and blocks one of the blood vessels in the lungs.