The condition known as thrombosis occurs when a clot of blood forms inside of a blood vessel and impedes the normal flow of blood through the circulatory system. When a blood vessel (a vein or an artery) is damaged, the body will use platelets (thrombocytes) and fibrin to form a blood clot in order to stop the flow of blood and prevent further injury. Under certain circumstances, the formation of blood clots can occur within the body even in the absence of an injury to a blood vessel. An embolus is a blood clot or a fragment of a clot that has broken off and begun to travel to other parts of the body. Thrombosis can take place in the arteries (arterial thrombosis) or the veins (venous thrombosis). Congestion will develop in the affected region of the body if the patient has venous thrombosis.