Thrombosis is the clotting of blood within a vein or artery, blocking the free circulation of blood. When a blood vessel is injured, the body uses platelets and fibrin to form a blood clot, as the first step in repairing it to prevent loss of blood. If that mechanism causes too much clotting, and the clot breaks free, a thrombus is formed.
But the more common causes of thrombosis are old age and obesity, which lead to the formation of plaques within the blood vessels. When a piece of plaque breaks off an artery wall, it turns into a clot that obstruct the flow of blood, causing thus a stroke or a heart attack, depending on the area of the circulatory system where this plug of bood ends up.
There are two kinds of thrombosis: 1) venous thrombosis; a blood clot that develops in a vein. 2) arterial thrombosis; a blood clot that develops in an artery. One of the most common types of venous thrombosis is deep vein thrombosis (DVT), which is a blood clot in one of the deep veins of the body. It most commonly affects leg veins, such as the femoral vein.