What is thrombosis? What effect does it have on the human body? What can predispose to thrombosis? Thrombosis is otherwise known as venous thromboembolism, which includes deep vein thrombosis and pulmonary embolism. Thrombosis (the formation of a blood clot in a vessel that prevents blood flow) occurs through slower blood flow (e.g. during pressure on a vein or through prolonged immobilisation), damage to the vessel wall and increased activity of blood clotting factors.