A thrombus may form whenever the flow of blood in the arteries or the veins is impeded.
Many factors can interfere with the normal flow of the blood.
Sometimes heart failure or physical inactivity retards circulation generally, or a change in the shape or inner surface of a vessel wall impedes the flow of blood, as in atherosclerosis. Any mass that has grown inside the body can exert pressure on a vessel, or the vessel wall can be injured and roughened by an accident, surgery, a burn, cold, inflammation, or infection. The blood may thicken in a reaction to the presence of a foreign serum or snake venom. Sometimes a thrombus detaches itself from the wall and is carried along by the bloodstream. A thrombus may form in the heart chambers.