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. Because blood normally flows more slowly through the veins than through the arteries, thrombosis is more common in the veins than in the arteries.
Venous thrombosis occurs most often in the legs or pelvis.
It may be a complication of phlebitis or may result from injury to a vein or from prolonged bed rest. Immediate medical attention is necessary in any case. Under no circumstances should the affected limb be massaged. In a thrombosis of the superficial veins, bed rest with the legs elevated and application of heat to the affected area may be all that is necessary. In a thrombosis of the deep veins, the affected part must be immobilized to prevent the clot from spreading or turning into an embolus, and anticoagulant drugs may be given.