Venous thrombosis most frequently develops in the lower limbs. It then is responsible for chronic swelling of the limb, the development of venous ulcers, etc. The post-thrombotic syndrome is a chronic handicap that can be debilitating and responsible for lenghty absences from the working place. Occasionally, fragments of a fresh venous thrombus are swept away by the blood stream, pass through the heart and become stuck in the circulation of the lungs. This is called pulmonary embolism.
Pulmonary emboli are a major cause of death following operation.
The importance of understanding the events leading to thrombosis and of means of preventing it thus is very obvious. Platelets and fibrin play a vital role on plugging defects in the closed circuit through which the blood flows. This process has been called haemostasis. Persons in whom fibrin fails to form, such as haemophiliacs, or person with too few platelets or defective platelets, have a pronounced tendency to bleed. In normal subjects, the haemostatic plug remains limited to the site of vessels leakage.