Thrombosis is the formation of a clot or thrombus inside a blood vessel, obstructing the flow of blood through the circulatory system. Thromboembolism is a general term describing both thrombosis and its main complication: dislodgement of a clot and embolisation.
Classically, thrombosis is caused by abnormalities in one or more of the following: (Virchow's triad):
- The composition of the blood
- Quality of the vessel wall
- Nature of the blood flow
The formation of a thrombus is usually caused by an injury to the vessel's wall, either by trauma or infection, and by the slowing or stagnation of blood flow past the point of injury. Occasionally, abnormalities in coagulation are to blame. Intravascular coagulation follows, forming a structureless mass of red blood cells, leukocytes, and fibrin.
There are two distinct forms of thrombosis:
- venous thrombosis
- and arterial thrombosis
If a bacterial infection is present at the site of thrombosis, the thrombus may break down, spreading particles of infected material throughout the circulatory system (pyemia, septic embolus) and setting up metastatic abscesses wherever they come to rest. Without an infection, the thrombus may become detached and enter circulation as an embolus, finally lodging in and completely obstructing a blood vessel (an infarction). The effects of an infarction depend on where it occurs.
Most thrombi, however, become organized into fibrous tissue, and the thrombosed vessel is gradually recanalized.