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A solid mass or blood constituents formed within the vascular system (rather than in non flowing blood). This is slightly different from coagulation, which is the process by which all clots are formed.

Three main factors

Damage to the endothelium

Usually the cause of arterial clots. Myocardial infarction and valve disease lead to endocardial damage and thus often to thrombosis. Atherosclerosis can lead to rupture of plaques and thrombosis.

Poor or turbulent blood flow

Most common cause of thrombus in venous circulation. Poor blood flow allows clotting factors to accumulate on the vein walls, since they are not washed away by fresh blood. Two main diseases caused by this are DVT and Pulmonary embolism – a major cause of death.


Genetic or acquired. A high platelet count is called thrombocytosis, or a high haemoglobin count is polycythaemia. Another cause can be a defect in factor V that stops it being repressed by protein C – this is a risk factor for oral contraceptives. Clotting abnormalities such as antiphospholipid syndrome also cause coagulation, with autoimmune antibodies making endothelium more clottable.


  • Prostacyclin and Nitric Oxide which are produced by undamaged endothelium.
  • Mast cells secrete Heparin. This combines with circulating antithrombin III to make a powerful thrombin inhibitor.