Causes of Thrombosis

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In general, thrombosis seldom occurs in a healthy artery, because the smooth inner lining prevents the clot from forming. Injury to a blood vessel, or any disease process affecting the smoothness of the inner lining, can start the process of thrombosis.

The commonest cause of thrombosis in arteries is atherosclerosis (see article on Atherosclerosis). Atherosclerosis causes strokes and heart attacks and is now the number one killer of the Western world. Atherosclerosis causes rough, raised plaques (which contain cholesterol) on the inner lining of arteries. These eventually tear, and the thrombosis starts to build up where the plaques are torn.

Even when arteries are normal, a clotting tendency can result from hormonal or biochemical changes in the blood. The tendency to thrombosis in arteries may be greater during pregnancy, in women using oral contraceptives, in people with cancer that has affected blood vessels, and in people whose blood is thicker than normal (those with polycythaemia, which means there are too many cells in the blood).

Thrombosis in veins is encouraged by local pressure, inflammation (thrombophlebitis) and stagnation of blood flow through inactivity.