Pulmonary embolism In thrombosis, a blood clot (thrombus) forms in a blood vessel. In embolism, a plug of material (embolus) travels though the bloodstream and becomes lodged in a blood vessel.
In both cases, blood flow is blocked. A thrombus may form when blood is flowing sluggishly, if the blood is prone to clotting due to a genetic condition, pregnancy, taking combined oral contraceptives or hormone replacement therapy, or as a result of the build-up of fatty deposits on artery walls (atherosclerosis).
Most emboli are pieces of blood clot that have broken off from a larger clot elsewhere, as may occur after a heart attack or as a result of deep vein thrombosis.
Other types include an air embolus, caused by air introduced into the bloodstream, and a fat embolus, caused by fat being released from a fractured bone.
The symptoms of thrombosis or embolism depend on which blood vessels are affected.
Blockage of the arteries to the brain may cause a stroke; blockage of the coronary arteries may cause a heart attack; and blockage of the pulmonary arteries to the lungs (pulmonary embolism) may cause breathing difficulty.

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