Pulmonary embolism is caused by a blocked artery in the lungs. The most common cause of such a blockage is a blood clot that forms in a deep vein in the leg and travels to the lungs, where it becomes lodged in a smaller lung artery.
Almost all blood clots that cause pulmonary embolism are formed in the deep leg veins. Clots also can form in the deep veins of the arms or pelvis.
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