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What Causes Pulmonary Embolism?

Major Causes

Pulmonary embolism (PE) usually begins as a blood clot in a deep vein of the leg. This condition is called deep vein thrombosis. The clot can break free, travel through the bloodstream to the lungs, and block an artery.

The animation below shows how a blood clot from a deep vein in the leg can travel to the lungs, causing pulmonary embolism. Click the "start" button to play the animation. Written and spoken explanations are provided with each frame. Use the buttons in the lower right corner to pause, restart, or replay the animation, or use the scroll bar below the buttons to move through the frames.

Install flash.

The animation shows how a blood clot in a deep vein of the leg can break off, travel to the lungs, and block blood flow.

Blood clots can form in the deep veins of the legs if blood flow is restricted and slows down. This can happen if you don't move around for long periods, such as:

  • After some types of surgery
  • During a long trip in a car or airplane
  • If you must stay in bed for an extended time

Blood clots are more likely to develop in veins damaged from surgery or injured in other ways.

Other Causes

Rarely, an air bubble, part of a tumor, or other tissue travels to the lungs and causes PE. Also, if a large bone in the body (such as the thigh bone) breaks, fat from the bone marrow can travel through the blood. If the fat reaches the lungs, it can cause PE


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July 1, 2011