The most common cause for pulmonary embolism is a blood clot that forms in a vein outside the lungs, called a deep venous thrombosis. The most common locations where these blood clots form is in the legs or pelvis.

If part of the clot breaks off of a deep venous thrombosis, it is carried by the bloodstream, back to the heart. Then, the clot passes through the heart, and is carried to the lung via the pulmonary artery. When the clot reaches the lung, it becomes lodged in one of the arteries that supply the lung. This clot, now called a pulmonary embolus, prevents blood from flowing to a portion of the lung. The lack of blood flow keeps this portion of the lung from transferring oxygen to the bloodstream. In turn, this results in a decrease in the supply of oxygen to the body.