A blood clot that travels through the blood vessels is called an embolus. A pulmonary embolism is a blood clot that blocks an artery in your lungs. It can be a life-threatening problem.

Inside the body, blood is normally a fluid. Occasionally, something goes wrong with the clotting system and blood clots form where they are not supposed to -- inside the blood vessels, usually in leg veins. The clot may form because you sit for long periods or have injured your leg, but often the cause is not known.

Clots tend to break into pieces. These pieces may float in the bloodstream until they block a blood vessel. A pulmonary embolus is a blood clot that breaks off from the wall of a vein and travels to the lungs.

Blood clots may form more easily in your blood vessels if your blood is flowing very slowly through your veins or if disease or medicines you are taking increase the tendency to clot. Your risk of developing blood clots increases if you smoke; if you are less active because of surgery, disability, or illness; if you have cancer or heart failure; if you have a broken leg or hip; if you have had a stroke or heart attack; if you sit for a long period of time; or if you use birth control pills.