What causes Pulmonary Embolism?

An embolism occurs when a clot moves through the bloodstream from the location where it was formed and becomes stuck in a blood vessel. An embolus, can be a blood clot, air bubble, or piece of fat, bone marrow, or tumor tissue.

The embolus in a pulmonary embolism is usually a blood clot. A blood clot that forms and remains in a vein is called a thrombus. It most often starts in a vein in the legs or pelvis.