A pulmonary embolism is a blockage of an artery in the lungs. It is caused by a clot that travels through the bloodstream to the lungs. Once the clot is stuck in a lung artery, it blocks the blood from nourishing that lung. The tissues on the other side of the blockage may die if it doesn't receive enough blood from other sources. The lung may become damaged and stop working properly. In severe cases this can lead to death.