Pulmonary embolism results from a clot formed in one of the deep veins of the body in a condition known as Deep Vein Thrombosis. Deep Vein thrombosis is more common in the deep veins of the legs and pelvis than elsewhere. The clot so formed travels all the way up into one of the pulmonary arteries of the lungs where it is trapped or wedged. This often leads to the death of lung tissue (pulmonary tissue). There are other conditions that may cause the thrombosis in deep veins. Good examples of them include injuries to bones or muscles, long periods of stagnancy of blood due to inactivity, therapy or drugs.
Other factors which can cause pulmonary embolism are air bubbles in the blood, staying parts of a tumor, or a lump of fat from the bone marrows.