Blockage of a branch of the pulmonary artery by a blood clot. The clot does not originate in the pulmonary artery itself, but comes from one of the large veins. Whenever a clot is loosened in a vein it is carried by the bloodstream to the right atrium and right ventricle of the heart, and from there to the pulmonary artery. There it may may stick in one of the large or small branches, according to its size, blocking the branch. Symptoms include constriction, chest pain and sometimes the coughing up of blood. Pulmonary embolism caused by a small clot usually causes little or no discomfort, but a large clot can block the pulmonary artery itself, and cut off the blood supply to the lungs, causing immediate death. Treatment is by the use of drugs to inhibit blood coagulation. This will not dissolve a clot that has already reached the lungs, but it will prevent the formation of new ones in the large veins. Surgery is almost always pointless: small clots cannot be removed in this way and in the case of a major, life-threatening pulmonary embolism even an emergency operation is rarely successful.