This pulmonary arterial branch is occluded by a blood clot. Note the lumen filled with red blood cells and layers of aggregated platelets composing the clot. Pulmonary arterial emboli are frequent; thrombi are rare.

Clots in the deep leg veins are the most common source of pulmonary emboli. Factors which predispose to such clotting include immobilization, venous congestion with low flow, vascular injury, and hypercoagulable states. Among the causes of secondary hypercoagulable states are pregnancy, malignancies – especially mucinous carcinomas, and tissue injury. The results of pulmonary embolization are variable depending on the size of the embolus and the underlying status of the patient. A large embolus which lodges at the bifurcation of the pulmonary arteries is fatal. Smaller emboli can be asymptomatic or result in transient acute dyspnea. A minority of smaller emboli cause infarcts. Repeated small infarcts may result in pulmonary hypertension and cor pulmonale. Pulmonary infarcts are likely to be wedge-shaped and in the periphery. They are usually hemorrhagic due to the dual blood supply in the lungs. With pleural involvement the patient will experience pleuritic chest pain and a friction rub may be heard. Pulmonary emboli can also arise from nonthrombotic sources such as bone marrow, fat, amniotic fluid, air, and foreign bodies.