Here is another large pulmonary thromboembolus seen in cross section of this lung. The typical source for such thromboemboli is from large veins in the legs and pelvis.

Pulmonary emboli can be classified by size as small, medium, and large. Small ones may be clinically inapparent. Medium-sized ones may not kill the patient, but may lead to pulmonary infarction. Large ones can be life-threatening. Recurrent pulmonary embolization can reduce pulmonary vascular flow and cause pulmonary hypertension.