Seen in the pulmonary artery in a left lung on cut section is a large pulmonary thromboembolus. Such thromboemboli typically originate in the leg veins or pelvic veins of persons who are immobilized. Other contributing factors include trauma to the extremities, hypercoagulable states (examples include Trousseaus syndrome in patients with carcinomas; protein C or S deficiency; use of oral contraceptives), heart failure, pregnancy, and older age.