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Since lung can obtain its oxygen from from the alveolar gas and has a second blood supply through the bronchial arteries, occlusion of a pulmonary artery does not usually produce tissue necrosis. Due to dual blood supply and free anastomosis between the pulmonary capillaries, small emboli fail to cause any infarction in a healthy lung. Tissue distal to the obstructed artery may be normal or merely show congestion, hemorrhage, and intra-alveolar fibrin with intact alveolar walls. However, inspite of the dual blood supply, from bronchial and pulmonary vessels, pulmonary infarction is very common. Embolus arising in any part of systemic circulation has to pass through the lungs. Infarction is common when the circulation of the lung is slowed down associated with increased pulmonary circulatory pressure - Eg. in chronic passive venous congestion, hypostatic congestion as seen in post-operative and post-natal period . Under such condition, when branch of pulmonary artery is blocked , force of the bronchial arteries is insufficient to supply the obstructed area due to increased pulmonary circulatory pressure. Blood drains into the area from all the connections and stagnate there. Vessels in the alveolar walls give way and blood escapes into the alveolar space and the whole area becomes dead and coagulated into a firm blood-filled solid, airless mass (hemorrhagic infarct). Big embolus blocking a big branch of pulmonary artery causes sudden death from shock, without infarction.
Cause: - Emboli that travel through the blood stream to the lungs and block a pulmonary artery. Emboli come from deep veins of legs, usually in post-operative and post-natal period. - Pulmonary thrombosis is usually due to atherosclerosis of the pulmonary artery. Gross: Image Link1 ; Image Link2 ; Image Link3 - Usually multiple, big or small, wedge-shaped and placed at the periphery of the lung . The area is dark red in colour with clean cut outline and feels firm. The cut surface is dry . Image Link4 - Overlying pleura shows patches of fibrinous pleurisy due to irritation. - With time the center of the infarct becomes brown and eventually pale as the hemorrhage breaks down and is removed.
Microscopic features: Image1 ; Image2 ; Image3 ; Image4 - Alveoli are filled up with blood and their outline is indistinct due to coagulation necrosis. Image1 ; Image2 ; Image3 - Small numbers of neutrophils may be present. - Pleura is congested with deposition of fibrin. - Over the next few weeks granulation tissue appears surrounding the necrotic tissue, which becomes encapsulated, gradually organizes, and is converted to a linear fibrous scar. During the first few weeks when granulation is taking place, there are often nests of metaplastic squamous epithelium associated with the granulation tissue, which should not be mistaken for squamous carcinoma.
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May 2009
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