Pulmonary sequestration is a congenital anomaly of the bronchopulmonary foregut in which there is a nonfunctional segment of lung. The blood supply is anomalous, arising from the systemic circulation (ie. the aorta or bronchial arteries ). By definition, there is no typical communication with the bronchial tree or with the pulmonary arteries. Intralobar and extralobar discrimination is based primarily upon venous drainage, and the presence of an independent pleural lining (as in the extralobar type). The intralobar variant drains to the pulmonary veins or left atrium, the extralobar type to the systemic venous system (the inferior vena cava or the azygous system). Both gain blood supply from the systemic arterial circulation, frequently from below the diaphragm, and identification of this confirms the diagnosis (although failure to identify the arterial supply does not exclude it).

In comparison, intralobar sequestrations usually become manifest during adolescence or adulthood, whereas extralobar sequestrations present at birth or soon thereafter (or may be suspected based upon prenatal ultrasonography). Intralobar sequestrations are three times more common with a male predominance, and a much lower incidence of associated anomalies (10% with associated skeletal, foregut, or diaphragmatic anomalies). Extralobar sequestrations may have associated anomalies in up to 65% of patients, including diaphragmatic defects, pulmonary hypoplasia, bronchogenic cysts, and cardiac anomalies. Intralobar sequestrations present with recurrent pneumonias, chronic cough, or dyspnea, or may be completely asymptomatic. Other complications of intralobar sequestrations include hemoptysis, and intrathoracic hemorrhage, and where there is marked vascular shunting, cardiac decompensation. Extralobar sequestrations, presenting at birth or infancy, may manifest as cyanosis, respiratory distress, feeding difficulties, failure to thrive, and recurrent pneumonias. Both predominantly are located in the left base in 60-90%. Treatment of both is resection.

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Animation showing origin of pulmonary vessel from descending thoracic aorta:
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