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Primary pulmonary lobule to include terminal bronchiole, respiratory bronchiole, alveolar sacs, alveolar ducts and surrounding capillary bed. This is the primary, functional unit of gas exchange in the lung. Approximately 3-5 primary pulmonary lobules are delimited by pulmonary lymphatics and venules to create secondary pulmonary lobules (not shown).

Photo credit: Patrick J. Lynch, medical illustrator