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a thin-walled saclike terminal dilation of the respiratory bronchioles, alveolar ducts, and alveolar sacs across which gas exchange occurs between alveolar air and the pulmonary capillaries.
Any of the terminal epithelial sacs of an alveolar duct where gases are exchanged in respiration. It is informally called an “air sac.” Synonym: alveolar sac
See also: alveolus