Medical Definition of alveolus
: a small cavity or pit: asa: a socket in the jaw for a toothb: any of the small thin-walled air-containing compartments of the lung that are typically arranged in saclike clusters into which an alveolar duct terminates and from which respiratory gases are exchanged with the pulmonary capillariesc: an acinus of a compound glandd: any of the pits in the wall of the stomach into which the glands open
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