alveoli

n. pit, small cavity (as in a honeycomb); air cell in a lung; socket of a tooth

An alveolus (plural: alveoli, from Latin alveus, "little cavity"), is an anatomical structure that has the form of a hollow cavity. Mainly found in the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood. Alveoli are particular to mammalian lungs; different structures are involved in gas exchange in other vertebrates.
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Alveoli Small air sacs or cavities in the lung that give the tissue a honeycomb appearance and expand its surface area for the exchange of oxygen [more]Alveoli - Community and Resources

Noun
1. a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
(synonym) air sac, air cell
(hypernym) sac
(part-holonym) lung
2. a bony socket in the alveolar ridge that holds a tooth
(synonym) tooth socket
(hypernym) socket
(part-holonym) alveolar ridge, gum ridge, alveolar process

n. alveolus, loculus