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alveolus

al·ve·o·lus

/alˈvēələs/

Noun

These small pockets of the alveolar ducts and sacs and terminal bronchioles through whose walls the exchange of carbon dioxide and oxygen takes place between the alveolar air and capillary blood. The bronchioles divide and become smaller until they terminate in the air sacs, known as alveoli. Each alveolus is surrounded by capillaries; like a capillary wall, the alveolar wall is also one cell thin. This allows gases like oxygen and carbon dioxide to pass through easily. Hundreds of millions of alveoli make up the bulk of the lungs.