A pulmonary alveolus (plural: alveoli, from Latin alveolus, “little cavity”) also known as an air sac or air space is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen is exchanged for carbon dioxide. Alveoli make up the functional tissue of the lungs known as the lung parenchyma, which takes up 90 percent of the total lung volume.

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