alveolar sac

1. terminal dilation of the alveolar ducts that give rise to alveoli in the lung; a small air chamber in the pulmonary tissue from which the pulmonary alveoli project like bays and into which an alveolar duct opens;

2. in birds, air-containing extensions of bronchi that connect with bone cavities. Syn: sacculus alveolaris [NA], air sac.