Part of a lung

• Lung compartment

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Air sac \Air" sac`\ (Anat.) One of the spaces in different parts of the bodies of birds, which are filled with air and connected with the air passages of the lungs; an air cell. An alveolus .

Wikipedia Word definitions in Wikipedia
Air sac generally refers to an organ or portion of an organ containing air within an animal's respiratory system. Air sac may refer to Pulmonary alveolus , small hollow cavities which are a part of the lungs within mammals Air sacs , structures unique to ...

WordNet Word definitions in WordNet
n. a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways [syn: alveolus , air cell ] any of the thin-walled extensions of the tracheae of insects any of the membranous air-filled extensions of the lungs of birds