air sac
1. Any one of a series of thin-walled sacs in birds that are connected to the lungs and increase the efficiency of ventilation. Some of the air sacs penetrate the internal cavities of bones.

2. A structural extension to the trachea in insects, which increases the surface area available for the exchange of oxygen and carbon dioxide in respiration.

air sac In birds, a thin-walled extension of the lung which penetrates into the body cavity, entering the bones and forming air spaces. The lightweight bones that result are particularly important to birds. Anhimidae (screamers) have air spaces that penetrate to the tibia.