air sac
In birds, a thin-walled extension of the lungs. There are nine of these and they extend into the abdomen and bones, effectively increasing lung capacity.
The sacs subdivide into further air spaces which partially replace the marrow in many of the bird's bones. The air space in these bones assists flight by making them lighter. In mammals, it is another name for the alveoli in the lungs, and in some insects, for widenings of the trachea.
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