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'Air sac' is an anatomical term with several meanings:

★ Pulmonary alveolus, informally known as an air sac, one of innumerable spherical outcroppings of the respiratory bronchioles in the mammalian lung, the primary sites of gas exchange with the blood

★ an anatomical structure continuous with the trachea found in some insects which can be used to actively pump air throughout the body

★ an anatomical structure unique to the dinosaur [1] and bird respiratory systems that allows unidirectional flow of air into the lungs and through the body