The anatomy of the avian respiratory system is quite complex compared to that of mammals. The avian and mammalian respiratory systems consist of the lungs which expand, gas exchange occurs and the air is exhaled. In birds the respiratory system also serves for the exchange of gases but is also important in eliminating heat from the body and has several non-respiratory functions such as the detoxification of metabolic products and vocalization. As in mammals, the upper respiratory system serves to filter air to trap dust. Unlike mammalian lungs, avian lungs are relatively rigid and do not move much during breathing.
Some air sacs penetrate the interior of the bones and even under the skin.
In mammals, inspired air goes into the lung as the lung expands and is expired when the lung contracts. In birds the lungs expand very little because the air goes through them into the air sacs and back through the lungs on expiration. Thus not only can a greater volume of air pass through the lungs, but since it passes through twice, gas exchange is more efficient. They also provide a cushion for the viscera when landing nor diving in to the water for food, and may be modified for courtship displays e.g. the neck bladders of the Frigate Bird and Prairie Chicken. Air sacs also serve to cool the body during vigorous exercise through the internal evaporation of water.