Figure 31.8: The Breathing System of a Bird (A) The lungs in the thorax are relatively rigid. The air sacs—of which there are usually nine—are located in the thorax and abdomen and serve as bellows to ventilate the lungs. (B) Arrays of numerous parabronchi in the lungs are the principal sites where gas exchange takes place. Air flows through the parabronchi unidirectionally. (C) Blind-ended air capillaries branch from the parabronchi. The interior surfaces of the air capillaries are the gas exchange membranes.