A mammal exhales oxygen depleted, carbon dioxide rich air via the same pathway the air entered. A primary bronchus, termed the mesobronchus, enters the lung substance. The parabronchi contain elongated vestibules that are branched and connect to each other to form air capillaries. These air capillaries are surrounded by blood capillaries, and are the site of gas exchange. Air sacs are large, thin-walled reservoirs for air.
Recurrent bronchi extend out from the air sacs and connect with the parabronchi.
This creates a passageway from the air sacs back through the tertiary and secondary branches to the mesobronchus and eventually out to the exterior environment. Upon inhalation, air enters a bird s trachea, goes into the primary bronchus, through the mesobronchus, and follows the system of branches until it reaches the small air capillaries of the parabronchi where gas exchange occurs. The air is then forced into the air sacs and subsequently leaves the air sacs through a different pathway then it entered. Air is forced out through the recurrent bronchi, then to more parabronchi where gas exchange occurs a second time. The air then moves through the system of secondary bronchi, to the mesobronchus, primary bronchus, and finally out to the external environment.