Figure 48.7 The Respiratory System of a Bird (A) Air sacs in the body cavity and air spaces in the bones are unique to birds. (B) Air comes in through the trachea, which divides into a left and a right primary bronchus. The primary bronchi go to the posterior air sacs. Secondary bronchi connect the posterior air sacs to the lungs. Air flows unidirectionally through the lungs in numerous parabronchi and flows in bronchi from the lungs to the anterior air sacs, then out through the trachea. (C) Air capillaries running between the parabronchi are the site of gas exchange.