Due to its system of nine air sacs connected to the lungs, the respiratory system of birds is arguably the most effective respiratory system of all air-breathers. Upon inhalation, air first flows into posterior air sacs, then into the lungs, and then into anterior air sacs on the way to being exhaled. Thus, there is one-way flow of air through the lungs, along thousands of tubules called parabronchi.
If the inner lining of the air sacs is neither thin nor highly vascularized, then what can be inferred about the air sacs?
They cannot be sites of gas exchange between air and blood.