How air flows in the bird lung

The finding, much against expectation, that most birds seem not to use the obvious anatomical shunt to bypass the lung when they pant, makes it even more desirable to clarify the patterns of air flow in their complex respiratory system.

The respiratory system of mammals is simple; the sac-like lungs and their subdivisions are ventilated by an in-and-out flow of air with diffusion playing the major role at the terminal units, the alveoli. In the bird lung, however, there is no need for a reversal of the air flow because the finest branches of the bronchi, the parabronchi (tertiary bronchi), are open at both ends and permit through-flow of air. This characteristic, in combination with the multiplicity of tubes, branches and connections to the air-sacs, has led to a great deal of speculation as to how air flows in this complex system. Virtually all imaginable possibilities have been suggested, including some that violate physical laws, observable fact, and physiological evidence. I should therefore like to summarize the rather strong evidence we now have that air flows unidirectionally through the lung both during inhalation and exhalation, and then discuss the consequences that this has for the function of the avian lung as an efficient organ for oxygen uptake. Although bird blood has no greater affinity for oxygen than mammalian blood, the arrangement of the avian lung permits a far better utilization of oxygen than in mammals at low atmospheric pressure, which is of importance to bird flight at high altitude.