Mechanics of Respiration in Birds

Birds ventilate their lungs not by expanding the lungs themselves, but by expanding the air sacs, a uniquely avian anatomical feature. The air sacs are balloon-like structures at the "ends" of the airway system, each with a wall of squamous epithelium. In the chicken, the "model" species, there are eight such sacs: an unpaired one in the cervical region; another unpaired one in the clavicular area; two paired sets (cranial and caudal) in the thorax; and a paired set in the caudal abdomen.

The key to the system is that distention and compression of the air sacs, not the lungs, moves air in and out and that at any given moment air may be moving into and out of the lung and being "parked" in the air sacs. The lungs are stiff and noncompliant, not at all like the distensible lungs of mammals. The air sacs act as "bellows" to suck air in and blow it out, and also to hold part of the total volume.

Here's the condition at the beginning of each inspiration of the respiratory cycle (only one side is shown). The cranial end of the system is shown to the left, and the trachea is shown. Air will enter and leave via the trachea as it does in mammals. All the air sacs are deflated at this stage. There is "resting" air in the lungs. Each one of a bird's paired lungs has a single large mesobronchus, essentially the intrapulmonary continuation of the trachea, which runs straight through it. This is a long straight airway, much like a mammalian bronchus in construction, which connects the trachea to the caudal air sacs. It also serves to ventilate the lung directly with part of each incoming breath.

At the moment of inspiration, air enters via the trachea and is routed to the mesobronchus. Half of the incoming air goes directly into the lung, and the other half goes directly (via the mesobronchus) into the caudal air sacs. The "resting" air that was in the lungs now moves out of them and into the cranial air sacs. Note the direction of flow indicated by the arrows, and the inflated state of the air sacs. They're inflated by muscular movements, as the chest expands and the ribs move ventrally and cranially. This distention pulls air into the caudal sacs via the inspiratory route, and into the cranial sacs from the lungs. Gas exchange occurs during this phase of the cycle, as half of the inspired air moves into the lung directly.

At the beginning of the exhalation, the air in the caudal air sacs (which are beginning to deflate) moves into the lungs (this was the "second half" of the air inspired before, remember). The air that was in the lungs and the cranial air sacs now moves out via the trachea. This movement results from compression of the chest and abdomen by muscle movement, and it's the reverse of the previous step.

At the beginning of the next inspiration, the situation is as it was in the first diagram, except that the air which was in the lungs at the beginning has been replaced by air from the caudal sacs; the cranial sacs are empty, and half of the air from the first breath is now in the caudal sacs. With the end of second inhalation, half of the volume of the first breath moves into the cranial air sacs; it will be expelled via the trachea in the next exhalation. As before, air moves from the caudal sacs into the lung. The cycle repeats in this way, with about one third the total volume of the system being moved from one place to the next through each inspiratory and expiratory phase.

Gas exchange in the lung is continuous throughout the process, with one half the inspired volume of any given breath being "processed" at any given moment to extract oxygen. This setup amounts to a "flow through" system, and rather than the in and out breathing cycle we have, birds respire more or less continuously. This has great importance because flight demands an enormous amount of oxygen exchange, far more than is needed for terrestrial locomotion, and the respiratory system of birds is very efficient at providing it. This is due only in part to the "bellows" action of the air sacs; it depends in great measure on the presence of a "countercurrent flow" mechanism for gas exchange in the lung proper, which we'll look at next.

I am greatly indebted to Dr. Charles J. McGrath of the VMRCVM's Large Animal Clinical Sciences Department for the illustrations used above.

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