Birds are busy animals. Consider insect-hawking flycatchers, far-flying shorebirds, and hovering hummingbirds, and you get a picture of how active birds can be. That’s why they favor high-energy foods and have high metabolic rates that demand huge oxygen levels.
Mammals’ respiratory systems are woefully inadequate to provide the amounts of oxygen that birds require.
In humans, for example, a tube called the trachea connects our nose and back of the mouth to our lungs. The trachea splits into two primary bronchi that enter the lungs, where they divide into secondary bronchi, which in turn branch into tertiary bronchi. These give rise to small tubes called bronchioles, which terminate in thin-walled, grape-like clusters called alveoli (shown at lower left in the diagram above). Surrounded by capillary networks, alveoli are the sites for gas exchange. Because of pressure differences, oxygen diffuses from the alveoli into blood capillaries, and carbon dioxide diffuses from blood capillaries into the alveoli.
The cul-de-sac nature of our lungs means that air normally flows in two directions — that is, from our nostrils through the trachea to the alveoli and back again through the same ducts. It follows that the first air breathed into the alveoli is left over and stale, having given up most of its oxygen before the last exhalation. Bidirectional airflow, therefore, is inefficient.
Birds employ a more efficient system, one in which thin-walled air sacs are connected to the lungs. As shown in the illustration of the cardinal, the air sacs fill the body cavity. They are not involved directly in gas exchange but function as bellows to direct airflow through the lungs in one direction, from back to front. This increases lung efficiency.
Another major difference between mammals and birds is that the grape-like alveoli are replaced by thin-walled, tubular structures called parabronchi (shown at lower right in the diagram). Like human alveoli, avian parabronchi are covered by a rich supply of capillaries and are the sites for gas exchange. Parabronchi are located throughout the lungs between secondary bronchi. Just as air moves in one direction through the lungs, it also flows in one direction through the parabronchi, from one secondary bronchus into another.
The genius of the air sacs is that they allow continuous, one-way flow during both inspiration and expiration. The air sacs are arranged in two groups: one coming off the front of the lungs (anterior) and the other off the back of the lungs (posterior). Here’s how the system works:
During inspiration, the posterior air sacs expand, pulling air into the primary bronchi, which terminate near the far end of the lungs. While some of the air is diverted through secondary bronchi near the back of the lungs and into parabronchi, most of it passes directly into the posterior group of air sacs. At the same time, the anterior air sacs expand, pulling air from the parabronchi through the secondary bronchi. This creates the one-way back-to-front flow through the lungs.
During expiration, the air sacs contract, forcing air from both air-sac groups. Air from the posterior air sacs moves through the parabronchi, while air from the anterior air sacs moves into the primary bronchi and trachea and then out of the body. Some stale air is left in the system but not enough to detract significantly from the overall efficiency. Notice that during both inspiration and expiration, air is flowing one way through the parabronchi.
Birds breathe differently from mammals because they lack a diaphragm. They move air in and out of their lungs and air sacs by means of special muscles that move the ribs and sternum downward and forward, expanding the body cavity and causing inspiration, and then up and backward, contracting the body cavity and causing expiration.
Thermoregulation is another essential function of the air sacs. The high level of avian activity generates excessive heat that must be dissipated. Birds, however, lack the heat-dissipating sweat glands that we possess.
Remember that sweat glands cool by producing a salty secretion that evaporates from the skin. The heat required to change sweat from a liquid to a vapor comes from the skin, thereby cooling it. Birds change water into vapor in a similar way in the air sacs, except that the heat required to vaporize the water comes from organs and tissues surrounding the air sacs.
A finely tuned respiratory system that moves air in one direction enables birds’ high activity level. And the air sacs help regulate temperature by providing a mechanism to dissipate excess body heat. The system is yet another example of the amazing biology of birds.
Eldon Greij’s column “Amazing Birds” appears in every issue of BirdWatching magazine. Subscribe. This article appeared in the April 2014 issue. Eldon is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founding editor of Birder’s World magazine.