Why is bird respiration efficient

When a bird draws in a breath of air, it travels through the nares or nostrils down the trachea into a series of posterior air sacs located in the thorax and rump—in their butts. When a bird exhales that same breath, it does not leave the body as it does with mammals but rather moves into the lung where oxygen is absorbed and carbon dioxide expelled.

When a bird inhales for the second time, that same breath of air moves from the lungs into the anterior air sacs. Every breath a bird takes requires two breathing cycles to complete a single breath, making the air passing through the lung unidirectional and always fresh and full of oxygen. Bird lungs are small and rigid, with the gas exchange region of their anatomy organized into a series of parallel tubes that bring deoxygenated blood into the lung at the opposite direction the air is flowing.

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.

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. New to BirdWatching? Sign up for our free e-newsletter to receive news, photos of birds, attracting and ID tips, descriptions of birding hotspots, and more delivered to your inbox every other week.

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