21.16: Birds have unusually efficient respiratory systems.

Figure 21.35: High-altitude respiration.

Birds take breathing to new heights. Because they often spend time in high-altitude, low-oxygen habitats, and fly for long periods of time, they need a lot of oxygen for ATP production (FIGURE 21-35). These extreme needs can be met by several key evolutionary adaptations that make it possible for birds to exchange gases much more efficiently than other terrestrial vertebrates.

For starters, birds’ respiratory adaptations enable them to keep oxygen-rich air flowing through the lungs twice as long as in mammals. In mammals, air is inhaled and reaches a dead end at the alveoli. On the exhale, it changes direction and is breathed out. During the exhale, no new oxygen reaches the lungs, a problematic situation during times of exertion. In birds, a more efficient system ensures that oxygen-containing air never runs into a dead end and the lungs never experience “stale” air. Here’s how they do it (FIGURE 21-36).

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When a bird inhales, some of the air passes through the lungs, where oxygen can diffuse into the blood. The rest of the air fills temporary holding structures called the posterior air sacs. Then, when the bird exhales, the posterior air sacs contract and push more oxygen-rich air through the lungs. With this system, even during exhales, when no “new” air is being breathed in, oxygen-rich air is passing through the lungs.

Figure 21.36: Meeting extreme needs. The highly efficient gas-exchange system of birds supports the oxygen demands of flight.

When a bird is inhaling, air moving through the lungs passes into other temporary holding structures, called the anterior air sacs. Then, as the bird exhales, oxygen-poor air passing from the lungs and oxygen-poor air from the anterior air sacs together move into the trachea, and this mix is expelled from the bird’s body.

With their circular system made possible by the air sacs and by the lungs’ unique pass-through channels, birds maintain a continuous and unidirectional flow of oxygen-rich air through the lungs. These adaptations make it possible for geese to fly over Mount Everest, nearly six miles above sea level. A mammal deposited at that height would pass out almost instantly from insufficient oxygen.

Q

Question 21.9

How is bird breathing less like mammalian breathing and more like fish “breathing”?

TAKE-HOME MESSAGE 21.16

Birds often spend time in high-altitude, low-oxygen habitats and may fly for long periods of time, both of which require a great deal of oxygen. These needs are met by a circular system of air flow, which makes it possible for birds to exchange gases more efficiently than other terrestrial vertebrates.

Compare the structure and efficiency of the human respiratory system with that of birds.

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