Birds extract O₂ efficiently to supply the considerable energy needed for flight. An efficient O₂ supply is particularly important for migratory birds that fly at high altitude where the air’s partial pressure of O₂ is relatively low. The lungs of birds are unique in that they are rigid and do not inflate and deflate. As a consequence, birds do not use tidal breathing. Instead, they benefit from unidirectional airflow that enhances gas exchange by maintaining larger concentration gradients for diffusion.

A bird’s respiratory system consists of two sets of air sacs: one located posterior to the lungs (behind them) and one located anterior to the lungs (in front of them). Air is pumped through these air sacs continuously in a bellows-like action (Fig. 39.9). As the set of posterior air sacs expand, air is drawn from the bird’s mouth into those sacs. The sacs are then compressed by surrounding muscles acting on the skeleton, pumping air through the bird’s rigid lungs. The lungs consist of a series of larger air channels that branch into fine air capillaries having a large surface area. The air capillaries direct the air so that it moves crosscurrent (that is, at 90°) to the path of blood flow through the walls of the air capillaries. This crosscurrent flow is not as efficient as the countercurrent flow of fish gills, but it enables birds to extract more O₂ from the air than mammals can.

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After losing O₂ and gaining CO₂, the air leaves the lungs and enters the set of anterior air sacs. Fresh air is again inhaled into the posterior air sacs, and then the stale air in the anterior air sacs is exhaled out of the bird’s trachea and mouth. As a result, birds achieve a continuous supply of fresh air into the lung during both inhalation and exhalation. In contrast to other air-breathing vertebrates, therefore, birds move air through their respiratory system in two cycles of ventilation.