Gas Exchange in Birds: Superior Efficiency!

Like reptiles and mammals, birds suck in air by increasing the volume of the body cavity. In birds however, most of the air drawn in during inhalation does not go directly to the lungs, but flows through the bronchus to the posterior air sacs; simultaneously, air already in the lungs moves forward into the anterior air sacs. During exhalation, air from the posterior sacs moves into the lungs, while air from the anterior sacs moves into the bronchi and flows out. Thus air moves forward through the lungs during both inhalation and exhalation. Instead of alveoli, bird lungs have tiny air ducts (parabronchi) running through the lung tissue, and it is across their walls that gas exchange takes place.

Birds are far more efficient than mammals in extracting oxygen from air, for two reasons: (1) because there is a continuous unidirectional flow of air through their lungs, and (2) because the blood in the capillaries associated with the parabronchi moves in a direction opposite to the flow of air and so provides some of the same benefits as the countercurrent exchange system of fish gills. This superior efficiency enables birds to fly actively at high altitudes, where the partial pressure of oxygen is low. Vance Tucker experimentally exposed sparrows and mice to an atmosphere simulating that at 6,000m altitude, and found that the sparrows could fly vigorously while the mice were unable to stand up and could barely crawl. Geese and swans can fly at altitudes up to at least 9,000 m.

Respiratory cycle of a bird. (A) During inhalation, new air (color) is drawn into the posterior air sacs; a small amount also enters the posterior portion of the lungs. Air already in the system (gray) is simultaneously moved forward through the lungs and into the anterior air sacs. (B) As air is exhaled from the anterior air sacs, air from the posterior air sacs moves forward into the lungs and across the gas exchange surfaces; this movement is effected by a contraction of the air sacs. The portion of air (color) inhaled in (A) will be exhaled during the following respiratory cycle. Note that during both inhalation and exhalation, oxygen-rich air is moving unidirectionally through the lungs. The blood in the parabronchi moves from left to right, opposite the flow of air, which allows the gas exchange to benefit from a countercurrent design.