BIRD LUNGS: TWO-CYCLE PROCESS
Lungs are structures specialized for gas exchange with air. All vertebrates have moist lungs to enable diffusion of oxygen across their surface. Vertebrate respiration has increased in efficiency through evolution of more complex surfaces that increase surface area. All vertebrate lungs work in concert with a circulatory system that transports freshly oxygenated blood around the body.

Because flying is a high-energy activity, birds generally have a high metabolic rate and require a great deal of oxygen. Thus, they have evolved a two-cycle breathing process involving both lungs. Eight or nine extensions of the lungs (air sacs) act as bellows that temporarily store air and then contract to force air through the system. This moves fresh air through the lungs when birds are inhaling, and also when they're exhaling.

On inhalation, both sets of air sacs inflate. Inhaled air flows down the trachea, bypasses the lungs, and fills up the posterior air sacs (b). At the same time, the anterior air sacs fill with stale air from the lungs (d).

On exhalation, both sets of air sacs deflate, forcing fresh air from the posterior sacs into the lungs (c), and stale air from the anterior sacs out through the trachea (e). Air takes two cycles of inhalation and exhalation to pass through the system and out of the bird.