The respiratory system of birds is amazingly efficient. In addition to the lungs, where gas exchange occurs, the bird's respiratory system includes groups of air sacs. This arrangement of respiratory structures allows air to flow in one direction through the lungs. Therefore, all air that passes through the lungs of a bird is fresh, with a high percentage of oxygen.
In the accompanying animation, we examine the route that air takes as it passes through the respiratory system of a bird.
The unique respiratory system of birds includes air sacs in addition to lungs. The lungs, but not the air sacs, function in gas exchange. The function of the air sacs is to allow air to flow unidirectionally through the bird's respiratory system. Here we show a full cycle of air flow, which requires two breaths.
To see this more diagrammatically, we group the many air sacs of the respiratory system into two main types: posterior and anterior air sacs.
During an inhalation, the air sacs expand. Air from the environment is drawn into the posterior air sacs while the air in the lungs is drawn into the anterior air sacs.
During an exhalation the air sacs compress, pushing air from the posterior air sacs into the lungs and the air in the anterior air sacs out through the trachea.
After a second full breath, air in the respiratory system completes the cycle. It leaves the lungs, passes through the anterior air sacs, and finally exits the body.
In birds, unlike in mammals, an inhalation fills the lungs with completely fresh air. The air passes through the bird's respiratory system in a single direction, bringing the fresh air first into the posterior air sacs, then into the lungs, and finally into the anterior air sacs before it exits through the trachea.
Contrast this to the respiratory system of mammals, such as humans, in which the flow of air is bi-directional. The fresh, inhaled air enters and mixes with a small amount of stale air that is never fully exhaled from the lungs. Therefore, the percentage of oxygen inside the lungs is always lower than the percentage of oxygen in the outside air. During an exhalation, the air leaves by the same path on which it came.
Because the respiratory system of birds is so efficient, birds can sustain physical activity at altitudes that would incapacitate a human. For example, some birds fly over the summit of Mount Everest, an altitude at which most humans would require oxygen tanks to survive.