Here’s a little more detail on how bird lungs work, compared to human lungs:
As I mentioned in the last post, avian “flow-through” lungs are much more efficient than human lungs. We humans pull in air and push it out of our lungs with every inhale and exhale, and in normal breathing, we only use a portion of our total lung capacity. During gas exchange, the fresh, oxygenated air we breathe in mixes with “old” air that’s been in the lungs for a while. Oxygen makes its way down to the alveoli (the little pockets where gas exchange between actually occurs); here, oxygen from the lungs diffuses out into the bloodstream, and carbon dioxide moves out into the lungs. Finally, carbon dioxide and a good deal of unused oxygen are expelled as we exhale.
This system works well enough, but it’s terribly inefficient. Birds, however, have evolved a much better system, illustrated below:
Step 1. Inhalation: Fresh air is inhaled into the first chamber
Step 2. Exhalation: Air is pushed across the lungs, where oxygen transfers over into the bloodstream (and carbon dioxide transfers into the lungs)
Step 3. Inhalation: Air, now deoxygenated, is pushed into the second chamber. Meanwhile, a new batch of fresh air is inhaled into the first chamber, as in Step 1.
Step 4. Exhalation: Deoxygenated air is exhaled out again. Meanwhile, the new batch of fresh air is pushed across the lungs, as in Step 2.
Step 5: The Cycle Continues…
Each “breath” of air actually remains in the bird’s lungs for two cycles of inhalation and exhalation, and is then fully expelled at the end. Fresh air flows across the lungs with every breath, and “old” air never mixes with new air (as it does in human lungs). Respiration is much more efficient, which allows birds to perform strenuous activities like flying that would be much more difficult with our paltry human lungs.