Avian breathing

Birds don't breathe like we do. They have a unidirectional airflow system that uses air sacs to keep oxygen-rich air constantly moving through the lungs.

When they inhale, fresh air first fills posterior air sacs. On exhalation, this air moves forward into the lungs, where oxygen exchange happens.

A second inhale pushes this now deoxygenated air into anterior air sacs, and the second exhale expels it.

So from the point of view of a single "packet" of air traveling through the system, there are four stages.

But from the point of view of the whole system, there are two stages:

• Inhalation fills the posterior air sacs from the outside, and the anterior air sacs from the lungs.
• Exhalation expels the posterior air sacs into the lungs, and the anterior air sacs to the outside.

This unidirectional airflow makes countercurrent exchange possible, where blood always flows opposite to the air moving through the lungs.

This maintains a constant oxygen gradient, allowing nearly 100% oxygen absorption, unlike mammalian lungs, which only extract about 50% due to early equilibrium.

This makes avian respiration vastly more efficient.

Credit to Robin Sloan's newsletter for sending me down this rabbit hole and Clint's Reptiles for explaining it