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. 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 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](https://www.robinsloan.com/newsletters/winter-reading/) for sending me down this rabbit hole and [Clint's Reptiles](https://youtu.be/DnLpLLTKyD0?si=TATQOupIRuMfiodj) for explaining it