Air Sac
Imagine trying to blow up a balloon by repeatedly inhaling and exhaling without taking the balloon off your lips. This process is similar to how air sacs in birds function, as they allow for a continuous flow of air through the lungs, much like how you need a constant flow of air to inflate the balloon efficiently. In both cases, the air sacs and your repeated breaths ensure that air is always moving in one direction, allowing birds to maximize oxygen intake without stopping, just as you need continuous airflow to blow up the balloon.
Practice Version
Air Sac: An air-filled space that moves air through a birds lungs. Air sac. An air sac is part of a bird's respiratory system that helps move air efficiently through their lungs for breathing.