How do birds keep their lungs filled with oxygenated air

Birds have air sacs that store inhaled air and push into the lungs like bellows. This process keeps the lungs filled constantly oxygenated air. Their lungs have millions of teeny-weeny passages that create a large surface area for gas exchange with the blood.

(P.S.) Bellows means, "A device with an air bag that emits a stream of air when squeezed together with two handles, used for blowing air into a fire." courtesy of Google's dictionary. ❤