Bird lungs are special, because they have an opening at each end.
Special air sacs before the lungs store air before it's ready to enter the lungs, and air sacs after the lungs store air until it's ready to be breathed out. Humans can only get fresh air in their lungs when they breathe in. Above is a simplified diagram of the bird respiratory system. The actual lung itself is the small, thin part between the two larger air sacs. As air passes through, oxygen is removed.