An air sac is part of the bird's respiratory system. It is a series of thin-walled sacs, usually eight or nine, that comprise the system along with the lungs. It makes the bird respiratory system more effective. When the bird inhales, the lungs fill up as well as the air sacs. This allows the bird to take in more oxygen. When the bird exhales, the air from the lungs exits the body, but the air from the air sacs enters into the lungs. When the oxygen from the air sacs enters into the lungs it is distributed to the body. Therefore, whether the bird inhales or exhales it is constantly getting a fresh supply of oxygen.