Birds
Birds have a unique repiratory system. Birds need a tremendous amount of oxygen to gain energy for flight, so they have developed a different repiratory system. Unlike mammals, the iungs of birds do not inflate or deflate as they breathe in and out. Instead, they have air sacs which are like tiny air bags in cars. These air sacs ventilate air, leaving their iungs full of air at all times. The process of oxygen circulating through the body in birds is done through small blood vessels.