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