Among the extant air-breathing vertebrates, the avian respiratory system is the most efficient gas exchanger. Novel morphological and physiological adaptations and specializations largely explain its exceptional functional superiority. Anatomically, the avian respiratory system is separated into lungs that serve as gas exchangers and air sacs that operate as ventilators. Utterly rigid, the avian lungs are deeply fixed to the ribs and the vertebrae. A thin blood-gas barrier (BGB), vast respiratory surface area and large pulmonary capillary blood volume generate high total pulmonary morphometric diffusing capacity of O