Structure and function of the avian respiratory system

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 2 . The weak allometric scaling of the thickness of the BGB indicates optimization for gas exchange; the negative scaling and strong correlation between the surface density of the respiratory surface area and body mass show the extreme subdivision of the gas exchange tissue; and the respiratory surface area, the pulmonary capillary blood volume and the total pulmonary morphometric diffusing capacity of O 2 correlate strongly and positively with body mass. The arrangement of the structural components of the exchange tissue form crosscurrent-, countercurrent-like- and multicapillary serial arterialization gas exchange designs. By synchronized actions of the air sacs, the palaeopulmonic part of the of the avian lung is efficiently ventilated continuously and unidirectionally in a caudocranial direction. This article is part of the theme issue ‘The biology of the avian respiratory system’.