Structure and Function of the Avian Respiratory System, with Observation on its Predisposition to Injury by Particulates and Pathogenic Microorganisms

Among the air-breathing vertebrates, the avian respiratory system, the lung-air sac system, is structurally the most complex and functionally the most efficient gas exchanger. The lung which is small and compact is firmly held between the ribs and the vertebrae while the air sacs are capacious transparent membranous structures which are widely spread in the coelomic cavity. The lung is ventilated unidirectionally and continuously by a bellows-like synchronized activity of the air sacs. It takes two inspiratory cycles and two exhalatory cycles for the air to enter and exit the lung. Morphometrically, the avian lung is highly specialized. Compared to a mammal of equivalent body mass, the bird lung has a respiratory surface area which is 15% greater and the blood-gas barrier is 62% thinner. Free (surface) macrophages are scarce on the the respiratory surface of the avian lung. Because of this it has been argued that birds have poor pulmonary cellular defences and are therefore highly vulnerable to injury by particulates and infection by pathogens. This conclusion has, however, been questioned. Diverse phagocytic cells which include subepithelial macrophages, pulmonary intravascular macrophages, bronchial epithelial cells, and free macrophages should provide adequate protection. The large tidal volume of birds and the extension of the air sacs to lie superficially, are some of the features which make birds susceptible to pulmonary afflictions and infections.