Path taken by gases through the respiratory system of the chicken

¹ Departments of Medicine, Environmental Medicine, Pathobiology, and Physiological Chemistry, The Johns Hopkins University, Baltimore, Maryland

The path of gases through the avian respiratory system was studied by comparing Pco₂ and Po₂ in expired gas and air sacs and Po₂ in left ventricular blood. A respiratory mass spectrometer was used for gas analyses and an O₂ electrode for blood Po₂. Findings were: a) Pco₂ and Po₂ in air sacs varied little during the respiratory cycle; they fell when N₂ was injected into the sac but returned rapidly to the original level in steps, a step with each inspiration. b) Po₂ of left ventricular blood and air sac gas were essentially the same and varied less than 5% of total Po₂ during the respiratory cycle. c) End-tidal Pco₂ and Po₂ were essentially the same as air sac Pco₂ and Po₂. The first two findings suggest that gas reaches the air sacs only via the respiratory surfaces of the lung. The third suggests that during expiration gas passes directly from air sacs to mouth without passing again over the respiratory surfaces. The path is therefore mouth-lung-air sacs-mouth, with almost complete valving despite the absence of demonstrable anatomical valves.