Efficiency of parabronchial gas exchange in deep hypoxia: measurements in the resting duck

Abstract

Cardio-respiratory parameters and air sac and blood gases were measured in the unrestrained, unanesthetized duck during exposure to varied levels of inspired hypoxia. As ventilation increased with hypoxia, the gas partial pressures in the air sacs and in arterial blood approached the inspired values, the differences being less than 3 Torr for air sacs and less than 5 Torr for arterial blood. To analyze the relative significance of ventilation, diffusion and perfusion in limiting parabronchial gas exchange, the conductances for ventilation (Gvent), diffusion (Gdiff = O2 diffusing capacity) and perfusion (Gperf) were calculated at all hypoxic levels. With increasing hypoxia, down to PIO2 = 50 Torr, all three conductances increased. Whereas Gvent continued to increase beyond this level, Gdiff remained constant, at about 0.27 mmol.min-1.Torr-1, while Gperf decreased, from a value of 0.22 mmol.min-1.Torr-1 at PIO2 = 50 Torr to 0.12 mmol.min-1.Torr-1 at PIO2 = 30 Torr. This reduction in Gperf may result from a direct hypoxic effect on the heart. Whereas ventilation is the main limiting factor for parabronchial gas exchange at rest down to hypoxia levels of PIO2 = 50 Torr, perfusion becomes the main limiting factor at deeper hypoxia. It is suggested that the higher tolerance of hypoxia exhibited by birds compared to mammals is not due to the higher efficiency of parabronchial compared with alveolar gas exchange, but reflects the ability of birds to tolerate lower arterial PCO2 level…