Gas exchange in the parabronchial lung of birds: Experiments in unidirectionally ventilated ducks

Abstract

Pulmonary exchange of O 2 and CO 2 was measured in unidirectionally ventilated ducks in an attempt to determine lung O 2 diffusing capacity, D O 2 . Perfusion shunt ( = venous admixture) was estimated from O 2 exchange in hyperoxia, and the ventilation shunt (ventilation of non-perfused parallel lung units) was estimated from exchange of the highly soluble inert gas, chloroform. Differences in the ventilation/perfusion ratio of parallel lung units were assessed from measurement of CO 2 exchange using a parallel two-compartment model. D O 2 values were calculated accounting for ventilation shunt, perfusion shunt, and inhomogeneity. Perfusion shunt averaged 2.7% and ventilation shunt, 9.4%. The ventilation/perfusion ratio in the two compartments differed on the average by a factor of 2.6. The uncorrected values of D O 2 , not accounting for lung inhomogeneities, progressively declined with increasing P O 2 , but this dependence was less pronounced after correcting for lung inhomogeneities. The corrected value of D O 2 averaged 100 μmol·min −1·torr −1 for ducks of 1.8 kg mean body weight. D O 2 did not differ when nitrogen was replaced by helium in the ventilatory gas indicating that diffusion within the air capillaries did not contribute a significant resistance to O 2 uptake. The results suggest that neither functional inhomogeneities nor diffusion between lung gas and blood limit O 2 uptake of the resting duck. Under conditions of elevated metabolism, however, these parameters may become rate-limiting for O 2 supply.