Cross-current gas exchange in avian lungs: Effects of reversed parabronchial air flow in ducks

Abstract

Experiments have been performed in ducks in order to determine which of both previously proposed physical models, the counter-current or the cross-current system, is best suited for analysis of gas exchange in parabronchial lungs of birds. Mesobronchi of both sides of the body were blocked between the origins of ventrobronchi and dorsobronchi using inflatable catheters, and both postthoracic air sacs were opened by introduction of large bore tubing through the body wall into their lumen. In this preparation a continuous parabronchial air flow could be achieved in either of both directions, from ventrobronchi to dorsobronchi by blowing air into the trachea, or in the opposite direction, from dorsobronchi to ventrobronchi, by blowing air from the air sacs into the bronchial system. During steady state, the pulmonary gas flow and P CO 2 and P O 2 in air as well as in arterial and mixed venous blood were measured. The results showed no dependence of partial pressures on the direction of parabronchial air flow. In particular, arterial P CO 2 was found to be lower than expired (=end-parabronchial) P CO 2 with both directions of parabronchial air flow. These findings cannot be easily explained on the basis of a counter-current gas exchange system, which has been proposed by Schmidt-Nielsen (1971), but are in good agreement with the assumption of a cross-current gas exchange system operative in the avian lung.