Effects of CO2 on pulmonary air flow resistance in the duck

Effects of CO2 on pulmonary smooth muscle were assessed by measuring the air flow resistance of secondary bronchi and parabronchi in ducks unidirectionally ventilated with a constant gas flow through the parabronchial lung, the bypass of the primary bronchus being occluded by a blocking catheter. Pressure differences across the blocking balloon (deltaP), corresponding to the pressure drop in the gas flowing through the mediodorsal and medioventral secondary bronchi (MD and MV) and parabronchi, were measured at flow rates (V) varied from 0.5 to 3 L-min-1 and at CO2 concentrations of ventilating gas (FICO2) varied from 0 to 10%. 1) deltaP increased more than linearly with V. The resulting flow resistance R(= deltaP/V) averaged 43 and 95 cm H2O-L-1-sec at V = 0.5 and 3 L-min-1, respectively. 2) Step changes in FICO2 at constant V were followed within 0.5 to 5 sec by changes in R. 3) Lowering FICO2 from 5% resulted in marked increases in R, the value at FICO2 = 0% being more than twice the average value at 5%. Raising FICO2 from 5% up to 10% was followed by only slight changes in R. 4) Vagotomy did not consistently change R at any level of CO2; it did, however, slightly increase the delay time for changes in R on step changes of FICO2. 5)The medioventral secondary bronchi and their orifices into the primary bronchus appeared to be mainly responsible for the resistance measured and its changes with CO2. The resistance offered by the parabronchi appeared to be much smaller and much less dependent on CO2. The results suggest importance of lung gas CO2 in aerodynamic valving of respiratory flow in avian lungs during normal breathing and particularly during thermal panting to prevent alkalosis.