Control of respiration in the chicken: effects of venous CO2 loading.

To determine if ventilation in unanesthetized chickens is adjusted sufficiently to prevent alterations in the partial pressure of carbon dioxide in arterial blood (PaCO2) when the CO2 content of mixed venous blood is changed, hypercapnic (PCO2 about 533 Torr) and hypocapnic (PCO2 less than 10 Torr) blood was infused into the left jugular vein of decerebrate chickens at 38 ml . min-1 for 30 sec. Ventilation and PaCO2 were assessed by determining respiratory frequency (f), tidal volume (VT), and the end-tidal CO2 fraction while serial samples of arterial blood were withdrawn from the sciatic artery. Infusion of hypercapnic blood resulted in an increase VT and minute ventilation (VE) as well as an increase in PaCO2. Infusion of hypocapnic blood resulted in a decrease in VT and VE and a small, transient decrease in PaCO2; the PaCO2 often returned to control levels before the end of the infusion period. The respiratory control system in the chicken appears to be better able to maintain a constant PaCO2 when perturbed by a reduced venous CO2 load reaching the lung than when perturbed by a reduced venous CO2 load reaching the lung than when perturbed by an increased CO2 load. These results are consistent with the hypothesis that intrapulmonary CO2 receptors, whose sensitivity to PCO2 is highest at low PCO2, are involved in the breath-to-breath control of breathing in birds.