Intrapulmonary CO2-rise time and ventilation in ducks

Many investigators have reported finding intrapulmonary chemoreceptors (IPCs) in several species of birds and reptiles; however, the role of IPCs in ventilatory control in birds has not been identified. This study was undertaken to assess the role of intrapulmonary CO2 dynamics on the control of breathing. Ducks were unidirectionally ventilated, and their breathing was monitored with a pneumotachograph connected to a personal computer. A gas-mixing system controlled by the computer adjusted the rate of rise of airway CO2 concentration ([CO2]). On inspiration, the computer removed CO2 from the ventilating gas for 1.5 s, followed by a controlled [CO2] rise. Breathing frequency was directly related to the rate of rise of airway [CO2]. Tidal volume, however, was not correlated with CO2-rise time but was related to the peak airway [CO2]. This response is likely mediated by IPCs because preventing airway [CO2] from falling during inspiration immediately altered that breath. An increase in CO2 production (as in exercise) will lead to an increase in the rate of CO2 excretion into the lung. The resulting alteration of breathing frequency would thus maintain acid-base balance. The observed response, therefore, may represent a link between ventilation and metabolism.