Role of mixed venous blood Pco2 in respiratory control

The P co2 of mixed venous blood has been increased abruptly by 5—49 mm Hg, without altering the volume or pressure of blood returning to the right ventricle, by infusion of blood with 550—700 mm Hg P co2 into the right atrium, and simultaneous withdrawal of equal volumes of blood from the inferior vena cava. Twenty-five such blood exchanges in four anesthetized cats, four anesthetized dogs, and three unanesthetized dogs failed, with one exception, to increase respiratory frequency or tidal volume if PaCO CO2 remained constant; respiration increased only when PaCO CO2 rose. Correlation of ventilatory minute volume with PvCO CO2 was poor ( r = 0.296; P > 0.1) but was good with PaCO CO2 ( r = 0.608; P < 0.001). Experiments on the innervated and denervated carotid body (dog) demonstrated that chemoreceptors can respond rapidly (0.5 sec) to changes in PaO O2 or PaCO CO2 . The absence of demonstrable ventilatory response to increased PvCO CO2 over periods of 0.5–3.0 min in animals in whom increments in PaCO CO2 led promptly to increased ventilation indicates that there are no CO 2 receptors in the precapillary pulmonary circulation of importance in the physiological regulation of respiration. Submitted on May 25, 1961