Role of mixed venous blood Pco₂ in respiratory control

¹ Cardiovascular Research Institute, University of California School of Medicine, San Francisco, California

The Pco₂ 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 Pco₂ 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 Pa_COCO2 remained constant; respiration increased only when Pa_COCO2 rose. Correlation of ventilatory minute volume with Pv_COCO2 was poor (r = 0.296; P > 0.1) but was good with Pa_COCO2 (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 Pa_OO2 or Pa_COCO2. The absence of demonstrable ventilatory response to increased Pv_COCO2 over periods of 0.5–3.0 min in animals in whom increments in Pa_COCO2 led promptly to increased ventilation indicates that there are no CO₂ receptors in the precapillary pulmonary circulation of importance in the physiological regulation of respiration.