Abstract
The reflex effects of increasing pulmonary ventilation on the responses of the hind-limb and systemic vascular resistances to stimulation of the carotid body chemoreceptors and carotid sinus baroreceptors and to distension of the urinary bladder have been studied in the anaesthetized dog. A preparation was used incorporating total cardiopulmonary bypass to maintain the arterial blood gas composition constant when alterations in pulmonary ventilation were made. The regions of both carotid bifurcations, the arch of the aorta and the cerebral circulation were independently perfused at constant pressure so as to exclude secondary reflexes from arterial baroreceptors. Four levels of pulmonary ventilation were used: 0.095, 0.285, 0.475 and 0.665 l min-1 kg-1 body weight, at a constant frequency of 19 cycles min-1. Increasing the pulmonary ventilation per se in steps from 0.095 to 0.665 l min-1 kg-1 resulted in a significant progressive reduction in hind-limb and systemic vascular resistances which were shown to be due to a reflex from the lungs. Stimulation of the carotid body chemoreceptors by hypoxic hypercapnic blood resulted in an increase in hind-limb and systemic vascular resistances when carried out at each of the four levels of pulmonary ventilation. The size of the increases in vascular resistances, however, was progressively and significantly reduced as the pulmonary ventilation was increased. This partial inhibition of the carotid body reflex vasoconstrictor response was dependent on the innervation of the lungs. Stimulation or unloading of the carotid sinus baroreceptors by altering the perfusion pressure in the vascularly isolated carotid bifurcation regions caused a significant decrease and increase respectively in hind-limb and systemic vascular resistances at all four levels of pulmonary ventilation. Unlike the responses to chemoreceptor stimulation, the size of these responses was unaffected by the level of pulmonary ventilation. Distension of the urinary bladder resulted in a significant increase in hind-limb and systemic vascular resistances. The size of these responses was also unaltered by changing the level of pulmonary ventilation. These results indicate that there is an interaction between the inputs from the lungs and the carotid body chemoreceptors in the control of hind-limb and systemic vascular resistances. In contrast the inputs from the carotid sinus baroreceptors and the urinary bladder were unaffected by the input from the lungs.