INVARIANCE OF THE RESISTANCE TO VENOUS RETURN TO CAROTID-SINUS BAROREFLEX CONTROL

Despite the well-established fact that the carotid sinus baroreflex sy stem has profound control over the physical properties of the systemic circulation, the resistance to venous return (RVR) seems to be invari ant of such control. We hypothesized that this apparent paradox may be explained from the baroreflex changes in systemic arterial compliance . In 12 pentobarbital-anesthetized mongrel dogs, RVR was measured at c ontrolled carotid sinus pressures (CSP) of 50 and 200 mmHg with normal and artificially increased arterial compliance. Arterial compliance w as determined from the arterial pressure decay when systemic blood flo w was stopped with total vena caval occlusion. Changing CSP between 50 and 200 mmHg changed RVR significantly only under the condition of ar tificially increased arterial compliance. A four-parameter lumped mode l of the systemic circulation revealed that the baroreflex changes in arterial compliance and arterial resistance, which occurred in opposit e directions, prevented a change in RVR when CSP was changed. The data also suggested that similar to 75% of RVR was attributed to large and conduit veins, the resistances along which were insensitive to barore flex control. We concluded that the invariance of RVR results from a c ombination of 1) baroreflex change in the arterial compliance, 2) baro reflex insensitivity of the resistance along large and conduit veins, and 3) spatially distinct location between the major site of reflex ch ange in capacitance and the major site of compliance.