Invariance of the resistance to venous return to carotid sinus baroreflex control

Despite the well-established fact that the carotid sinus baroreflex system has profound control over the physical properties of the systemic circulation, the resistance to venous return (RVR) seems to be invariant 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 controlled carotid sinus pressures (CSP) of 50 and 200 mmHg with normal and artificially increased arterial compliance. Arterial compliance was determined from the arterial pressure decay when systemic blood flow was stopped with total vena caval occlusion. Changing CSP between 50 and 200 mmHg changed RVR significantly only under the condition of artificially increased arterial compliance. A four-parameter lumped model of the systemic circulation revealed that the baroreflex changes in arterial compliance and arterial resistance, which occurred in opposite directions, prevented a change in RVR when CSP was changed. The data also suggested that ~75% of RVR was attributed to large and conduit veins, the resistances along which were insensitive to baroreflex control. We concluded that the invariance of RVR results from a combination of 1) baroreflex change in the arterial compliance, 2) baroreflex insensitivity of the resistance along large and conduit veins, and 3) spatially distinct location between the major site of reflex change in capacitance and the major site of compliance.