Changes in systemic vascular volume in response to the carotid sinus baroreceptor reflex are primarily driven by active changes in vascular capacity rather than blood flow redistribution.
To quantify the relative contribution of blood flow redistribution and active changes in vascular capacity in the regulation of cardiac output, blood flow and volumes in two parallel vascular beds were measured in response to varying carotid sinus pressures. In nine dogs, carotid sinuses were isolated and intrasinus pressure was controlled. Two external reservoirs were placed between the caval veins and the right heart to measure changes in vascular capacity in splanchnic and extrasplanchnic vascular beds. At intrasinus pressures of 50 and 200 mm Hg, we have simultaneously measured arterial resistances, compliances, changes in flows, and "unstressed vascular volume," and time constants of venous drainage in the splanchnic and extrasplanchnic vascular beds. Compliances and time constants of venous drainage were found to be nearly equal in the two beds. A decrease in intrasinus pressure from 200 to 50 mm Hg resulted in a small redistribution of blood flow (about 5% of cardiac output) from the extrasplanchnic compartment to the splanchnic vascular bed. Changes in reservoir volumes were found to be around 7.0 ml/kg. The splanchnic vascular bed was responsible for a greater change in reservoir volume for a given change in intrasinus pressure. With any change in intrasinus pressure, the change in arterial resistance in the extrasplanchnic vascular bed was greater than that of the splanchnic vascular bed. Blood flow redistribution was not found to be a significant factor contributing to changes in reservoir volume. The changes in reservoir volume seen, must have been due to active changes in vascular capacity in the two channels chosen.
Brunner et al. (Sun,) studied this question.