Dynamic changes in venous outflow by baroreflex and left ventricular distension.

We examined the dynamic changes in venous outflow from the splanchnic and extrasplanchnic vascular beds in response to carotid sinus (CS) baroreflex and left ventricular (LV) distension in 12 dogs anesthetized with pentobarbital sodium. Splenic sympathetic nerve activity was measured in an additional group of six dogs. A heart-lung bypass was used with constant cardiac output and constant venous pressure. LV distension was produced by inflating a balloon in the LV. LV distension and an increase in CS pressure from 50 to 200 mmHg decreased blood pressure by 26 +/- 5 and 30 +/- 6 mmHg and increased vascular capacitance by 5.5 +/- 0.9 and 4.5 +/- 1.2 ml/kg, respectively. Splanchnic venous outflow exhibited a transient decrease, whereas extrasplanchnic venous outflow showed a transient increase, in response to LV distension and increasing CS pressure, accompanied by a sustained decrease in splenic nerve activity. The results indicate important differences between splanchnic and extrasplanchnic components of the total venous system in terms of the regulation of venous capacitance. It is suggested that changes in venous capacitance produced by LV distension and CS baroreflex are primarily due to active changes in splanchnic venous tone.