Reflex Venoconstriction and Capacity Vessel Pressure-Volume Relationships in Dogs

Reflexogenic control of vascular capacity was studied by measuring the mean circulatory pressure (P MC ) at various blood volumes. P MC was obtained by fibrillating the heart for 10 seconds and rapidly transferring blood from the aorta to the vena cava until the pressures equilibrated. It was measured 0.5, 2, and 5 minutes after randomized changes in blood volume of 0 to ± 30% of the control volume in chloralose-urethane anesthetized dogs. Active venoconstriction was suppressed by administering hexamethonium and enhanced by administering norepinephrine. The effective total vascular compliance, expressed as the reciprocal of the slopes of P MC vs . blood volume curves in the controls, was linear. Compliance values differed significantly from each other and increased between 0.5, 2, and 5 minutes after a change in blood volume (2.60, 3.47, and 4.17 ml/kg body weight mm Hg -1 , respectively). With ganglionic blockade, only an 8-ml/ kg hemorrhage was required to bring P MC from the control level of 7.8 mm Hg to 4 mm Hg within 0.5 minutes. With reflexes intact, a 17-ml/kg hemorrhage reduced P MC from a control of 10.7 mm Hg to 4 mm Hg. To maintain this pressure for 5 minutes, an additional 8.2 and 9.1 ml/kg, respectively, had to be hemorrhaged, suggesting that the compensation after about 30 seconds was mostly from passive viscoelastic creep and fluid shifts. Less than half of the compensation for hemorrhage during the first 5 minutes came from the reflex venoconstriction.