Brooksby, G. A.

In dogs anesthetized with α-chloralose, passive alterations in splanchnic blood flow over a range of flows between 10 ml/kg min -1 and 50 ml/kg min -1 produced by a blood flow pump or by a change in perfusion pressure were associated with changes in splanchnic blood volume of 0.19 ± 0.1 (SD) ml/kg and 0.43 ± 0.15 ml/kg, respectively, for a change in flow of 1 ml/kg min -1 . Mechanical increase in splanchnic outflow resistance raised portal venous pressure from a control mean of 6.9 ± 4.1 mm Hg to 15.4 ± 3.5 mm Hg and increased splanchnic blood volume by 8.1 ± 2.2 ml/kg. Of the total change in splanchnic blood volume during an 18-second supramaximal stimulation of the thoracic splanchnic nerves, an estimated 65% was released "passively." The magnitude of this passive release was related linearly to splanchnic blood flow. During increased resistance to splanchnic outflow, the passive release was absolutely and relatively reduced, but the volume of blood actively released by constriction of capacitance elements was absolutely and relatively increased. During hemorrhage (7.2 ml/kg body weight), an average of 4.7 ± 0.6 ml/kg of blood (21% of the calculated splanchnic blood volume) was mobilized, and arterial blood pressure decreased an average of 5%. Repetition of the bleeding after bilateral splanchnicectomy passively mobilized 3.25 ± 0.9 ml/kg of blood (68% of the innervated value), but arterial blood pressure declined by an average of 20%.

Arterial inflow to and venous outflow from the vascularly isolated splanchnic region were measured simultaneously by using cannulating electromagnetic square-wave flow transducers in 25 anesthetized dogs with intact abdomens. Change in splanchnic blood volume (SBV) was calculated as the instantaneous difference between inflow and outflow. Addition of the respective increases and decreases in SBV induced by changing carotid sinus pressure from the control value (mean, 136 mm Hg) to 200 or to 40 mm Hg gave an average total volume of 93 ml of blood ( SD , ±21.7) that could be mobilized reflexly from the splanchnic circulation, Electrical stimulation (15 v, 2 cps, 3 msec duration) of the left thoracic splanchnic nerve for 2 to 10 minutes decreased SBV by 159 ml of blood ( SD , ±84). These mean values represent 32 and 48%, respectively, of the calculated total SBV. In both situations the translocation of blood was rapid, 50% of the total change in splanchnic blood flow and SBV occurring within 30 seconds. During a 2-minute hemorrhage (7.2 ml/kg), a mean volume of 56 ml of blood ( SD , ±16) was mobilized from the splanchnic region. This represented 54% of the total volume bled. Under these circumstances, blood moved out of the splanchnic circulation at a fairly constant rate.