Volume, pressure, and flow relationships in the peripheral venous system

In order to study the pressure flow (P-Q) relationships in the peripheral venous system as a function of blood volume (BV) blood was removed from the right atrium of 7 dogs and passed through an extracorporeal system from which venous return (Q) was measured and returned directly into the pulmonary artery. Right atrial pressure (Pra) was maintained at 0. Guyton has previously shown that Q = (Pms-Pra)/Rv, where Pms = mean systemic pressure and Rv = resistance to venous return. By stopping the extracorporeal circulation, within 12-15 sec Pra rose to a plateau which was used as an estimate of Pms. Pms and Q were varied by incremental changes in BV. Surprisingly, the relationship of Q to Pms was nonlinear and could best be described by a parabola. In a typical example the P-Q relationship is described by the following regression equation: Pms = -0.57 + 4.14Q + 1.31Q² (r = .97). The constantly decreasing slope of the P-Q curve could only minimally be explained by convective acceleration losses within the venous system and is not the result of changes in venous compliance; thus Rv must be increasing as the animal is transfused. Continual measurement of abdominal pressure (Pabd) with a balloon placed at the base of the abdomen has shown a rise in Pabd as Pms was increased by transfusion. Thus, a possible mechanism for the nonlinear P-Q curves is a decrease in the transmural pressure of those abdominal veins which serve as venous resistance vessels.