Regional control of venous return: liver blood flow.

The aim of the study was to determine whether closing pressures or vascular distensibility can be used to describe liver venous hemodynamics when right atrial pressure is raised. The study was performed using a vascularly isolated pig liver preparation that allowed the independent control of portal vein and hepatic artery inflows and of outflow pressure (Pout). Pressure-flow (P-Q) relationships of both liver vessels were generated at multiple levels of Pout. At Pout of 0 mm Hg, the portal vein P-Q relationship was linear, with a convexity toward the pressure axis at low flows (5 to 10 ml/min/kg). The zero flow pressure was 1.5 +/- 0.2 mm Hg, greater than Pout (p < 0.05). On raising Pout from 0 to 15 mm Hg, the shape of the portal vein P-Q relationships became progressively more linear, with a decrease in slope; no difference between zero flow pressure and Pout was observed. At Pout of 0 mm Hg, the hepatic artery presented a zero flow pressure > Pout. Raising Pout from 0 to 15 and 30 mm Hg resulted in a zero flow pressure always > Pout (p < 0.05). The behavior of the liver vein system is characterized by a zero flow pressure mimicking a classic vascular waterfall and by distensibility, once the waterfall is exceeded. Both factors act to minimize the reduction in venous return with an increased central venous pressure. Flow through the hepatic artery is affected by an increase in backpressure occurring upstream from the sinusoids, reducing arterial inflow for a constant perfusion pressure.