Passive autoregulation of portal venous pressure: distensible hepatic resistance.

Hepatic resistance to portal blood flow is extremely low and both the pre- and postsinusoidal resistance sites are distensible. Both isolated in situ and in vivo vascular circuitry were used in cats to demonstrate the principle of distensible resistance as a mechanism for the observation that blood flow was able to be decreased from 50 to 20 ml.min-1 x kg-1 while intrahepatic pressure decreased by only 1.4 +/- 0.2 mmHg and portal pressure by 2.0 +/- 0.4 mmHg. Presinusoidal resistance increased by 226% and hepatic venous resistance by 57%, thus accounting for passive autoregulation of portal pressure. The relation between vascular resistance and the distending blood pressure that acts on the resistance is predictable from the relationship IC = R.Pd3, where IC is the index of contractility (does not change passively, but does change with active vascular tone changes), R is vascular resistance (changes actively and passively), and Pd is distending blood pressure (estimated as the average of pressure on either side of the resistance vessels). The relatively minor effect of portal flow on portal pressure is accounted for by a combination of factors including the low basal resistance, the distensible resistance, the hepatic arterial buffer response, and hepatic blood volume compliance. By calculation of IC, the venous distensibility can be quantified and the passive effect of flow changes on portal and intrahepatic pressure determined.