Effects of hematocrit on oxygenation of the isolated perfused rat liver

The isolated perfused rat liver is used ubiquitously for metabolic and endocrine studies of hepatic function, yet few data are available regarding the inadequacy of the oxygenation of such preparations. Moreover, the isolated rat liver is usually deprived of its arterial supply and perfused via the hepatic portal vein with low-hematocrit or cell-free solutions. To investigate the efficacy of the oxygen supply, we determined the effect of hematocrit on the relation between oxygen consumption and perfusate flow. We then attempted to define a hematocrit at which hepatic oxygenation was maximal. Livers of male rats anesthesized with pentobarbital sodium were perfused via the portal vein with fresh canine red blood cells suspended in Krebs-Ringer-bicarbonate buffer. Perfusions were carried out at various flow rates, and the relation between perfusate flow and oxygen uptake was determined. At flow rates above 100 ml X min-1 X 100 g liver-1, oxygen uptake was independent of flow but below that value was flow limited, regardless of whether the hematocrit was 10, 20, or 40%. To determine the optimal hematocrit for hepatic oxygen uptake, hepatic portal venous and hepatic venous pressures were held at 10 and 0 mmHg, respectively. The hematocrit was lowered in steps from 80 to 10%. Blood flow increased exponentially as hematocrit fell while oxygen uptake increased to a maximum at approximately 20%. It is concluded that an hematocrit of approximately 20% provides the optimal combination of blood flow and oxygen-carrying capacity while maintaining physiological perfusion pressures, e.g., 10 mmHg.