A radioactive isotope dilution technique, in which two radioactive tracers were used simultaneously, was employed to investigate the nature of the hepatic arterial vascular bed or arterial vascular "space" in 22 isolated perfused canine livers. The technique continuously monitored the blood leaving the liver for radioactivity so that no assumptions had to be made regarding the representative nature of samples. The concept of an arterial vascular space was defined in terms of the mean circulation time of labeled red cells through the perfused organ, and the variation of this parameter with variations in arterial flow was considered; agreement with other published work was shown. The experiments on the size of the arterial vascular space showed a linear relationship between the arterial fraction of the blood occupying the total hepatic vascular bed and the arterial fraction of the total hepatic blood flow. The evidence from these experiments supported the idea that the artery and the portal vein share a large proportion of the total hepatic vascular bed.ADDITIONAL KEY WORDS isotope dilution arterial vascular "space" mean circulation time hepatic vascular bed portal vein

Summary. The changes in the drculatoTy pathways were studied in an isolated perfused canine liver, when the venous circulation was reversed. A radioactive isotope dilution technique was employed, in which two radioactive tracers were used simultaneously, to investigate the nature of the hepatic vascular bed under normal and reversed conditions. The results indicate that when the venus circulation is reversed the hepatic arterial blood fills far less of the vascular bed than under normal conditions. This indicates that there may be a low impedance pathway between the hepatic artery and the portal vein, which enables the hepatic arterial blood, under reversed flow conditions, to reach the portal tract without it traversing the normal arterial pathways through the liver.

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