Dynamics of arterial and portal venous flow interactions in perfused rat liver: an intravital microscopic study.

Intravital epifluorescent microscopy was used to quantitate microvascular parameters in the single-pass, dually perfused rat liver preparation. Livers perfused via the hepatic artery (HA) and portal vein (PV) at physiological pressures and perfusion rates responded to vasoactive agents and exhibited the HA buffer response. The distribution of arterial blood was found to be highly heterogeneous, whereas PV flow was distributed uniformly. The intrasinusoidal velocity of fluorescein isothiocyanate (FITC)-labeled red blood cells (RBCs) arriving from the HA was higher than that for RBCs arriving from the PV, indicating a shorter transit time for the arterially delivered FITC-RBCs. Experiments on livers perfused simultaneously via the HA and retrogradely via the hepatic vein revealed the presence of arteriovenous shunts, with some of the arterially delivered FITC-RBCs reaching the terminal hepatic venules via direct channels without traversing the sinusoidal bed. In livers perfused portally only, changes in PV flow rate (from 8 to 20 ml/min) produced small changes in perfusion pressure but large changes in vascular diameters, while portal pressure and transit time of portal blood remained relatively constant. In experiments designed to identify the location of hepatic vascular resistance, it was observed that hepatic venular diameters measured in the preparation under identical pressure and flow conditions were greater during retrograde than during prograde perfusion, suggesting that the site of hepatic vascular resistance is presinusoidal or sinusoidal.