Compliance of the fetal sheep liver

To estimate the importance of the liver as a fetal blood reservoir, we measured the compliance of the liver in eight near-term fetal sheep in utero. Balloon-tipped catheters were positioned above and below the entry of the hepatic veins into the inferior vena cava. Then the hepatic artery and portal vein were ligated and a flow probe placed around the common umbilical vein, the only remaining blood supply to the fetal liver. The balloons were inflated to block hepatic outflow momentarily while flow into the liver and pressure at the outflow of the liver were recorded continuously. Compliance was obtained from the pressure--volume curves and averaged 3.74 +/- 0.49 (SEM) ml/mmHg per 100 g of liver or 1.02 +/- 0.09 ml/mmHg per kg body weight. We were able to simulate these responses with a simple mathematical model of the hepatic circulation with constant compliance and pre- and post-sinusoidal resistances to blood flow. In a second group of none fetal sheep in utero whole body vascular compliance was estimated by measuring the venous pressure rise following the infusion of 60 ml of warm 5% (w/v) glucose in water during a 20 s interval. Whole body compliance averaged 3.66 +/- 0.37 ml/mmHg per kg body weight. Thus, the liver is responsible for about 28% of the whole body compliance on the fetal sheep and may function, therefore, as a significant blood reservoir.