Fluid shift across the placenta: I. The effect of dextran T 40 in the isolated guinea-pig placenta.

In an effort to understand better the transplacental water flux induced by osmotic forces, 17 near-term guinea-pig placentae were perfused on both sides with an artificial cell-free fluid containing dextran. Maternal and fetal arterial inflow rates were kept constant by syringe pumps and inflow and outflow pressures were recorded continuously. Precise measurements of outflow rates were made by weight determination and transplacental fluid shifts calculated by weight changes during placental passage. The content of dextran T 40 in the perfusion fluid was altered in stepwise manner from 0 to 60 to 120 g/l, and the effects of all possible combinations of fetal--maternal concentration differences on the transplacental fluid shift were observed. On the average, about 1 X 10(-3) ml/min/g placenta was moved per 1 g/l concentration difference. In two experiments venous colloid osmotic pressures were recorded continuously. In ten experiments the oncotic fluid shift was counterbalanced by an additional hydrostatic pressure difference; an average 1 mmHg hydrostatic pressure counterbalancing 15 g/l of dextran concentration osmotic force. The calculated hydraulic conductance, Lp, was approximately 0.017 ml/min/mmHg/g and the reflection coefficient, s, for dextran T 40 was 0.1. Both parameters characterize the isolated guinea-pig placenta as leaky.