Water and electrolyte acquisition across the placenta of the sheep.

Fetal plasma is known to be hyperosmotic with respect to maternal plasma. Although some solutes are actively transported across the placenta and occur in higher concentrations in fetal than in maternal plasma, the total concentration difference is reversed by an opposite difference in the concentrations of electrolytes. Calculations on the basis of the Kedem and Katchalsky equations for a homogeneous membrane demonstrate that active solute transfer and a physiologically plausible hydrostatic pressure difference account for the known rates at which water and electrolytes are accumulated in the fetus. Best estimates of the membrane parameters (per kilogram fetal weight) are PSNa, Cl = 0.23 ml-min-1-kg-1, LpS = 5-10(-6) cm5-min-1-kg-1-dyne-1, and sigma NaCl approximately equal to 0.5. (where PS is the permeability-surface area product, LpS is the filtration coefficient, or sigma is the Staverman reflection coefficient). The driving forces for water and electrolyte transfer (active transfer and hydrostatic pressure) are not used as regulators. Rather, electrolyte permeability, which is the major constraint on fetal growth, and which continuously increases during gestation, makes possible the exponential increase in fetal weight during gestation.