AJP - Heart and Circulatory Physiology, Vol 233, Issue 4 475-H487, Copyright © 1977 by American Physiological Society
Water and electrolyte acquisition across the placenta of the sheep
E. E. Conrad Jr and J. J. Faber
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.
