Model study of placental water transfer and causes of fetal water disease in sheep.

The purpose of the computer simulation was to use experimentally measured parameters of placental water transfer to compute conceptual water acquisition during the last third of ovine pregnancy and to evaluate the possible role of each of these parameters in the pathophysiology of polyhydramnios. Total conceptual water at birth was almost insensitive to the value of the placental filtration coefficient. It was more sensitive to fetal and maternal placental blood flows, the concentrations of actively transported or metabolically produced solutes in fetal plasma (bicarbonate, fructose, alpha-amino acids, urea, lactate), and the hydrostatic pressure difference across the placental barrier. It was quite sensitive to the NaCl reflection coefficient and permeability. We conclude that the actively transported or produced solutes in fetal plasma constitute a primary driving force. The opposing diffusion gradient of NaCl, and to a much lesser extent that of glucose, are essential to restrain the process. The causes of polyhydramnios in this species are, in order of probability, an increase in the placental diffusion permeability of NaCl, a decrease in the placental reflection coefficient for NaCl, or an increase in the concentration of an osmotically effective solute in fetal plasma, by active transport or metabolic production.