Novel determination of filtration coefficient of ovine placenta and intramembranous pathway.

We recently reported that distilled water injected into the amniotic compartment is rapidly absorbed by the fetal blood, which perfuses the fetal membranes and the fetal surface of the placenta, i.e., by the "intramembranous pathway." In the present study, we infused warmed distilled water at 2 ml.min-1.kg estimated fetal wt-1 into the amniotic compartment for 6-8 h in 12 chronically catheterized sheep. A steady state was achieved in amniotic fluid, fetal blood, and maternal blood osmolalities in 5.1 +/- 0.3 (SE) h. During the steady state, all of the infused water must pass to the ewe. Because the amount of water directly crossing the fetal membranes into the maternal circulation was not significantly different from zero (3.3 +/- 5.8% of total), we assumed all water crossed the placenta. The placental filtration coefficient was then calculated during the steady state as the infusion rate divided by the changes in the transplacental osmotic gradient and averaged 0.0260 +/- 0.0055 ml.min-1.mmHg-1.kg fetal wt-1. Attempts to correct this value for placental reflection coefficients less than 1 were unsatisfactory; thus the uncorrected value appears to provide the best estimate of the placental filtration coefficient. Similar calculations yielded a value of 0.00137 +/- 0.00025 ml.min-1.mmHg-1.kg-1 for the filtration coefficient of the intramembranous pathway. Thus this method provides a relatively simple new technique for estimating the filtration coefficient of the ovine placenta and intramembranous pathway under steady-state conditions.