Placental transfer of water and nonelectrolytes during a single circulatory passage

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Abstract
A bolus of a 14C-labeled nonelectrolyte and tritiated water was injected into the maternal arterial system supplying the uterine circulation. Blood was sampled immediately on the fetal side of the placenta from the umbilical vein. A placental transfer index (PTI) was calculated from the ratio of the tracers in the fetal blood divided by the ratio of the tracers in the injectate. Placental transfer was measured for 11 nonelectrolytes in anesthetized guinea pigs and 8 nonelectrolytes in unanesthetized sheep. Placental transfer in both species was determined by molecular weight and lipid solubility. The PTI measured for the lipophilic solutes antipyrine and 1,6-hexanediol were less than 1, and those for methanol and ethanol were greater than 1. We conclude that placental transfer for water and small (mol wt less than 200) lipid-soluble nonelectrolytes is limited by diffusion as well as by blood flow.