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1. The sheep placenta was perfused in situ through the umbilical circulation with a dextran solution containing carbon monoxide and nitrous oxide. 2. The concentration of carbon monoxide in the perfusion fluid after passage through the umbilical circulation was found to be 19±3% of its concentration in the entering perfusion fluid. This represents "shunted umbilical flow," the percentage of the perfusion fluid (and presumably of the fetal blood) which does not exchange substances with the maternal blood. 3. The concentration of nitrous oxide in the perfusion fluid after passing the placental membrane and before admixture of shunted perfusion fluid was found to be 14±4% of its original concentration in the perfusion fluid when the maternal arterial concentration was zero. This leads us to the conclusion that the perfusion fluid did not at any time reach the maternal arterial concentration and that no functionally effective countercurrent flow pattern exists in the sheep placenta. 4. Reversal of the flow direction in the fetal umbilical vessels resulted in a slight increase of the N2 O concentration in the perfusion fluid after passing the placental membrane and before admixture of the shunted perfusion fluid.5. We suggest that the placenta contains a mixture of countercurrent and concurrent flow units. Diffusion of gases along the capillaries within the comparatively thick placental membrane of the sheep is also presented as a possibility.

Transfer of Carbon Monoxide and Nitrous Oxide in the Artificially Perfused Sheep Placenta