The Oxygen Pressure Gradient Between the Maternal and Fetal Blood in Pregnant Sheep *

Recent studies on the relations of the fetal and maternal blood vessels in the cotyledonary placenta of the sheep (Barcroft and Barron') have demonstrated that the vessels are so arranged that the streams flow th.rough parallel cords of capillaries in opposite directions. This arrangement of the two sets of capillaries brings the fetal blood leaving the placenta in -relation to the maternal blood entering; similarly, the maternal blood leaving the placenta is adjacent to the blood delivered by the umbilical arteries. Further, it provides for the maintenance of the maximum average pressure difference between the freely diffusible constituents of the fetal and maternal bloods for the entire length of the parallel capillaries. The gases in fetal blood emerging from the cotyledon should be in equilibrium with those in the entering maternal blood. The object of the experiments to be reported here was to estimate the magnitude of the oxygen pressure difference between the fetal and maternal bloods and to discover whether or not this difference aliters as gestation advances and the fetal and maternal capillaries 'become more and more closely approximated. The gas pressures were estimated by referring the percentage saturation of the blood samples to fetal and maternal dissociation curves appropriate for the several stages of gestation. Some of these curves (to be published) were prepared earlier; others were determined from bloods o-btained in the course of this investigation. In each case the blood was exposed to varying oxygen pressures, while th'e carbon dioxide pressure was held as nearly as possible to 50 mm. Hg and the temperature was maintained at 380 C. For this study nine iewes were available. They were stocked on known dates and kept on a farm near New Haven until shortly before they were to be sacrificed, then they were brought to the