Physiological evidence consistent with the presence of a specific O2 carrier in the placenta

In 30 experiments we perfused the fetal side of the sheep placenta with fluids which had different solubilities for argon, N2, and O2 (dextran, blood, and fluorocarbon emulsions). In some of the experiments we partially exchange-transfused the ewe with the fluorocarbon emulsion. By these procedures we were able to change the physical solubility of argon and N2 severalfold in the fetal perfusion fluid and maternal blood. We found that the diffusing capacity for argon and N2 did not increase with increases in physical solubility in the fetal perfusion medium or in maternal blood. This indicated that the rate-limiting step in the placental transfer of these gases is the small diffusing capacity of the placenta. In contrast, O2 diffusing capacity increased markedly with increased solubility in the fetal perfusion medium. Also the Po2 was frequently the same in the venous blood leaving both sides of the placenta. This indicates that O2 may reach equilibrium between maternal and fetal capillaries in one pass through the placenta. The results are compatible with the presence of specific O2 carrier in the placenta.