Factors affecting O2 transfer in sheep and rabbit placenta perfused in situ

In these experiments we have studied three factors that affect placental O2 transfer. The fetal artery of an isolated cotyledon of the sheep placenta (or one of the umbical arteries in flow studies in rabbits) was perfused in situ with blood of varying PO2 and at different flow rates while the ewe was administered varying inspired O2 concentrations. Measurements were made of the PO2 of inflowing and outflowing umbilical blood, and the O2 transfer rate was calculated by the Fick principle. Changes in individual factors could be studied, since most compensations tending to maintain normal O2 delivery were not operative in the isolated preparation. Results indicate that a 20% change in umbilical arterial PO2, a 14% change in umbilical blood flow, and a 20% change in maternal arterial PO2 would be equivalent in causing a 10% change in placental O2 transfer. Small changes in umbilical arterial PO2 are sufficient to maintain the rate of placental O2 transfer equal to the rate of fetal consumption. Maternal arterial PO2 becomes progressively more critical to fetal oxygenation as its level falls. The experimental results are compared to those predicted by a mathematical model of placental exchange.