Utilization of substrates by the ovine placenta in vivo

The uptakes of oxygen, glucose, and amino acids by the pregnant uterus via the uterine circulation and by the fetus via the umbilical circulation have been measured in sheep during the last month of gestation. The umbilical uptakes of oxygen and glucose are approximately 55% and 28%, respectively, of the total uterine uptake. This discrepancy between uterine and umbilical uptakes is due primarily to a large utilization rate of oxygen and glucose by the placenta. Part of the placental utilization of glucose can be accounted for by placental lactate excretion into both maternal and fetal blood. In marked contrast to the pattern of glucose utilization, the comparison of uterine and umbilical amino acid uptake shows that the bulk of the amino acids taken up by the pregnant uterus is transferred to the fetus. The placenta utilizes glutamate of fetal origin and produces ammonia, which is excreted primarily into the maternal circulation.