The oxygen consumption of the placenta and foetal membranes in the sheep.

1. In anaesthetized sheep near term the O(2) uptake of the foetus from the placenta was reduced to zero either by replacing the foetus with a mechanical pump, or by ventilation of the foetus after delivery or in utero. When umbilical arterial and venous P(O2) were made equal, they were much less than maternal arterial P(O2).2. This difference in P(O2) was attributed to O(2) consumption both within the placental cotyledons and the extra-cotyledonary foetal membranes.3. The foetal membranes and cord in the sheep are supplied by branches of umbilical blood vessels and constitute an oxygen-consuming by-pass on the foetal side of the placenta.4. The O(2) consumption of individual placental cotyledons was measured in vivo.5. The combined O(2) consumption of the whole placenta and foetal membranes in vivo was a considerable fraction of foetal O(2) consumption.6. The consequences of these observations are discussed in relation to O(2) transfer across the placenta.