As seen in the linked diagram, maternal arterial hemoglobin is nearly 100% saturated at a PO2 of 80-100 mmHg (Figure 3.04). The pH is 7.40 due to maternal hyperventilation. Oxygen is unloaded at the placenta and the blood returns through the uterine vein, with a PO2 of 35 mmHg and pH of 7.36. The A-V O2 difference is approximately 4.5 ml/100 ml. In contrast, umbilical vein blood, having been arterialized in the placenta, remains relatively desaturated, at a PO2 of 20 mmHg and a pH of 7.32.
The venous blood coming to the placenta through the umbilical arteries has an even lower PO2 of 15 mmHg. The pH (7.24) of this blood is distinctly acid. The A-V O2 difference is only 3.5 ml/100 ml.
Note that although the fetal blood has a greater potential oxygen carrying capacity than that of the mother (i.e. due to high hemoglobin conc.), it never achieves anything close to 100% saturation. In fact, arterial blood in the fetus may be equal to or only slightly more saturated than mixed venous blood in the mother (i.e. 75%).
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