The degree of increased fetal oxygen affinity varies from species to species. For example, the ODC of the fetal sheep is greatly left-shifted as compared to the adult sheep, while the ODC of the fetal rabbit is only slightly left-shifted as compared to the adult rabbit (Figure 3.03).
As can be seen, the difference in P50 values between maternal and fetal bloods in the sheep is 17 mmHg, while in the rabbit it is only 4 mmHg. It is the same in the human. The P50 value is the partial pressure of oxygen that produces a 50% saturation of the hemoglobin.
In contrast, various species differ greatly in the oxygen carrying capacity of the maternal versus fetal blood (Table 3.05). In sheep there is very little difference (+2 ml oxygen/100 ml blood), while in humans the difference is great (+7 ml oxygen/100 ml blood).
Published ODC's may be misleading if measurements were not made at physiological pH values. For example, the ODC of human fetal blood appears greatly left-shifted in many diagramsrelative to the maternal, but when the fetal acidosis is taken into account the ODC moves a bit closer to that of the maternal blood due to a positive Bohr shift.
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