Oxygen transfer from the maternal blood pool to the fetal capillaries is totally dependent upon diffusion. Thus oxygen is always moving down its concentration gradient. Because the membranes separating these two blood systems are thicker than those in the adult lung, diffusion is slower and requires a larger concentration gradient. If it were not for the higher oxygen affinity of fetal blood, the percent saturation of fetal hemoglobin would be much lower than is actually the case.
A number of factors affect the maternal to fetal oxygen exchange (Table 3.06). First is the difference in the ODC's of the two hemoglobins, maternal and fetal. Relative affinity can be expressed by the P50 value, generally lower for fetal bloods. The degree of maternal hyperventilation influences the position of the ODC; a greater hyperventilation increases alkalosis, which shifts the ODC to the left (a reverse Bohr effect). The greater this effect, the closer together the maternal and fetal ODC's become. This tendency, however, is largely counteracted by 2,3-diphosphoglycerate synthesis.
In a similar fashion, the degree of fetal acidosis affects the position of the fetal ODC; more acidity pushes the curve to the right through a Bohr shift. As discussed above, the difference in oxygen carrying capacities of the two bloods are also important. Fetal blood usually has a higher hemoglobin concentration than maternal blood, which allows it to hold more oxygen when fully saturated (ml oxygen/100 ml blood).
Finally, substances that incapacitate hemoglobin as an oxygen carrier, such as carbon monoxide found in cigarette smoke, affect the maternal to fetal transfer of oxygen. High altitude which lowers the PO2 in the air, limits the saturation of the maternal, and, in turn, the fetal hemoglobin.
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