Maternal-Fetal Oxygen Transfer in Lower Vertebrates

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There exists a difference in oxygen affinity between fetal and maternal bloods in almost all vertebrates examined and this difference in affinities probably facilitates oxygen transfer to the fetus. It is likely that the high oxygen affinity of fetal blood represents a biochemical pre-adaptation from an ancestral oviparous embryo for oxygen uptake in a relatively hypoxic environment. In most cases, the maternal-fetal difference in blood oxygen affinities is due to the characteristics of the fetal red cell and not due to any changes in the adult red cell during pregnancy. These characteristics are based on the presence of a unique fetal hemoglobin with an intrinsically high affinity for oxygen or on the absence of high red cell concentrations of organic phosphates—allosteric modulators of hemoglobin function. However, in several species of snake, representing different families, it appears that pregnancy is associated with apronounced decrease in the oxygen affinity of the adult red cell. This suggests that the blood of the pregnant female is better able to unload oxygen to the fetus than could the blood of thenonpregnant adult. The maternal-fetal difference in blood oxygen affinities in these species isprobably due to the characteristics of the fetal red cell as well as to the change in the affinity of the adult cell during pregnancy. Nonetheless, although the magnitude of the pregnancy-associated change in oxygen affinity of the adult cell in these snakes suggests that it is physiologically significant, the actual significance remains to be determined.