The earliest applications of techniques for chronic catheterization of fetal blood vessels included simultaneous measurement of maternal and fetal blood concentrations of gases, nutrients, and metabolites under different nutritional and physiological conditions. It soon became apparent that for some important nutrients, such as glucose, fetal concentration closely parallels maternal concentration with a gradient favoring continuous transfer from dam to fetus in sheep, cattle, horses, and pigs (Fig. 2). For other vital nutrients, such as amino acids and calcium, relations between maternal and fetal concentrations are much less apparent. For calcium and many amino acids, fetal concentration is persistently greater than maternal concentration. Under normal conditions, maternal and fetal blood oxygen tensions and concentrations and acid base status are tightly regulated, and a strong, positive maternal fetal oxygen gradient is maintained. However, maternal hypoxemia induced naturally by altitude or artificially in hypobaric chambers is associated with development of fetal hypoxemia and acidemia.
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