FETAL OXYGEN SUPPLY AND DEMAND: COMPARATIVE ASPECTS

ABSTRACT

Experimental evidence in six placental mammals (horse, cattle, sheep, man, rhesus monkey, and guinea pig) indicates that fetal O₂ uptake per unit fetal body mass does not vary markedly from small to large animals, in sharp contrast to the fact that small adults have a much greater O₂ consumption rate per unit body mass than large adults. As a consequence, in an 800-g guinea pig, the mass-specific O₂ consumption of the pregnant uterus is less than the mass-specific O₂ consumption of the maternal organism, whereas the opposite is true in a 50-kg sheep. This effect of body size on the relationship between maternal and uterine O₂ consumption may provide a physiological explanation for the evidence that small mammals are capable of producing a greater total newborn mass in relation to maternal body mass than large mammals.

There are major differences among pregnant mammals in the rate of uterine perfusion which is used to supply O₂ to the fetus. Variability in placental structure accounts in part for these differences. In a countercurrent exchange placenta, uterine perfusion can be much smaller than in a venous equilibration exchange placenta and yet provide the same level of fetal oxygenation.