FETAL OXYGEN-SUPPLY AND DEMAND - COMPARATIVE ASPECTS

Experimental evidence in six placental mammals (horse, cattle, sheep, man, rhesus monkey, and guinea pig) indicates that fetal O-2 uptake pe r unit fetal body mass does not vary markedly from small to large anim als, in sharp contrast to the fact that small adults have a much great er O-2 consumption rate per unit body mass than large adults. As a con sequence, in an 800-g guinea pig, the mass-specific O-2 consumption of the pregnant uterus is less than the mass-specific O-2 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 ute rine O-2 consumption may provide a physiological explanation for the e vidence that small mammals are capable of producing a greater total ne wborn mass in relation to maternal body mass than large mammals. There are major differences among pregnant mammals in the rate of uterine p erfusion which is used to supply O-2 to the fetus. Variability in plac ental structure accounts in part for these differences. In a countercu rrent exchange placenta, uterine perfusion can be much smaller than in a venous equilibration exchange placenta and yet provide the same lev el of fetal oxygenation.