Placental oxygen transport in sheep with different hemoglobin types

To study the effect of genetic differences in the maternal oxyhemoglobin dissociation curve on fetal O2 supply, we compared eight pregnant ewes homozygous for high O2 affinity hemoglobin (A) with eight pregnant ewes homozygous for low O2 affinity hemoglobin (B). Each ewe carried a single fetus. Fetal weights were not significantly different (A, 3,000 +/- 170 g; B, 3,070 +/- 270 g). The A ewes had significantly higher arterial O2 saturation (95 vs. 89.4%), uterine blood flow per kilogram of fetus (464 vs. 374 ml/min), uterine venous O2 saturation (78.1 vs. 67.5%), and placental-to-fetal weight ratio (0.107 vs. 0.085). Uterine venous PO2 was significantly less in A ewes (41.7 vs. 47.6 Torr), but umbilical venous and arterial PO2 and fetal O2 uptake were virtually equal in the two groups. We conclude that the difference in O2 affinity between A and B hemoglobins is fully compensated for by differences in arterial O2 saturation, in the rate of perfusion of the pregnant uterus, and in the degree of PO2 equilibration between the uterine and umbilical circulations so that the single fetuses of A and B hemoglobin carriers have equal levels of oxygenation.