Fetal and maternal blood oxygen affinity: a comparative study in llamas and sheep.

We compared blood oxygen affinity (P50) and hemoglobin concentration among fetal and maternal llamas and sheep, as respective examples of species native to high and low altitudes. P50, hemoglobin concentration and blood oxygen content were determined at sea level in 16 pregnant llamas, 6 pregnant sheep and their respective fetuses. P50 was similar in fetal llamas and sheep, but maternal llamas had higher blood oxygen affinity than maternal sheep. As a consequence, the P50 difference between mother and fetus was less in llamas than in sheep. Fetal llamas had higher hemoglobin concentrations than fetal sheep. In contrast, the maternal hemoglobin concentrations were similar. The blood oxygen content was higher in fetal and maternal llamas than in fetal and maternal sheep. We conclude that the llama, a species native to the altiplano, has a higher blood oxygen content than the sheep, as determined in the fetus by a high hemoglobin concentration and in the mother by a low P50.