Oxygen affinity and phosphate compounds of red blood cells during intrauterine development of rabbits

1. It is known that the oxygen affinity of blood from newborn mammals is higher than that from the adult animal. To obtain information on possible changes in oxygen affinity and its regulation in the course of early mammalian ontogeny we have investigated P₅₀ of blood and hemoglobins, intraerythrocytic phosphate concentrations and hemoglobin composition of the rabbit during intrauterine development. 2. We have found that the oxygen affinity of red blood cells markedly increased during intrauterine development. P₅₀ fell from 3.9 kPa (29 Torr) 14 days after conception to 2.7 kPa (20 Torr) near term. This raise in oxygen affinity was accompanied by a concomitant decrease in the concentration of 2,3-diphosphoglycerate, adenosine triphosphate and inorganic phosphate. In erythroblasts from 14 days old embryos we have found embryonic hemoglobins which are characterized by a high oxygen affinity. Embryonic pigments were completely replaced by adult hemoglobin from day 22 after conception onwards. 3. From these results we conclude that at the very early ontogenetic stages the low oxygen affinity of red cells favours oxygen delivery to the highly metabolizing embryonic tissues. At later stages of intrauterine development placental oxygen supply becomes poorer, since the growth of the fetus increases more than placental blood flow and placental weight. Near term hypoxic hypoxia is alleviated by a high oxygen affinity of the fetal blood.