Blood oxygen affinity in relation to yolk-sac and chorioallantoic gas exchange in the developing chick embryo

Abstract

A complete profile of blood O₂ affinity throughout chick development from the inception of blood formation to adulthood shows three clearly marked phases. In Phase I from incubation day 4 to 8. P₅₀ increases from 38 to 52 torr; in Phase II from day 8 to day 18 (pipping) P₅₀ falls from 52 to 30 torr; in Phase III P₅₀ rises from 30 torr at pipping to 47 torr in the adult chicken. Phase I, concurrent with the replacement of red cells that derive from yolk-sac hematopoesis to definitive red cells formed in the embryo, is not associated with hypoxie stress to the embryo. High P₅₀ of Phase I is an adaptation that assures higher mixed venous O₂ partial pressures in the presence of significant mixed venous shunt fractions. Phase II is coincident with chorioallantoic gas exchange, definitive red cells, and progressive hypoxia as the O₂ consumption of the embryo increases. The increase in blood O₂ affinity, due to progressive decrease in red cell [ATP], is adaptive in ensuring adequate O₂ saturation of arterialized blood. In Phase III, the transition to convective air-breathing and removal of the ${P_{{O_2}}}$ , restriction on loading, the increase in P₅₀ is an advantage for O₂ delivery to tissues. This increase in P₅₀ is due to the rise in intra-erythrocyte [IPP]. P₅₀ changes during chick development illustrate the interplay between two major factors affecting the O₂ pressure at which delivery of O₂ to tissue occurs: large shunt fractions and a diffusive limitation on hemoglobin loading. When hypoxia threatens adequate loading, lower P₅₀ secures the required arterial O₂ saturations. In the absence of an hypoxie limitation on loading, as in Phases I and III, blood O₂ affinity decreases in favor of maximizing tissue O₂ delivery.