Oxygen affinity and equilibrium curve shape in blood of chicken embryos

Oxygen equilibrium curves of blood from 4- to 18-day chicken embryos were investigated at 38 degrees C. Curves were recorded at the PCO2 measured in the air cell of each egg. Since arterial PCO2 is known to closely approximate that of the air cell, these curves reflect in vivo arterial pH. Curves were also recorded at a second, higher PCO2, allowing calculation of the curve at estimated venous PCO2 and of physiological curves by interpolation. Half saturation PO2 of physiological curves increased from 38 Torr at 4 days to 52 Torr at 8 days, and then decreased to 31 Torr at 18 days. Increasing blood oxygen affinity late in development favors oxygen loading at the falling PO2 that exists at the chorioallantoic surface, while higher affinity before 8 days may be related to diffusion resistance of the inner shell membrane early in incubation. In blood from 4- to 6-day embryos, the Hill coefficient, nH, of curves at air cell PCO2 increased from 1.5 at oxygen saturation 0.1 to 6.5 at saturation 0.85. After 6 days, nH steadily increased at low saturation and decreased at high saturation. By 18 days, nH varied only from 2 to 3.4. A biphasic equilibrium curve shape (hump at the low end of the oxygen equilibrium curve) developed in 4- and 5-day embryo blood after a period of storage on ice, and was accentuated if some of the cells were intentionally lysed.