On the Oxygen Affinity of Bird Blood

In oxygen affinity characteristics bird blood appears to have several features that distinguish it from mammalian blood. For domesticated species at least the range of oxygen half saturation values is extremely wide. A difference in the shape of the oxygen dissociation curve has been recorded by several authors with an increase in sigmoidocity with increasing oxygen saturation. There is evidence that the oxygen affinity determining organic phosphate of bird red blood cells inositol pentaphosphate (IP5) is relatively metabolically inert. This suggests that modulation of blood oxygen affinity is primarily achieved by altering the IP5 hemoglobin interaction rather than varying IP5 levels per se. In contrast to mammals carbon dioxide has no direct effect on whole blood oxygen affinity for some bird species (hen chick goose) or it may cause the oxygen affinity to increase (pigeon flamingo). Carbon dioxide is a blood oxygen affinity modulator of some flexibility its effect in both direction and magnitude being dependent on the hemoglobin type red cell pH and organic phosphate levels. The physiological significance of these distinguishing features is discussed.