The optimal oxygen equilibrium curve: a comparison between environmental hypoxia and

SYNOPSIS. Internal hypoxia in vertebrates occurs during anemia, when blood oxygen (O2) carrying capacity is reduced, or during exposure to environmen-tal hypoxia. In non-altitude adapted vertebrates, exposure to environmental hypoxia results in a change in blood O2 affinity which, in some cases is ben-eficial to tissue O2 delivery. In contrast, the elevation in blood O2 carrying capacity observed in almost all vertebrates is always beneficial to tissue O2 delivery (assuming no large changes in blood viscosity) and may be more im-portant than changes in blood O2 affinity in maintaining tissue O2 delivery. Experimental anemia in vertebrates results in a decrease in blood O2 af-finity which is always beneficial to tissue O2 delivery. The reduction in affin-ity is brought about by an increase in the organic phosphate to hemoglobin ratio (NTP:Hb) within the red cell. In fish NTP:Hb decreases during envi-ronmental hypoxia but increases during anemia indicating that NTP regula-tion is quite different between these treatments despite the similarity of these treatments at the tissue level