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the oxygenated form of hemoglobin (HbO2) that results from the reversible combination of oxygen and reduced hemoglobin (Hb). Oxyhemoglobin carries O2 from the respiratory organs to the tissues and imparts a bright red color to arterial blood. The O2 molecule is bonded to Hb by the Fe2+ atom in heme; the valence of Fe2+ remains unchanged during this process, that is, no actual oxidation takes place. The combination of O2 with one of the four heme groups alters the three dimensional Hb structure and the affinity of the remaining heme groups for O2.
The formation and dissociation of oxyhemoglobin are influenced by several factors, including CO2 concentration and pH. The structure of heme in the Hb molecule is the same for all animals, but the protein fraction—globin—differs from species to species in size, amino-acid composition, and physical properties. The structure of globin is another factor that affects the affinity of Hb for O2. Species differences in globin structure are associated with natural habitat: in general, an increase in the supply of available O2 decreases the affinity of Hb for O2 and consequently increases the partial pressure of O2; this increase is necessary to effect Hb saturation and formation of HbO2. For example, the Hb of land animals exhibits a lower affinity for O2 than that of aquatic animals; fishes that inhabit running water have Hb with a lower O2 affinity than species dwelling in stagnant water. A given species may even have several Hb types, which replace each other during ontogenesis; for example, HbO2 forms more readily in the human fetus than in the adult.
REFERENCESProsser, L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. Pages 238–79. (Translated from English.)
Korzhuev, P. A. “Problema oksigenatsii gemoglobina.” Uspekhi fiziologicheskikh nauk, 1973, vol. 4, no. 3.