Hemoglobin's affinity with oxygen - carbon dioxide, temperature and bisphosphoglycerate (BPG)

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• The carbon dioxide gas is temporarily converted to carbonic acid in red blood cells by the enzyme carbonic anhydrase, and then further converted to hydrogen and bicarbonate ions.
• The result of increased carbon dioxide is decreased pH causing the Bohr effect.
• Elevated carbon dioxide levels enhance unbinding of oxygen from oxyhemoglobin thereby making oxygen available for actively metabolizing cells.
• By contrast, decreased carbon dioxide, as in the alveolar spaces, increases affinity of hemoglobin for oxygen and promotes oxygen loading and transport.

• To a limited degree, changes in temperature affect the association and dissociation of O2 with hemoglobin.
• The oxygen carrying ability of hemoglobin is unaffected by normal temperatures.
• Near metabolically active cells, blood temperature rises, increasing the thermal motion of molecules which promotes the unloading of O2 to continue fueling aerobic metabolism in the tissue cells.
• When temperature lowers, metabolism slows and the need for O2 in cells lessens. More O2 remains bound to the hemoglobin.

• Red blood cells do not have mitochondria so they do not undergo aerobic metabolism, using only glycolysis to generate ATP.
• A special product of glycolysis, bisphosphoglycerate, or BPG, accumulates in red blood cells in low oxygen situations.
• The hormones thyroxine, human growth hormone, epinephrine, norepinephrine, and testosterone can increase the production of BPG.
• The higher the level of BPG in the red blood cells, the more O2 that is unloaded from the hemoglobin.