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The Haldane effect describes how oxygen concentrations determine hemoglobin's affinity for carbon dioxide. For example, high oxygen concentrations enhance the unloading of carbon dioxide. The converse is also true: low oxygen concentations promote loading of carbon dioxide onto hemoglobin. In both situations, it is oxygen that causes the change in carbon dioxide levels.

The Bohr effect, on the other hand, describes how carbon dioxide and H+ affect the affinity of hemoglobin for oxygen. High CO2 and H+ concentrations cause decreases in affinity for oxygen, while low concentrations cause high affinity for oxygen.