In addition to changes in PO2, the loading and unloading reactions are influenced by changes in the affinity of hemoglobin for oxygen. Such changes ensure that active skeletal muscles will receive more oxygen from the blood than they do at rest. This occurs as a result of the lowered pH and increased temperature in exercising muscles.
The affinity is decreased when the pH is lowered and increased when the pH is raised; this is called the Bohr effect. When the affinity of hemoglobin for oxygen is reduced, there is slightly less loading of the blood with oxygen in the lungs but greater unloading of oxygen in the tissues. The net effect is that the tissues receive more oxygen when the blood pH is lowered (table 16.8). Since the pH can be decreased by carbon dioxide (through the formation of carbonic acid), the Bohr effect helps to provide more oxygen to the tissues when their carbon dioxide output is increased by a faster metabolism.
When oxyhemoglobin dissociation curves are graphed at different pH values, the dissociation curve is seen to be shifted to the right by a lowering of pH and shifted to the left by a rise in pH (fig. 16.35). If the percent unloading is calculated (by subtracting the percent oxyhemoglobin saturation for arterial and venous blood), it will be seen that a shift to the right of the curve indicates a greater unloading of oxygen. A shift to the left, conversely, indicates less unloading but slightly more oxygen loading in the lungs.
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