At arrow A in the accompanying graph, the hemoglobin is only 20% saturated, and at arrow B it is 97% saturated. The difference is due to the fact that:

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The difference in saturation between arrow A and B is due to the reversible binding of oxygen with hemoglobin, with varying affinity due to cooperative binding. At arrow A, the conditions are such that oxygen has a lower affinity for hemoglobin, resulting in only 20% saturation. At arrow B, conditions favor higher oxygen affinity for hemoglobin, resulting in 97% saturation. pH differences in muscle and pulmonary capillaries do affect oxygen binding, but not to the extent seen in the graph. Oxygen does not bind irreversibly to hemoglobin in the lungs.

At Arrow a in the Accompanying Graph, the Hemoglobin Is