Hemoglobin without oxygen, or deoxyhemoglobin, can bond with oxygen to form oxyhemoglobin.This "loading" reaction occurs in the capillaries of the lungs.The dissociation of oxyhemoglobin, or "unloading" reaction, occurs in the tissue capillaries.The bond strength between hemoglobin and oxygen, and thus the extent of the unloading reaction, is adjusted by various factors to ensure an adequate delivery of oxygen to the tissues.
If the lungs are functioning properly, blood leaving in the pulmonary veins and traveling in the systemic arteries has a PO2 of about 100 mmHg, indicating a plasma oxygen concentration of about 0.3 ml O2 per 100 ml blood. The total oxygen content of the blood, however, cannot be derived if only the PO2 of plasma is known. The total oxygen content depends not only on
■ Figure 16.32 The oxygen content of blood. Plasma and whole blood that are brought into equilibrium with the same gas mixture have the same PO2, and thus the same number of dissolved oxygen molecules (shown as black dots). The oxygen content of whole blood, however, is much higher than that of plasma because of the binding of oxygen to hemoglobin.
the PO2 but also on the hemoglobin concentration. If the PO2 and hemoglobin concentration are normal, arterial blood contains about 20 ml of O2 per 100 ml blood (fig. 16.32).
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