PO2 at which hemoglobin is 50 per cent saturated P50

The hemoglobin dissociation curve is shifted to the left by hypothermia, alkalosis, or increased base deficit, and to the right by hyperthermia, acidosis, or base excess. These changes can have very significant effects; thus with a right shift of the curve, hemoglobin requires a greater alveolar oxygen tension to saturate as the blood passes through the lung, but conversely in the tissues the same right shift promotes greater unloading of oxygen at higher tensions. Because FiO 2 can generally be increased to compensate for the lung exchange, the tissue effects are more important. Acidosis may be beneficial and alkalosis detrimental to tissue oxygen delivery under critical circumstances. The above influences are caused by physicochemical changes in the hemoglobin molecule, and an additional influence is due to the presence of erythrocyte 2,3-diphosphoglycerate (2,3-DPG). Binding of this organic phosphate to hemoglobin reduces its affinity for oxygen, and therefore decreased 2,3-DPG (as in stored blood) causes a left shift of the dissociation curve. The latter effect is seldom important in clinical practice, except during massive transfusion.

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