Oxygen is transported to the tissues in two forms: combined with hemoglobin (Hb) in the red cell or physically dissolved in the blood. Approximately 98% of the oxygen is carried by hemoglobin and the remaining 2% is carried in the physically dissolved form. The amount of physically dissolved oxygen in the blood can be calculated from the following equation:
Dissolved O2 (mL/dL) = 0.003 (mL/dL per mm Hg) X Pao2 (mm Hg) (4)
If Pao2 equals 100 mm Hg, then dissolved O2 = 0.3 mL/dL.
Binding Affinity of Hemoglobin for Oxygen. The hemoglobin molecule consists of four oxygen-binding heme sites and a globular protein chain. When hemoglobin binds with oxygen, it is called oxyhemoglobin (HbO2). The hemoglobin that does not bind with O2 is called deoxyhemoglo-bin (Hb). Each gram of hemoglobin can bind with 1.34 mL of oxygen. Oxygen binds rapidly and reversibly to hemoglobin: O2 + Hb ^ HbO2. The amount of oxyhemoglo-bin is a function of the partial pressure of oxygen in the blood. In the pulmonary capillaries, where Po2 is high, the reaction is shifted to the right to form oxyhemoglobin. In tissue capillaries, where Po2 is low, the reaction is shifted to the left; oxygen is unloaded from hemoglobin and becomes available to the cells. The maximum amount of oxygen that can be carried by hemoglobin is called the oxygen carrying capacity—about 20 mL O2/dL blood in a healthy young adult. This value is calculated assuming a normal hemoglobin concentration of 15 g Hb/dL of blood (1.34 mL O2 /g Hb X 15 g Hb/dL blood = 20.1 mL O2ML blood).
Oxygen content is the amount of oxygen actually bound to hemoglobin (whereas capacity is the amount that can potentially be bound). The percentage saturation of oxygen (So2) is calculated from the ratio of oxyhemoglo-bin content over capacity:
Hb O2 content Hb O2 capacity
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