| In mixtures of gases each component makes a specific contribution, known as its partial pressure that is directly proportional to its concentration. It is customary to use the partial pressure of a gas as a measure of its concentration in physiologic fluids. For atmospheric O2 at a barometric pressure of 760 mmHg (760 torr, 101.3 kPascal (kPa), the partial pressure of oxygen, pO2, is 150-160 mmHg, i.e. air contains about 20% oxygen. The amount of O2 in solution is, in turn, directly proportional to its partial pressure. Thus, in arterial blood the pO2 is ∼100 mmHg (13.3 kPa), which produces a concentration of dissolved O2 of 0.13 mmol/L (4.2 mg/L).
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| The major fraction of O2 in blood is transported in blood and stored in muscle in complexes with the proteins Hb and Mb, respectively. Hb is a tetrameric protein with four O2-binding sites. In arterial blood with an Hb concentration of 150 g/L (2.3 mmol/L), the contribution of Hb-bound O2 is about 275 mg/L (8.6 mmol/L). The overall effect is a dramatic 60-fold increase in the O2 content of this physiologic fluid, yielding almost 200 mL dissolved O2/L of blood.
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