Oxygenation capacity is the volume of oxygen at standard pressure and temperature carried by 100 mL of blood after saturation with room air.
Oxygenation capacity includes the oxygen in combination with haemoglobin (1.39 mL/g) and oxygen in solution in plasma (0.3 mL).
It should also be mentioned that most of the oxygen that dissolves in plasma diffuses into the red cell and binds to hemoglobin.
One gram of hemoglobin can bind 1.36 mL O2 (numbers between 1.34 and 1.39 are used). This means that 1 L of blood with a hemoglobin content of 150 g/L can bind 204 mL of O2 if fully saturated.
The Oxygenation capacity with a saturation of 98%, which is normally achieved in arterial blood, the hemoglobin-bound oxygen amounts to 200 mL/L of blood. This should be compared with the mere 3 mL of O2 that is physically dissolved in 1 L of blood at a Pao2 of 100 mm Hg (13.3 kPa).
The hemoglobin-bound oxygen creates no pressure in plasma, which is important because it allows much more oxygen to diffuse over the membranes before a pressure equilibration is reached. Anemia reduces and polycythemia increases diffusion capacity.
Many consider alveolar concentrations to be homogeneous. However, if the alveolar dimensions grow, by expansion or confluence of several alveoli as in emphysema, the diffusion distance can be too great to allow complete mixing of inspired and alveolar gas during ordinary breathing.
Oxygenation capacity is influenced a lot by the hemoglobin content of the blood as well as the hemodynamic status.