Once Hb is completely saturated with oxygen (HbO 2) oxygenation is increased either by increasing its concentration on air (fraction inspired oxygen-FiO 2) or by increasing the pressure into the alveoli with which is administered in the air-oxygen mixture (intra-alveolar pressure). O 2 loose in the blood which is not combined to Hb accounts instead for the component of partial pressure of oxygen (PaO 2) that can be increased.
Because oxygen has a low solubility in plasma, it is oxygen carried by red blood cells (RBC) specifically, and not ‘blood’ (plasma and RBCs) in general, which determines arterial oxygen content (CaO 2) in the oxygen delivery equation. The delivery of oxygen to cells (DO 2) depends on the content of oxygen in the blood mainly combined to hemoglobin (Hb), which is regulated by the respiratory system, and from its transport in the blood, which is function of the cardiac output/mean arterial pressure/peripheral vascular resistance inter-relationships and of the oxygen extraction/consumption rate at cellular/tissues level, the latter being the main driver, controller and modulator of the oxygen delivery system.
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