Cellular health requires that oxygen (O2) supply be matched appropriately with the tissues O2 needs. If O2 supply is not aligned with needs, ischemia will supervene, and tissue injury results. The cellular delivery of O2 is a finely regulated system. Once having passed the alveolar–capillary membrane, O2 is carried in blood to the tissues. According to the formula: O2 content (CaO2) = [(hemoglobin concentration) × saturation of O2 (SaO2) × 1.39 + 0.003 × arterial O2 tension (PaO2)], most of the O2 transported in blood is bound to red cell hemoglobin, and the amount of physically dissolved O2 is negligible under physiologic conditions. Whole body oxygenation is determined by the CaO2, cardiac output, and O2 extraction ratio. The total amount of convective O2 delivered to the organs (DO2) then is calculated as: DO2 = cardiac output × CaO2.
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