Figure 36-1 Oxygen transport cascade. A schematic view of the steps in oxygen transport from the atmosphere to the site of utilization in the mitochondrion is shown here. Approximate PO₂ values are shown for each step in the cascade, and factors determining those partial pressures are shown within the square brackets. There is a distribution of tissue PO₂ values depending on local capillary blood flow, tissue oxygen consumption, and diffusion distances. Mitochondrial PO₂ values are depicted as a range because reported levels vary widely. (Adapted from Nunn JF: Nunn's Applied Respiratory Physiology, 4th ed. Boston, Butterworth-Heinemann, 1993.)