O2 is transported from atmospheric air to the mitochondria of tissue cells along a pathway with several diffusive and convective steps. O2 is transported from atmospheric air to the mitochondria of tissue cells along a pathway with several diffusive and convective steps. The steps in this O2 cascade are breathing, O2 diffusion across the blood–gas interface, circulation of O2 throughout the body, O2 diffusion across the blood–tissue interface to the mitochondria, and O2 utilization to generate ATP by oxidative phosphorylation. Jay F. Storz et al. J Exp Biol 2010;213:4125-4136 © 2010.