Figure 1: Diagrammatic representation of oxygen levels in adult brain cells. PO2 (oxygen tension) varies with the cell type and the microenvironment of the respective organ. In the brain, physiological oxygen levels range between 2-5% whereas in the bone marrow physiological oxygen levels have been calculated to be highest (~5%) near the sinuses and lowest (1%) inside the cortical bone. Adapted from David M. Panchision (2009).