Multicellular life is based on O2 for efficient generation of high-energy compounds and O2 consumption increases with the mass and metabolic activity of the organism. However, exposure to O2 must be limited due to the damaging effects of reactive oxygen species (ROS) on cellular macromolecules. Thus all of the major physiological systems of mammals participate in complex homeostatic mechanisms that are designed to maintain the O2 concentration to which each cell is exposed within a narrow range.
