Oxygen physiology and mechanisms of oxygen toxicity: a narrative review

Oxygen inhaled through respiration is consumed in the mitochondria, mainly for oxidative phosphorylation to produce energy. Too little or too much oxygen can be extremely harmful to humans. Insufficient oxygen supply to tissues and organs can result in either dysfunctions or necrosis. However, when the oxygen supply is over supplied, the body is unable to consume the excessive oxygen, which puts the cells in a state of hyperoxia, leading to the production of a large number of reactive oxygen species, which can further cause oxidative damage to the cell membranes and organelles, leading to oxygen toxicity. Although the body has several oxygen-sensing mechanisms to prevent organs and cells from being exposed to hypoxia- or hyperoxia-induced oxidative stress, the relevant capacity and duration of action are relatively limited. Thus, continuous and real-time individualized monitoring and guidance is particularly important in oxygen therapy, especially in the elderly, in order to correct hypoxemia and tissue hypoxia while avoiding or reducing oxygen toxicity caused by hyperoxia. This review aims to briefly summarize the physiology of oxygen and to update the latest progress regarding the mechanism of oxygen toxicity, providing theoretical insights on oxygen therapy practice.