Hypoxia

The dependence of many cells on aerobic respiration as an obligatory energy source requires a variety of responses to oxygen lack or hypoxia.

The appearance of oxygen in the Earth's atmosphere provided an opportunity for the use of oxygen for energy provision by many living organisms. The reduction of oxygen to water provides an efficient source of energy even at relatively low concentrations of oxygen, whilst the stability of oxygen allows for much higher concentrations to be tolerated by living systems. These biochemical advantages have enabled the evolution of complex multicellular organisms. Such organisms have evolved elaborate organs for the delivery of oxygen from the external environment to their cells. They also utilize oxygen in critical metabolic processes other than energy provision. However, the toxicity of oxygen, arising from the generation of active free radical species, necessitates mechanisms to minimize such toxicity. Furthermore, the dependence of many cells on aerobic respiration as an obligatory energy source requires a variety of responses to a lack of oxygen, or hypoxia. Oxygen homeostasis is therefore a key consideration in the anatomy and physiology of living organisms, and many pathological processes are associated with hypoxia.