Hypoxia is the physiological state characterized by decreased oxygen levels in organs and tissues compared to ambient oxygen concentration. While oxygen is essential for normal aerobic metabolism, low oxygen concentrations influence a variety of cellular responses. The oxygen concentration influences, besides the cellular control of O2 consumption, cell growth, metabolism as well as development.
Normoxia is used to describe the partial pressure of oxygen in the inspired gas to that of air level, with oxygen tensions between 10–21%. Hyperoxia describes for those above 21%, while hypoxia is a condition of low oxygen tension, typically in the range 1–5% O2.
Within most organs the physiological oxygen level of the tissues is hypoxic. In fact, oxygen in arterial blood is only about 13%. Oxygen concentrations within tissues vary the farther away from arterial blood vessels. For example, oxygen ranges from 0.5 to 7% in the brain; 4–12% in the lung, fat, skin, bone and cartilage.