Hypoxia is lack of oxygen in the blood and body tissues to such an extent that it results in physiological and psychological disturbances. Asphyxia, lung disease, paralysis of respiratory muscles, and some forms of coma prevent enough oxygen reaching the blood. Diseases of the heart or circulation may also lead to tissue hypoxia. Irreversible brain damage follows prolonged hypoxia.

Temporary hypoxia may result from scarcity of oxygen in the air being breathed or from the inability of the body's tissues to absorb oxygen under conditions of low barometric pressure. It produces a variety of reactions in the body from mild intoxication and stimulation of the nervous system, to progressive loss of attention and judgment, unconsciousness, and eventual brain damage. The continuing physiological requirement for oxygen is a critical consideration in high-altitude flight and spaceflight.