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Hypoxia is a reduction in oxygen supply to one or more areas of the body. Total absence of oxygen is called anoxia. Cells throughout the body depend on oxygen to survive; when the oxygen supply is reduced, cells begin to starve and eventually die.

The brain, which is the body's single largest consumer of oxygen. Although the brain represents only about 2% of the body's weight, it utilizes about 20% of the body's oxygen. As a result, the brain is especially sensitive to hypoxia. After about 4 to 6 minutes without oxygen, large numbers of brain cells begin to die. Prolonged hypoxia (e.g. suffocation) results in death.

Shorter intervals of hypoxia may result in lesser degrees of brain damage. The effects of hypoxia are especially severe in the hippocampus. Individuals who survive an hypoxic episode often sustain hippocampal damage and anterograde amnesia.

Ischaemic hypoxia is caused by a circulatory problem. Normally, oxygen is carried throughout the body via the bloodstream. When the flow of blood is reduced or blocked, cells which depend on that bloodflow are deprived of oxygen (and other nutrients). Ischaemic hypoxia can be caused by a variety of conditions including cardiac arrest, stroke or aneurysm, and massive bleeding.

Anemic hypoxia occurs when oxygen levels in the blood fall below normal levels. In this case, bloodflow continues to reach its target areas, but does not bring the needed oxygen with it. Anemic hypoxia can be caused by conditions including carbon monoxide poisoning, respiratory failure or obstruction (e.g. choking or near-suffocation), sleep apnea, or anesthesia overdose.