The brain consumes 20% of total body oxygen.

Most cerebral oxygen consumption (60%) is used to generate adenosine triphosphate (ATP) to support neuronal electrical activity.

The cerebral metabolic rate (CMR) is usually expressed in terms of oxygen consumption (CMRo2) and averages 3-3.8 mL/100 g/min (50 mL/min) in adults.

CMRo2 is greatest in the gray matter of the cerebral cortex and generally parallels cortical electrical activity.

Because of the relatively high oxygen consumption and the absence of significant oxygen reserves, interruption of cerebral perfusion usually results in unconsciousness within 10 sec, as oxygen tension rapidly drops below 30 mm Hg.

If blood flow is not reestablished within 3-8 min under most conditions, ATP stores are depleted, and irreversible cellular injury begins to occur.

The hippocampus and cerebellum seem to be most sensitive to hypoxic injury.

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