Brain death

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The brain is organised so that the unconscious functions (breathing, balance, posture and walking for example), are nearest to the base of the brain and the brain stem. The more sophisticated functions associated with consciousness (speech, language, complex thought, and imagination for example), are nearer the upper and outer surfaces.

The outer layer of the brain in which most of the higher functions take place is called the cerebral cortex. The cerebral cortex is the most vulnerable part of the brain, as it needs a lot of oxygen and energy to function. If oxygen is cut off, the cerebral cortex can die in just a few minutes. It is therefore possible for the brain stem to still be alive after the cerebral cortex has died.

Brain death is the complete and irreversible loss of brain stem function. If there is still activity in the brain stem, a person is considered to have sustained brain damage, rather than brain death. For example, a person in a persistent vegetative state has a working brain stem, but the higher levels of the brain may have been destroyed. Although unconscious, they are able to breathe unaided. There may even be some spontaneous movement in response to stimuli such as loud noises or pain.