Brain death is a legal definition of death that refers to the irreversible end of all brain activity (including involuntary activity necessary to sustain life) due to total necrosis of the cerebral neurons following loss of blood flow and oxygenation. It should not be confused with a persistent vegetative state.
When the brain is injured, it responds in much the same way as an injury like a twisted ankle – it swells. Unlike the muscles and tissue of the ankle, however, the brain is in a confined space – the skull – and has no room to swell.
Brain death may result in legal death, but still with the heart beating, and with mechanical ventilation all other vital organs may be kept completely alive and functional, providing optimal opportunities for organ transplantation. Most organ donation for organ transplantation is done in the setting of brain death.