JonathanB, The brain can survive for up to about six minutes after the heart stops. The reason to learn cardiopulmonary resuscitation (CPR) is that if CPR is started within six minutes of cardiac arrest, the brain may survive the lack of oxygen. After about six minutes without CPR, however, the brain begins to die, and there may be lasting effects even with subsequent recovery. Medication and mechanical ventilation permit tissue oxygenation, but severe brain damage or a prolonged period without oxygen or glucose causes the death of the brain. By definition, brain death is when the entire brain, including the brain stem, has irreversibly lost all function. The legal time of death is that time when a physician(s) has determined that the brain and the brain stem have irreversibly lost all neurological function. Just as a matter for your interest - the brain is not able to do three things – 1. it cannot feel pain, but it can feel pain from all over the body. 2. The brain cannot store oxygen. Stand up too quickly, and you may faint – this is the brain sensing the lack of blood flow/oxygen supply. 3. The brain cannot store blood glucose /sugar. This means that diabetics who give themselves too much insulin may drop their blood sugar level and faint, and without immediate glucose infusion the brain can die. ALL ANSWERS SHOULD BE THOROUGHLY RESEARCHED, IN ANY FORUM AND ESPECIALLY IN THIS ONE. - MANY ANSWERS ARE FLAWED. It is extremely important to obtain an accurate diagnosis before trying to find a cure. Many diseases and conditions share common symptoms. The information provided here should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Hope this helps matador 89