Brain death is considered an irreversible form of death (per National Health Services U.K.) Even if people are on life support where failing organs are kept alive by modern medical interventions, someone who is brain dead will never regain consciousness, per Better Health Channel.
Brain death occurs when oxygen and blood supply to the brain is cut off and this results in brain tissue dying (per Life Source). Without the proper functioning of your brain, your entire system starts to shut down. Your brain, heart, and other vital organs die from a lack of oxygen, and this results in biological death. Just how soon biological death comes is dependent on a few factors. For example, someone who drowned might have more minutes within which they can be revived because of hypothermia which necessitates less oxygen to the brain. But 11 minutes after clinical death, the brain is usually damaged without any hope of reversal (per American AED CPR Association).
According to the associate investigator in the Cardiovascular Health Across the Lifespan Program at the RI-MUHC in Montreal, Canada, Dr. Sam Shemie (via McGill University Health Center), the second time we die is in fact actual death. “In the end, it comes down to one simple truth: the only organ that we can neither artificially support nor replace is the brain. And when the brain completely stops working, the person is dead,” he explained. Despite this, however, there is some controversy globally over when someone can be pronounced dead.