system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is physically preventing a dead person from being brought back to life?
gpt
Cells in the brain start to die from lack of oxygen that comes from the blood.   Once those cells are gone,  the is no more 'you'.

Physical processes in the body can continue (coma) or be forced to continue with machines,  but once the brain starts to die there is no going back.