As technology helps us to live longer, it will also shape how we die. With a higher-brain definition of death, you're gone when your personality is. With a whole-brain definition of death, you've lost the ability to breathe on your own again. Each breath provides the oxygen necessary for survival to the rest of the body. Very simply, dying starts to happen when your body doesn't get the oxygen it needs to survive.
Different cells die at different speeds, so the length of the dying process depends on which cells are deprived of oxygen. The brain requires a tremendous amount of oxygen but keeps very little in reserve, so any cutoff of oxygen to the brain will result in cell death within three to seven minutes; that's why a stroke can kill so quickly.
