Once you stop breathing, or the heart stops beating, the body is unable to provide oxygen to your cells or remove carbon dioxide. The lack the oxygen and build up of carbon dioxide and other toxins leads to rapid cell death. You can be brain dead and still keep other parts of the body "alive" as long as the body is able to circulate fresh blood and remove carbon dioxide and other toxins. That's why death is usually defined as cessation of circulorespiratory function. About how fast various cells die, from the following site:

https://www.sciencefocus.com/the-human-body/when-we-die-does-our-whole-body-die-at-the-same-time/

>The brain and nerve cells require a constant supply of oxygen and will die within a few minutes, once you stop breathing. The next to go will be the heart, followed by the liver, then the kidneys and pancreas, which can last for about an hour. Skin, tendons, heart valves and corneas will still be alive after a day. White blood cells, which are more independent, can keep going for almost three days.