system
Given the following post, respond as if you're commenting on Reddit:
human
Why are cells (e.g brain cells) irreparably damaged so quickly when deprived of oxygen?
gpt
One of the things the other commenter didn’t mention is alternative metabolic pathways. When there’s no oxygen, your body will use other stuff to make energy, and that stuff can create byproducts that rapidly kill the cell. If a little gets created during normal function, the cell can clean it up. But no oxygen, no cleanup. 

Apoptosis (cell death) takes some time - minutes or even 10’s of minutes depending on the cell type because different cells work and live at different paces. Reactions slow down significantly as temperatures drop, which is why some people survive long periods of drowning at low temperatures.