system
Given the following post, respond as if you're commenting on Reddit:
human
Do humans instantly die when the heart stops?

As someone who is not expert in biology *but* has a know-how in mechanics (totally unrelated, I know) I find it strange that in movies, when someone's heart stops they instantly collapse like a ragdoll. Why? Only the "fuel" line got severed, not the nerves. The muscles have internal oxygen reserves as well. 

They say that if you can restart a heart within 5 minutes, the brain does not suffer damage. So why can't we move for 5 minutes after the heart stops?
gpt
Your brain and other organs don't die instantly, but you do go into shock immediately. When your heart stops, your blood pressure collapses, triggering a wave of biochemical responses. Your body has a strict hierarchy of organs to protect, and skeletal muscles are not high on the list. They get shut down while all of the body's remaining (rapidly dwindling) resources are focused on saving the brain, heart, and other internal organs starting with the lungs, liver, and kidneys.

Heart tissue is extremely resilient, so if something happens that causes it to stop, it is probably severely damaged and will be difficult to get started again. Possibly it has been operating at a lower capacity for quite some time, which would mean that the other tissues in the body would also be suffering from lowered blood flow -- wastes and toxins build up, nutrient caches (including oxygen and glucose) run low, electrolyte balances are off, etc. 

With no oxygen delivery at all, you have about 2 minutes before brain damage begins, and another 4 or so before parts of your brain begin to die. If your brain is getting some perfusion, you can last longer, although you may still have some damage. If your heart is still partly functioning, you can live a lot longer, although you'll still probably experience damage to lower-priority organs/systems such as the gut.