system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How long can you function once your heart stops?

Often in movies and such when a person is struck in the heart they drop like a sack of potatoes. In real life if someone were to shoot you in the heart how long would you be able to function normally?
gpt
Almost not at all.

Your brain needs a constant supply of oxygen. Even when you're, say, holding your breath, your heart is still circulating your blood (so it's still pulling oxygen from the air left in your lungs), so you can hold out for a little while. But if your heart stops, your brain consumes the available oxygen in a matter of seconds, and you almost immediately pass out (in a matter of a couple seconds at most). Your brain cells can survive for around a minute or two in this state before they start taking irreversible damage (the exact time depends on things like the temperature), and you're dead a few minutes after that.

An example of this in a case where the heart *doesn't* stop is fainting. If you have low blood pressure or certain disorders, standing up quickly causes your heart to briefly not be able to lift blood to your brain, and you faint on standing up. In this case, the brain doesn't stay without oxygen for long enough to do any damage, but you lose consciousness almost instantly.