system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it possible for someone to hold their breath for a very long time?

The world record is 24 min 37.36 sec

How can they stay conscious for so long on a single breath? Doesn't the body need a constant supply of oxygen?
gpt
Two main things. The first is the *mammalian dive reflex*. Whenever your nose is submerged, your body automatically does a lot of tricks to reduce your need for oxygen. Your heart beat slows down and organs that aren't immediately necessary for your survival get less oxygen - enough to keep them alive, but not enough for them to be fully "on". Even your brain gets a reduced supply, and parts that you don't immediately need get less and work much more slowly. 

The other thing is saturating their blood through hyperventilation before diving. Hyperventilating - breathing very quickly - can be bad for you and make you dizzy. If you train for it and do it properly, though, you don't give your blood much time to use up the oxygen it already has. The rapid breathing causes more oxygen to dissolve into your blood until it's completely saturated with way more than you need. Right before diving under, these divers hyperventilate so that they are carrying "extra" oxygen in their blood. 

And, of course, there's just good old fashioned training. Training their lungs to expand and hold as much as possible, training their body to deal with lactic acid buildup a bit better, training their moments to be as efficient as possible to use very little oxygen...