Your brain starts to die without any oxygen, but not breathing is not the same as not giving your brain any O2. Your blood cells buffer amount of O2 for use an

Your brain starts to die without any oxygen, but not breathing is not the same as not giving your brain any O2. Your blood cells buffer amount of O2 for use and CO2 ready for removal, which is how an untrained person is able to hold their breath and not die. When you stop breathing, consumption of O2 continues until CO2 builds up to a threshold point, triggering a desperate need to breathe (humans cannot actually detect O2 levels). If you try and hold your breath for too long, either the buildup of CO2 will cause the need to breathe, or lack of O2 will cause blackout and the autonomous nervous system to take over and resume normal breathing. While I am not a doctor, I don't see how it's possible to deprive your brain of enough O2 to damage it and not lose consciousness. It's possible to train long breath holding in two ways: increasing O2 stores and increasing efficiency. Hyperventilating can result in you purposefully purging extra CO2 and over oxygenating your blood. This will let you hold your breath for a very long time, I can comfortably go from a 30s hold to a 2m hold by pre-hyperventilating (see: Wim Hof Method). It also adds the risk of letting O2 levels drop before CO2 rises, causing blackout, which is basically one of the things that kills free divers. DO NOT DO THIS IN OR NEAR WATER, BLACKING OUT IN ANY AMOUNT OF WATER KILLS. The other thing is that our systems that consume O2 can be made much much more efficient. Training probably helps, but there are some autonomous systems capable of reducing O2 consumption that all of us carry around. The mammalian diving reflex appears to automatically cut O2 consumption and limit max heart rate once your face is under water, for example. tl:dr: You carry extra O2, and you can increase how much on purpose. The rate you use O2 isn't fixed, and if you run out of O2 you black out and start breathing normally (which kills you if your underwater).