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Your urge to breathe is CO2
What drives the urge
Hold your breath and a nagging need to breathe arrives after 30 or 45 seconds. It comes from increasing CO2 levels. At that point the pain feels like a limit, yet the body is actually fine: it is the brain's defense mechanism. Free divers train their CO2 threshold higher, push past the signal, and find that things open up for a while once it is muted.
When divers push past it
The reflex exists for a reason, and divers who override it can pay. A free diver's result is not official until a full inhale at the surface, because some float up passed out. Divers who dove 300 or 350 feet and surfaced with blood on their faces, passed out, have been seen, and one was dead for a few minutes.
Listening to the body
The free divers who stay around and stay happy are the ones who always listen to their bodies. If they feel ready to go down and grab the tag, they do it, and if they don't, they never force it.
How does the body sense carbon dioxide?
Central chemoreceptors in the brainstem detect changes in the acidity of the fluid around the brain, which shifts as CO2 crosses into it and forms carbonic acid. Peripheral chemoreceptors in the carotid and aortic bodies detect CO2, acidity, and low oxygen in the blood. Together they set the breathing rate. In healthy people, CO2 is the main driver of the urge to breathe, with oxygen mattering only when it falls very low.
Why do free divers black out near the surface?
As a diver rises, the surrounding pressure drops, and so does the pressure of oxygen inside the lungs. The oxygen supply, already low after the dive, can fall below the level that keeps the brain conscious during the last meters. This is called shallow-water blackout. Competition rules from governing bodies such as AIDA require a diver to give an OK sign and say 'I'm OK' after surfacing.
Why does hyperventilating before a dive raise blackout risk?
Fast breathing blows off CO2, which delays the urge to breathe, while oxygen keeps falling at the same rate underwater. A swimmer can therefore reach dangerously low oxygen before any strong signal arrives. Swim safety organizations have documented blackouts among swimmers who hyperventilated before underwater laps in pools. The urge to breathe is driven by CO2 and not by oxygen, which is why the warning arrives late.
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