And what about people like David Blaine or Tom Sietas? Sietas held his breath underwater for over 22 minutes (world record). I know they train for it like months and even years, but doesn’t holding your breath = no oxygen to brain?
Permanent brain damage apparently occurs just after 4 minutes of lack of oxygen to the brain, so why are freedivers left generally unscathed after 8 or 10 minutes without air?
In: Biology
As you dive down, the water pressure increases and the Partial Pressure of oxygen in your lungs and blood also increase, keeping it at sustainable level. Even as you metabolise O2 to CO2, the remaining O2 is still at sustainable partial pressure. As you swim up, the pressure drops and there’s risk of shallow water blackout.
It is 4 minutes after the blood oxygen level is below a critical level. Your body doesn’t actually react to lack of oxygen it reactors to increased amount of CO2, that is what gives you the feeling of needing to breathe, it doesn’t mean that you are running out of oxygen.
Now imagine that you are running, you start to breath faster than if you were walking. This is because you are using more oxygen. Now. Try running and hold your breath, as long as you can. You can keep your breath for longer when walking. Makes sense doesn’t it?
Now what free divers and people who do extreme feats that require you to hold breath do is that they relax and calm their bodies, and use breathing techniques to pump as much oxygen in to their body as they can. They have also learned to deal and resist against the urge and struggle caused by build up of CO2.
You can actually try that yourself. Sit on a comfortable chair. Close your eyes, breathe deeply few times and relax as much as you can. Then just hold your breath. You can practice this skill and get quite good at it. This is often used in things like meditation and yoga.
Now. When your body runs out of oxygen, that is below the critical level, you pass out. This is where the timer of 4-6 minutes start. After that it is safe to assume that damage to the brain and other vital organs start to happen. Each passing second increases the probability.
Now. What is interesting is that if you cool your body temperature, the chemical reactions that happen in your cells slow down. This means they’ll use less oxygen. The chemical reactions. This is actually used a lot in medicine, during long surgeries if blood supply has to be cut for some reason, or if there has been severe trauma.
Now. If you do cold water free diving. Your body temperature drops, which gives you an edge. You use less oxygen.
The current world record holder is Finnish Johanna Nordblad (Torille!) who dove 103 meters under ice in the time of 2m 42s, without fins or a wetsuit.
I want to add to the other comments that when talking about 4-6 minutes without oxygen, it is also without blood supply (e.g. in case of cardiac arrest / ventricular fibrillation). While the divers still have a working circulation, oxygen is still supplied to the brain. Though indeed over time the saturation gets less, it’s not like the moment you hold your breath your saturation becomes 0. So there is technically still oxygen supply there.
Aside from all the things already mentioned there are specific techniques free divers can use to gather large quantities of oxygen in their lungs and bloodystem prior to diving, like breathing techniques and such.
Think of it like a survival video game where you have both an oxygen meter and a health meter. When underwater, your oxygen meter depletes slowly. Once the oxygen meter is empty then your health meter starts to go down until you die. The “4-6 minutes without oxygen” is the health meter part, you can’t really train that. You can, however, train your body to use up your oxygen meter more slowly and you can do things like breathing air with extra oxygen for a while to make your oxygen meter bigger. Those two combine so that you can give yourself a long time of oxygen meter before your health meter even comes in to play.
Free divers do suffer accumulating brain damage even if they don’t black out. Essentially the more they practice the more efficient their bodies become at using oxygen but it does affect some of them negatively.
They will use breathing techniques to improve oxygen utilisation and they will learn to slow their bodies metabolism to use less oxygen.
For some deep dives the pressure can cause blood to enter the lungs and oxygenate them feeding it back to the rest of the body.
Some people also just suffer a little brain damage. You can live with a little brain damage.
When you breathe in and out, you don’t change 100% of the air in your lungs. Also, the standard air is about 79% nitrogen and only 20% oxygen (and then a bit of carbon dioxide and other stuff).
When the amount of CO2 in your lungs goes up, that compels the need to breathe. But there is still oxygen in your lungs. An exhaled breathe is about 4% carbon dioxide. So you still breathe out lots of oxygen (and nitrogen). So it takes time to actually use up all that oxygen in your lungs, and so even though you aren’t breathing in and out, your blood is still able to switch out oxygen for CO2 for some time. That is significantly expanded if you just breath in pure oxygen before a dive (so instead of 80% nitrogen and a bit less than 20% oxygen, you have a breathe of 100% oxygen).
Long story short, holding your breath doesn’t stop oxygen going to your brain. Your blood keeps circulating while your heart is pumping, getting that sweet sweet oxygen to wherever it’s needed.
Because they have a beating heart. Apparently there is enough oxygen in the bloodstream to support the brain for a period of time as long as it is actively being moved through the brain. This is why they tried to go to the breathless CPR a few years ago (namely because people wouldn’t do CPR because they didn’t want to put their mouth on another person’s mouth) and it would be better than nothing.
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