Eli5: how can free divers hold their breath for so long without causing brain damage?


If it only takes 4 minutes without oxygen to cause brain damage, how can they stay underwater for 4+ minuets and not hurt themselves.

In: Biology

One part of the answer is that divers pack air before diving. They hold their nose and literally take large gulps of air and swallow them (it appears). Then, they somehow access those pockets of extra air while under water. It gives them more time. Hence, brain receives additional oxygen. Probably more to it though

The brain isn’t without oxygen when they dive, they are relying on residual oxygen in their lungs, blood, and tissues.

Consciousness is very expensive and generally a lack of oxygen will put them to sleep well before permanent brain damage occurs.

They start by oxygenating their blood. Lots of heavy or deep breathing. You can have two deposits or oxygen, your blood/body, and your lungs. The lungs are kind of like a the secondary source that feeds the primary source. If you can fill the primary source as well as the secondary source, you can last longer.
Also, they use techniques to lower their heart beat, and thus/also oxygen consumption. Things like meditation. Reduced consumption coupled with increased supply equals longer times.

Free divers often purposefully hyperventilate before diving in order to fill their bloodsteam with as much oxygen as possible. This oxygen is then used by the brain to function.


Free divers also tend not to use a lot of muscular movement, which would result in the use of oxygen, some have reported that their limbs feel heavy or unuseable after a long while.


The brain, obviously, knows its not getting much oxygen, and it takes quite a bit of mental control to override the impulse to breathe, apparently.


It’s a trained skill and takes a lot of practice, I think.

It’s a fun little trick called the Mammalian Diving Reflex.

All mammals can do it to drastically increase the time they can spend under water. It’s what allows dolphins, whales, seals and sea lions, among others, to stay under water for so long.

Basically, as far as I understand it, they slow down all bodily functions to use up less oxygen than normal.

With enough training, anyone can stay under water for much longer than what people commonly think is safe.

I free drive! Something I take advantage of is what’s known as the Mammalian Dive Response, wherein when your face is immersed in water, it helps to elicit a calming affect. If you are very comfortable in the water you can basically go into a meditative like state before your dive, almost like you are sleeping. So you use much less oxygen than if you are in a normal waking state, out of the water. You can feel yourself relax and your heart rate slow down. Then you dive down, maintaining this tranquil state.

A clear ocean looks so amazingly big from deep down!

But make sure you know your limits and have a buddy with you who can help because it’s easy to black out under water, especially if you’re going for your personal best.

As they dive deeper, the increase in pressure on the body actually makes the lungs shrink, but also pushes the oxygen into the blood stream. So there’s some actually complex physics going on.

“Diffusion is the movement of gas molecules from a region of higher concentration to a region of lower concentration until they are equal. This means that the deeper you go, the more oxygen will dissolve into your blood. Then, as you ascend, the volume of air inside your lungs increases, oxygen comes out of the blood, back into your lungs.”

It’s because they train their lungs. They have larger lungs that they hyperventilate their VO2 capacity is larger

Two things. First off. your blood and lungs will still hold oxygen. In fact, the air you normally exhale still has most of it’s oxygen. Some VERY rough math suggests you’ve got at least 10 breaths worth of oxygen in your blood at any given time.

Second thing is the Mammalian Diving Reflex. Basically, when your face is under water your heartrate slows. Just a weird quirk of your body so you use less oxygen when underwater.

Check out Wim Hoff, he has breathing videos where if you follow along with his breathing you can double the time you’re able to hold your breath. In a few minutes even. He was on the JRE podcast in case you want more and longer content.