At times, the free divers in The Deepest Breath seem almost superhuman.
But the big reason they can go this deep is because of these
amazing changes that kick in inside any mammal's body when they go underwater.
It's called the Mammalian Diving Reflex.
We're going to get into the hard science of it,
but basically it gives you much more time and much more endurance underwater.
So while you might not believe it, every human is capable of free diving
to some extent. This is the real science of what you see
in The Deepest Breath.
Up until the 1960s, when the Mammalian Dive Reflex was still being explored
scientists thought humans would actually die if they dove down
past 38 meters or 125 feet.
Free divers proved them wrong. In 1952,
for instance, Italian pioneer Raimondo Bucher dove to 39 meters.
So we actually don't know now if humans have a hard limit underwater.
But what we do know is that across free diving disciplines -- and some use ropes,
some use fins, some use weights -- records have continued plunging downwards,
with one weighted discipline hitting lows lower than 200 meters.
Scientists did know back then, though, was that pressure changes
can be bad for humans, and that still holds true.
The human body was built for sea level.
We're used to having the Earth's atmosphere on top of us, and that's it.
But seawater is heavy -- 840 times more dense than air at sea level.
So as you dive into the ocean, the stuff pressing down on top of you quickly
adds up.
Divers reaching 102 meters like Alessia Zecchini would be subject
to a difference of about ten atmospheres of pressure.
That's 11 Earth's atmospheres on top of you.
That's a significant change in a short span.
If you climb 9000 meters to the top of Mount Everest,
you still wouldn't even change one whole atmosphere of pressure.
That much pressure change does things to the gases inside your body,
which you keep in your lungs, sinuses, ears and intestines.
There's a law in physics --
I know, I know. Bear with me.
Boyle’s L aw, which basically says that when pressure goes up,
gases lose volume. They get packed in tighter together.
So the gases that fill up your lungs like a balloon on the surface compress
as you go down in the water. Your lungs shrivel to the size of fists.
That's why scientists thought 38 meters was fatal.
It was where Boyle's Law predicted your lungs would actually implode.
And this pressure change has real world consequences.
Think of your lungs as balloons again, pulling you up towards the surface
in the water. When they suddenly shrink under pressure,
as happens at a certain depth, you sink instead of float.
Maybe the weirdest thing is that pressure changes can affect your mind.
There's something called Henry's Law.
Sorry. Physics again.
Basically, a gas interacts with any liquid underneath it.
The higher pressure
that gas has, the more gas molecules you will find dissolved in the liquid.
So as air reaches a higher pressure in your lungs,
gas molecules flow to your liquid blood.
Your blood suddenly has more nitrogen and carbon dioxide dissolved in it.
Both can lead to narcosis.
A kind of high. Divers sometimes report being distracted
by the pretty fishes or even forgetting where they are.
Finally, there's bodily integrity.
When you go up, you're changing pressures again.
This time, the gas in your lungs
expands rapidly and the resulting stretch can cause tissue damage.
This is the infamous lung squeeze they mentioned in the documentary.
But pressure isn't the only thing you've got to worry about.
You still need oxygen in an airless environment
because unlike, say, energy in our fat cells,
humans aren't built with large emergency supplies of O2.
Lung capacity for men at the very peak of life is about six liters.
Just three large soda bottles.
But humans can use four liters of oxygen per minute if they're exercising.
So there goes most of that and record holding free divers like the late
Natalia Molchanova would stay under for 9 minutes or more.
You have a huge problem if you burn through your reserves
of oxygen while diving.
First, you'll blackout in the ocean.
And after that brain death happens quickly, assuming
you don't involuntarily open your mouth and drown first.
And it can be even worse on the final ascent because the problems
of pressure and oxygen combine in this perfect storm.
Remember Henry's Law?
Well, it works both ways.
As you ascend, the compressed gas in your lungs expands, effectively slurping
out the gas molecules that were dissolved in the liquid underneath your blood.
And what was one of those molecules? Oxygen.
So the brain loses its oxygen reserves and shuts down just as you're surfacing.
It's called latent hypoxia, and it's everywhere in the doc.
But despite all those dangers, people are still able to free dive.
And the reason why is because of the Mammalian Diving Reflex.
It was first recognized in our aquatic cousins,
but so far it seems like every mammal has some version of it.
You can think of it like your body
automatically going into power saving mode when you hit the water.
The trigger is simple.
Immerse yourself, especially your face in cold water.
What happens is extraordinary.
First, your heart slows down by a lot.
In some studies, participants slowed their heart rate
to five beats per minute underwater. With a slower heart rate,
you don't burn as much oxygen.
But there are ancillary benefits too. If you've ever wondered
why people who take an ice plunge report feeling less stress,
it's because of this. A lower heart rate equals lesser feelings of anxiety.
Second, you stop sending as much blood to your extremities.
You keep it for your vital organs.
and that increase flow of blood to your lungs pushes back
against the higher pressure that's working to compress them.
The Mammalian Diving Reflex is the very reason why they don't implode,
as was once predicted.
And the cherry on top is the spleen.
Honestly, I don't know what it does normally, but during the Mammalian
Diving Reflex, the spleen contracts, releasing
oxygen rich red blood into your body like it's a fire hydrant.
It's an amazing second wind.
Don't get me wrong, free divers are athletes.
They build on top of this biological miracle with training.
The breathing and stretching exercises they use look kind of like everyday yoga.
You could do all of them in your living room.
But they're designed to control muscles
in the body that constrain the lungs, boosting their capacity.
And it works like you wouldn't believe.
One champion free diver
had a lung capacity, a full four and a half liters larger than average.
But it's not just oxygen storage.
The exercises also get a person used to dealing with excess CO2,
which triggers a desire to exhale so strong
you can sometimes see it in the form of contractions.
But none of it would be possible without that magic switch that happens underwater.
Obviously, the Mammalian Dive
Response doesn't make you invulnerable as we know too well.
Humans still are lost in the depths in places like the Blue Hole.
But it is the mechanism by which humans, like Alessia Zecchini,
can achieve the feats that you see in The Deepest Breath, feats
that seem at first impossible, like going underwater 102 meters.
So the next time you're in the ocean or a pool or even just a cold shower,
think about the Mammalian Diving Reflex and what it does for you.