are going to be taking over this channel.

We start with Rohin Francis, a cardiologist who runs Medlife Crisis.

His video includes footage from his diving trip to Borneo

and a couple of jokes that I am deeply uncomfortable with.

Rohin, over to you.

- How long can you hold your breath?

One, two minutes?

I'm sorry, homo sapiens are just pretty pathetic

next to the diving world champions

who can stay underwater for one or two hours.

So how do they do it?

Well, there are a few different ways,

some you can actually take advantage of by channelling your inner dolphin.

[dolphin squeak]

Others involve a little bit of gentle evolution,

so maybe a bit of a tall order for most of us

unless you're a member of an Asian community who have evolved into real life Aquamen.

[tranquil music]

There are five ways we can spend longer underwater.

Number one is to increase your tolerance to carbon dioxide.

The first thing that makes you want to take a breath

is the rising level of carbon dioxide or hypercapnia.

"Hyper from the Greek, meaning too much or above,

"and kapnos, meaning smoke."

This acidic waste gas builds up,

and the body is so acutely sensitive to any changes in pH,

even though it's tiny, that it starts screaming at you to...

Breathe, god damn you!

Incidentally, this is why any diet claiming to alkalinize your blood is basically

[cow moo]

Now you can improve your carbon dioxide tolerance quite quickly.

I was able to go from about 90 seconds breath-hold

to five minutes with just a few days' worth of snorkelling

and some tips from an experienced free diver.

The next obstacle though is a lot harder to get around

and that's hypoxia or low oxygen.

Hypoxia kills cells.

No matter how you die, ultimately it's hypoxia

at the cellular level, i.e. your cells being starved of oxygen,

that is the cause of your inevitable death.

Unlike training your body to become more tolerant to hypercapnia,

increasing your tolerance to hypoxia takes years of training.

And of course, there is a level past which no human can actually go.

So what is the solution for a longer dive?

Well, to maximise oxygen delivery and minimise oxygen use.

Which brings me to number two.

The reason that this steak is red is a protein called myoglobin,

which is found in the muscles of pretty much every mammal

and it's responsible for storing oxygen.

However if we were tucking into a steak that came from

a seal it would be almost black in appearance because

they've got ten times more myoglobin than humans or cows.

This is not a human steak.

So why can't we just pack in more myoglobin

and store more oxygen like the seals do?

The problem with proteins is that

[child burbles]

The problem with proteins is that when they g--

[child burbles]

The problem with proteins is th--

[child drums]

The problem with proteins is when they get too tightly

packed they start to clump together and lose their function

but marine mammals have evolved a very clever way to deal with this.

Their variant of the myoglobin molecule has a positive electric charge.

As I'm sure you know, positive repels positive

and as a result the proteins can get very close together

without forming those clumps.

Number three is to adjust your blood flow to preferentially supply

the heart and the brain at the expense of things like the extremities

which are much more tolerant to having a reduced blood flow.

And number four is to slow your heart down.

Your heart is very oxygen-hungry organ

so by reducing your heart rate you're immediately

buying yourself more time under water.

I've mentioned these two together because they form part of the dive reflex.

Erroneously sometimes called the mammalian dive reflex.

It's actually been found in pretty much every air-breathing vertebrate

that's been studied.

It's stimulated by submersion in water,

particularly cold water.

And remember, below 200m depth, water is cold

no matter where you are in the world

and of course that's where most mammals do their hunting.

So what is the dive reflex?

Well let's take a look.

[bells]

[blows conch shell]

That's the wrong video.

I've not tried this before but I filled up a basin with ice water.

I'm going to hold my breath and submerge my face

in the water probably just for about thirty seconds or so

and I'll see if anything happens to my heart rate.

I've got a pulse oximeter that you can see.

Okay. Here goes.

Agh. That was cold.

So my heart rate has dropped right down to 45.

It's still staying at 45.

I don't know how long I was under

maybe 30, 40 seconds.

Now you can see it's slowly starting to climb again.

This is just a cheap £10 oximeter that I bought online

so I don't know how low the heart rate goes.

I don't know when it starts becoming inaccurate but a lot of

these commercial devices will have a cut off in the forties

because most people's heart rate doesn't go that low.

But I think it was a pretty clear demonstration.

In diving mammals this is far more pronounced

with Weddell's seals, for example, dropping the heart rate

as low as four beats a minute.

This effect is mediated by my favourite nerve, the vagus.

Don't tell me you don't have a favourite nerve.

Number five is also part of the dive reflex because

it has another feature and that's to squeeze your spleen.

The spleen is normally a small organ

which is involved with the immune system and filtering the blood

as such stores a little reservoir of blood.

Now, in a healthy human with a normal sized spleen,

that volume will be around 160ml of blood or 5% extra oxygen carrying ability,

which can be squeezed out as part of the dive reflex when needed.

However, diving mammals can have significantly enlarged spleens

representing a much bigger reservoir of blood

to be provided when necessary.

I'm sorry, it looks like this one's out of reach for most of us.

But that doesn't mean that there aren't any members

of our species that haven't taken advantage.

However, to meet them, we're going to need to take a short trip

to Southeast Asia.

Years ago I saw a BBC documentary that stuck in my mind permanently,

which was about a fisherman who was so negatively buoyant

he essentially sank twenty metres and walked along the bottom of the seabed,

and what's more, he almost effortlessly held his breath for several minutes.

He was member of the Bajau community, who are an ethnic group

indigenous to oceanic Southeast Asia, the Philippines, Indonesia and here.

I'm in north Borneo at the moment,

in Sabah where they're one of the biggest ethnic groups

and my diving instructors here tell legendary stories of Bajau

who can hold their breath for fifteen minutes at a time under water.

So what makes them so special?

The Bajau have led a nomadic subsistence lifestyle

for at least a thousand years, literally living out at sea,

eating what they catch and rarely setting foot on land.

Earlier this year, a research team led out of Copenhagen

published an incredible study which demonstrated that

the Bajau have evolved larger than average spleens

when compared to nearby non-diving communities.

Free divers develop big spleens through training.

But the enlarged spleens in the Bajau are seen in members

of the community who don't do any fishing or diving

so we know that this is a genetic adaptation.

You might wonder how a genetic pressure has been exerted

until you consider the fact that the Bajau who dive

spend five hours under water per day.

So you can easily imagine how any mutation to confer an

increased diving ability would have been positively selected.

They identified many genetic variations seen in the Bajau

but two snappily titled genes stood out.

PDE10A is a gene noted to have differences amongst the Bajau,

specifically the part of the gene that is responsible

for thyroid function and spleen size.

And the BDKRB2 gene is involved in the blood redistribution part

of the dive reflex that we talked about earlier.

Rather like the Sherpa in the high altitude Himalaya, this is

evidence of human evolution from our fairly recent past.

And now we're in the era of genetic analysis becoming commonplace,

we're able to trace specific mutations

and how they've travelled through time and geographically

to make us the people we are today.

But does research like this actually benefit patients here in hospital?

Well I see the effects of hypoxia secondary to disease

on a daily basis, where it causes death and disability.

The hope is that by understanding how the Bajau or the Sherpa

have adapted to life in a low oxygen environment, might guide

development of new treatments to help the critically ill.

Honey I've, uh, thought of a name.

Just hear me out here.

What do you think of Tom Scott Francis?

Ow!

- [clears throat] Thanks, Rohin. Go subscribe to Medlife Crisis!

I would recommend starting with his Minute Medicine video

on why you shouldn't run every medical test even if you can.

Next week, a maths puzzle for you.