WEBVTT
Kind: captions
Language: en

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- I'm taking January off, and while I'm gone,
some amazing creators

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are going to be taking over this channel.

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

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His video includes footage
from his diving trip to Borneo

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

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Rohin, over to you.

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- How long can you hold your breath?

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One, two minutes?

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I'm sorry, homo sapiens
are just pretty pathetic

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next to the diving world champions

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who can stay underwater
for one or two hours.

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So how do they do it?

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Well, there are a few different ways,

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

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[dolphin squeak]

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Others involve a little bit of gentle evolution,

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

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

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[tranquil music]

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There are five ways we can
spend longer underwater.

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Number one is to increase your
tolerance to carbon dioxide.

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The first thing that makes
you want to take a breath

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is the rising level of carbon
dioxide or hypercapnia.

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

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"and kapnos, meaning smoke."

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This acidic waste gas builds up,

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

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

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Breathe, god damn you!

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

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[cow moo]

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Now you can improve your carbon dioxide
tolerance quite quickly.

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

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

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and some tips from an
experienced free diver.

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The next obstacle though is
a lot harder to get around

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and that's hypoxia or low oxygen.

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Hypoxia kills cells.

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No matter how you die,
ultimately it's hypoxia

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

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that is the cause
of your inevitable death.

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Unlike training your body to
become more tolerant to hypercapnia,

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increasing your tolerance
to hypoxia takes years of training.

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

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So what is the solution for a longer dive?

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Well, to maximise oxygen delivery
and minimise oxygen use.

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Which brings me to number two.

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

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which is found in the muscles
of pretty much every mammal

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and it's responsible
for storing oxygen.

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However if we were tucking
into a steak that came from

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a seal it would be almost
black in appearance because

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

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This is not a human steak.

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So why can't we just
pack in more myoglobin

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and store more oxygen like the seals do?

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The problem with proteins is that
when they g--

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[child burbles]

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The problem with proteins is th--

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[child drums]

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The problem with proteins
is when they get too tightly

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packed they start to clump
together and lose their function

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

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

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As I'm sure you know,
positive repels positive

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and as a result the proteins
can get very close together

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without forming those clumps.

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Number three is to adjust your
blood flow to preferentially supply

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

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

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And number four is to
slow your heart down.

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Your heart is very oxygen-hungry organ

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so by reducing your heart
rate you're immediately

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buying yourself more time under water.

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

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Erroneously sometimes called
the mammalian dive reflex.

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

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that's been studied.

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It's stimulated by submersion in water,

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particularly cold water.

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And remember, below 200m depth,
water is cold

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no matter where you are
in the world

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

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So what is the dive reflex?

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Well let's take a look.

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[bells]

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[blows conch shell]

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That's the wrong video.

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

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

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in the water probably just
for about thirty seconds or so

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

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

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Okay. Here goes.

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Agh. That was cold.

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So my heart rate has dropped
right down to 45.

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It's still staying at 45.

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I don't know how long I was under

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maybe 30, 40 seconds.

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

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

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

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

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these commercial devices will
have a cut off in the forties

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

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But I think it was a
pretty clear demonstration.

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In diving mammals this
is far more pronounced

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

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as low as four beats a minute.

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

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

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Number five is also part
of the dive reflex because

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

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The spleen is normally a small organ

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which is involved with the immune system
and filtering the blood

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as such stores
a little reservoir of blood.

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

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

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

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However, diving mammals can
have significantly enlarged spleens

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representing a much
bigger reservoir of blood

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to be provided when necessary.

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

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

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of our species that
haven't taken advantage.

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

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to Southeast Asia.

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

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which was about a fisherman who was so
negatively buoyant

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

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

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

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

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I'm in north Borneo at the moment,

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

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and my diving instructors
here tell legendary stories of Bajau

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

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So what makes them so special?

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The Bajau have led a nomadic
subsistence lifestyle

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

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eating what they catch and
rarely setting foot on land.

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Earlier this year, a research
team led out of Copenhagen

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published an incredible
study which demonstrated that

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the Bajau have evolved
larger than average spleens

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when compared to nearby
non-diving communities.

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Free divers develop big
spleens through training.

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But the enlarged spleens in
the Bajau are seen in members

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

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so we know that this is
a genetic adaptation.

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You might wonder how a
genetic pressure has been exerted

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until you consider the fact
that the Bajau who dive

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spend five hours under water per day.

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So you can easily imagine
how any mutation to confer an

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increased diving ability would
have been positively selected.

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They identified many genetic
variations seen in the Bajau

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but two snappily titled genes stood out.

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

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specifically the part of
the gene that is responsible

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for thyroid function and spleen size.

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And the BDKRB2 gene is involved
in the blood redistribution part

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of the dive reflex that
we talked about earlier.

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

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evidence of human evolution
from our fairly recent past.

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

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we're able to trace specific mutations

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and how they've travelled
through time and geographically

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to make us the people we are today.

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

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Well I see the effects of
hypoxia secondary to disease

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

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

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

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development of new treatments
to help the critically ill.

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

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Just hear me out here.

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What do you think of Tom Scott Francis?

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Ow!

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

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I would recommend starting
with his Minute Medicine video

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

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Next week, a maths puzzle for you.