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

  • 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.

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

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10分間息を止める漁師たち (The Fishermen That Hold Their Breath For 10 Minutes)

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    林宜悉 に公開 2021 年 01 月 14 日
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