This is kind of weird. We're too warm
under the duvet, too cold without it,
and the best temperature is reached when we have one foot sticking out.
What I wonder is why the foot? Why is it that when we are too hot
it’s the foot that helps us regulate our temperature? Why not
the left knee, a shoulder blade or a bicep?
And if we venture into the specifics of the foot in relation to the temperature,
we will also have to tackle the reverse problem:why some people have
ice-cold feet? And again it comes back to the
same question:why feet?
Even before starting my research, I already know, as you probably do, how
our body regulates its temperature: we sweat when we are too hot, we shiver and complain
when we're too cold.
This is what can be seen from the outside, but inside, our first rampart
against thermal variations is vasodilation and vasoconstriction.
That is, the fact that our body adjusts the diameter of our blood vessels to
let more or less blood pass.
That’s why we have swollen fingers in the summer, because they carry
a lot of blood.
And when it's cold it's the other way around: all our extremeties get smaller
because they are "less full", in a way.
And when I say “all our extremeties”, I mean *all* our extremeties.
This blood flow rate adjustment mechanism is well known and exists
almost in every part of the body.
But that does not explain what makes feet so special (or hands for that matter,
because it's exactly the same thing).
There are 3 big reasons that make our feet and hands so special.
You may know the first two, but it is with the last one
that everything is at play.
First, our extremities have a lot of surface in relation to their volume.
A bit like a potato.
A hot potato will need a lot more time to cool down than
a bunch of fries.
So in the script I wrote that this would be "fries" but this turned out more like
mashed potatoes. Just imagine that I am showing you fries right now, okay?
And so this cools down a lot faster when it's shaped like fries simply because
the fries have much more contact surface with the air and therefore much more
opportunities to cool down.
Our fingers, our toes are exactly in this situation.
They are like fries, when our torso is more of a potato type.
Even without the fingers, our hands and feet are very small for such a
large contact surface compared to the rest of the body.
For the next two reasons, we're not talking about the whole foot or the whole hand.
Because the really special parts are the hairless areas;
the bottom of the foot, a large part of the toes and the entire palm of the hand.
These hairless areas have denser vascularity
than the rest of the body.
Look.
These are the blood vessels that we found at our fingertips on the skin surface.
We can distinguish the lines of our blood vessels.
And these lines correspond to the lines of our fingerprints.
Look at your fingers, at every line there are hundreds of blood vessels
that bring blood to the tissues.
And to show you the density of the blood vessels, look at this:
This time it's a cut view.
So the top area is the surface of the finger and the bottom is deeper
inside of the finger.
Here again, we can see the formation of lines as for fingerprints.
All this to say that there is a great blood supply in our hands and in our
feet and therefore many opportunities to bring blood to our fries to keep them cool faster.
The last reason that distinguishes our extremities the rest of the body, and in my opinion the most
interesting... They have something that exists nowhere else in the body:
arterio-venous anastomoses.
It is a complicated name for a blood vessel that links an
arteriole and a venule.
So, I agree, it doesn't sound like anything special yet, but these bridges
are the main players in our thermal regulation thanks to vasoconstriction.
And we will go into a little detail because it's really in the details
that things get interesting.
When we are hot and we need to evacuate heat, we send the blood, which carries heat,
to our fries.
The blood flow in our extremities ìs about 7-8L/min, i.e. between 110 and 133 mL/s.
It's a lot.
It’s this much blood, every second! When we're cold, our first mechanism,
the one we were talking about is vasoconstriction, the fact that the diameter of our blood vessels
is reduced ... and this can reduce the blood flow to 0.
And 0 is not a lot.
And if there is no more blood flow in the extremities, it means that the blood
mostly stays in the potato and has fewer opportunities to cool down.
So we keep our warmth.
This vasoconstriction, which is really key in this case, if you have learned the same
thing as me at school, was taught like this:
You have an arteriole, a venule, between them we have a metarteriole and lots of
capillaries around.
These capillaries, we have them everywhere in the body and it is they who are responsible for disseminating
nutrients to our tissues like glucose or dioxygen.
They also help to get rid of waste like CO2.
This is the normal behavior that you already know.
Now when it's cold, we were told that here, on the capillaries,
there are muscles, sphincters that can close to cut or reduce blood flow.
That is very true, but it is almost anecdotal in terms of
thermoregulation in the palm of the hands and bottom of the feet.
Because in those areas there is another blood vessel.
The famous anastomosis.
We'll call her Anna, it'll be easier this way.
Anna is actually huge.
She is 10 times the size of an arteriole.
All the Anna's of our hands and feet receive 80-90% of the total blood flow,
when the capillaries receive the rest... and use it mostly for nutritional purposes.
Anna's only role is to regulate the temperature. That's it.
She has a very impressive muscular structure that allows our
nervous system to control the diameter and thus blood flow with great precision.
That's the vasoconstriction that really matters!
So the model we learned in school is true, but it is incomplete when we talk
about thermoregulation of the hands or feet.
This is mainly used to feed tissues and that is specially made
to regulate temperature.
To tell you the truth, we have Anna's all over the body, but in the hands and the feet
is where we have the biggest number of them.
We consider that there are something like 600 anastomoses/cm2 at the end of our
fingers, (just right there, there's 600) and 100/cm2 at the bottom of the feet.
So that's why we stick out our feet and not our knees, because there
are specific vessels, Anna's, whose job it is to regulate the temperature.
Our 4 extremeties are in fact thousands of radiators.
Right.
Welcome to the frustrating part of the video.
There is one question that I don't have not answered: the fact that on the contrary,
there are people who suffer from cold hands and feet, that's a fact.
Actually, this disorder affects primarily women with 30% of women
from 20 to 40 years old compared to 7% of men in the same age group.
There is an average difference of 1.5C between men's hands
and women's hands.
Why would there be a difference around gender for this? Good question.
There are loads of answers but I must admit that I am not convinced by many of them.
There are papers that say this difference comes from the difference in fat and muscle, since
it is a fact that men and women are on average different on this, but
other articles refute it.
These differences in fat and muscle, I find them relevant
at the scale of the body but not so much for the extremities, since these are not
very fatty nor very muscular areas.
There is also the hormonal hypothesis since there is a correlation with adolescence
and menopause but which is still lacking support from further research.
Others think it's a character trait that can be inherited.
And the last guess I could find is connected to something that I never
would have thought of.
The vasoconstriction that occurs in our hands is not necessarily tied
to the outside temperature... but it's also regulated by our emotions.
For example anger, anxiety, depression, guilt, fear of abandonment,
and more, are emotions that come with a drop in temperature in the fingers
(that's for both men and women).
Conversely, when we are emotionally detached, when we have a feeling of security, of
comfort, superiority and control, our fingers tend to be warmer.
And the way we deal with our emotions is socially constructed,
for example it is generally more acceptable for a man to be aggressive or angry
than it is for a woman.
Well, our emotional management... may also be reflected in
the temperature of our hands and feet.
While the fact that our emotions impact our bodily temperature is proven,
it's harder to make a clear connection with the differences in socialization
between men and women.
That said, even if one could prove its veracity, I think it would work in combination with
other factors and wouldn't be the one and only reason for it.
I warned you that this would be the frustrating section of the video.
I don't have a definitive answer to give you.
But if my synthesis does not satisfy you, I will link the articles in the description,
and you can make up your own mind.
What is certain is that yes, there is a difference between the male and
the female temperature average in our extremeties.
And to top it all off, the people who deal with ice-cold feet will also have
a hard time falling asleep because of it.
Falling asleep can only happen when the body is relaxed.
Which implies vasodilation,
an opening in our blood vessels and therefore an influx of heat especially at the ends of our limbs.
So be kind to people with frozen feet.
Not only is it unpleasant but it also keeps them up at night.
It's a double penalty.
So let's love each other and accept our temperature differences.
#tolerance
Well if you're still here, I'll keep going.
I have a little bit more info that isn't necessary to the video but which I'd love
to mention because I think it's cool, it's about heat preservation.
We talk a lot about vasoconstriction as the key factor and all,
as I just did myself... but when we look at the mechanism as a whole,
it's even more impressive and ingenious.
What I tried to say there was ingenious.
Do you know the type of heat exchange that goes counter-current?
This is a blood vessel that is inside the body.
Here we have the skin, then this is the outside air which we consider to be very cold.
And so we are somewhere in the finger there.
So we've got hot blood coming here, which cools there and goes back up.
When the blood cools in the extremities, it then goes up through the veins.
But in reality, we don't want to have ice-cold blood move back up into the
body, that would be completely counterproductive.
This would cool the organs, and they can't afford to do that.
So instead, the blood goes up near the artery that brings
hot blood.
This proximity means that there will be exchanges of heat here.
So the hot blood will be pre-cooled before it arrives at the extremities and the colder blood
will be preheated before going back up in the body.
This way, the cold remains very localized.
And it goes even further.
When it's cold out, the blood comes back via deeper tissue level, to benefit
from this counter-current mechanism by being extra close to the arteries whereas when it
it's hot out, the blood moves through a more superficial tissue to get away from the hot
arteries and have a larger contact surface with the outside world.
This was the second episode of season 2 of “I got lost on the Internet again”,
I hope that you liked it.
I invite you to subscribe because we still have 8 episodes of ridiculous questions to cover.
And if you really don't want to miss anything
regarding Scilabus (from radio, social media, the secondary channel etc),
I have relaunched my newsletter, the link is in the description.
And obviously I have no intention of spamming you.
I am changing the subject completely, this has nothing to do with the channel but I wanted to
quickly mention that a few months ago, I was a jury member for a French language
competition in video science communication organized by the French Embassy
in Canada and open to all students of the country.
Since we couldn't have an award ceremony for obvious reasons,
I just wanted to take a moment at the end of this video to thank all the
participants for their involvement and to congratulate the winner whose video you will find
in the description.
I invite you to go watch it just to see what was done.
I will see you again soon, talk to you next time, bye.