- [Voiceover] Let's say this is you,
and let's say it's a nice day outside.
We've got a sun shining.
Let's say it's about 70 degrees outside.
And let's say you've been doing a bunch of exercise.
One thing that's gonna happen when you do exercise
is your body is gonna produce a lot of heat.
And the reason for that is you have muscles,
like this quadriceps in your leg
and like your heart that are gonna be very active
when you're doing exercise.
And all the reactions that make them active
end up releasing heat.
And so a problem that your body might face in this situation
is how does it maintain its core body temperature
of 98.6 degrees Fahrenheit?
Now, one of the ways that your body can get rid
of this excess heat produced by your muscles is to sweat.
But there's another way that it gets rid of heat
that I'd like to talk about here.
And that way also involves the skin.
And so to find out more about it,
let's look at a zoomed in picture
if a little piece of your skin.
So here we're gonna draw your skin
in this zoomed in frame here.
So here is the surface of your skin,
which we'll draw in pink.
The skin is pink.
And below your skin a little bit deeper,
you're gonna have some blood vessels.
Let's say this is maybe an arteriole.
Let's call it arteriole.
And this arteriole will be supplying your skin
with blood and with oxygen via the blood.
The way it's gonna do this is to send off
little capillary beds into the skin,
which I'll try to draw here.
Here are our little capillary beds.
And these capillary beds, of course,
consist of very fine blood vessels
with high surface area that allow exchange of oxygen
between the blood and the tissue.
But what you have to realize about the skin for our purposes
is that it kind of acts as insulation.
And the reason for this is that oftentimes
it's not a nice day out.
And oftentimes you're not doing exercise.
So when it's a cold day out
and you're maybe just sitting outside,
you don't want to lose too much heat.
And so in those cases it's great to have this insulation
that keeps the heat from down below
and from this arteriole, for instance,
that keeps that heat from diffusing out too quickly.
So it's really just like your house,
which during winter you want to have insulation
to prevent loss of heat.
But these little capillaries
that go to the surface of the skin,
as you can tell, they kind of bypass all this insulation,
and they go right to the surface.
And so the heat from the blood going to these capillaries
will have a much easier time of getting out.
And this analogy to the house
is that these little capillaries are like little windows
that allow heat out past the insulation
in the walls of your house.
So one thing you're gonna do
when you're trying to get rid of heat,
so now we're no longer talking about winter,
is, well, what would you do in your house,
if it were really hot inside the house
and you had these windows that were just kind of,
you know, not open too wide, what you would do
if you were feeling really hot in your house
is you'd open up those windows wider.
So that is exactly what your body is going to do
when your body is overheating.
So here we've got our skin again.
And here we've got our arteriole
running through the deeper part of the skin.
What your body's gonna do is
it's gonna open up that window wider.
And so it's going to dilate these capillaries over here.
It's gonna make them much wider than they were.
And this is known as vasodilation.
Vasodilation.
It'll vasodilate those capillaries
because what's gonna happen
when these capillaries are vasodilated?
Do you think that more or less blood
is going to be passing through these capillaries
in this situation or in this one?
And obviously more is going
to pass the capillaries in this situation.
So here I'm just drawing in
the insulation of our skin again.
And now in this case we have a lot more blood
going up into these capillaries
and therefore, a lot more loss
of heat to these surroundings.
And so this is what happens when it's hot out
and when your body wants to get rid of heat
so that it can maintain its nice core temperature of 98.6.
And by the way, something we should ask is
what's so special about this 98.6?
Why do we want that temperature?
And the answer is that your body has, as you know,
tons of chemical reactions occurring all of the time.
And if you've taken some chemistry,
you might have learned that temperature affects
the way that reactions are carried out,
how fast they're carried out,
where the equilibrium is, et cetera.
And that might sound, you know, fancy or confusing
if you don't know much chemistry.
But the point is that all the machinery
and all the chemistry of your body
works optimally at this temperature.
And that's why you want to maintain it.
Now, this is what happens when it's hot out.
And when it's cold out, what do you think is gonna happen?
Well, when it's cold out,
you're gonna wanna close the windows.
You're gonna wanna close these windows that allow heat out.
So you're gonna do the reverse.
You're going to make your capillaries
as small as possible.
And so as a result you're gonna have
less heat at the surface of your skin.
And that's why if it's a cold day outside
and you walk around, when you feel your hands,
they feel extremely cold.
And that's because your body has made
all the capillaries in your skin as thin as possible
so that you lose very little heat.
And because they're thin, this whole surface area
of your skin becomes very cold.
Now, as a kind of review, we can also ask the question,
maybe this has been bothering you
for the whole video so far,
but how does your body regulate the size of these vessels?
And the answer is that all these blood vessels
have little bits of smooth muscle around them,
which I'll just draw in orange,
little smooth muscle cells around them.
And these smooth muscle cells,
when they contract, when they squeeze,
they make the vessels smaller.
And when they relax, they allow the vessels to be bigger.
And the way that your body tells these smooth muscle cells
to squeeze or not to squeeze is just through nerves.
So I'm gonna draw a nerve in purple.
And it might be kind of hard to see,
but I think you get the point.
Your body literally sends impulses through these nerves
that tell these smooth muscle cells
whether they should be contracting
or whether they should be relaxing.