Arteries, arterioles, venules, and veins | Health & Medicine | Khan Academy

February 12, 2020

I want to figure out how blood
gets from my heart, which I’m going to draw here,
all the way to my toe. And I’m going to draw my
foot over here and show you which toe I’m talking about. Let’s say this toe right here. Now, to start the
journey, it’s going to have to go out of
the left ventricle and into the largest
artery of the body. This is going to be the aorta. And the aorta is very,
very wide across. And that’s why I say
it’s a large artery. And from the
aorta– I’m actually not drawing all the
branches of the aorta. But from the aorta, it’s going
to go down into my belly. And it’s going to branch towards
my left leg and my right leg. So let’s say we follow
just the left leg. So this artery over
here on the top, it’s going to get a
little bit smaller. And maybe I’d call this
a medium-sized artery by this point. This is actually now getting
down towards my ankle. Let’s say we’ve gone quite
a distance down in my ankle. And then there are, of
course, little branches. And let’s just follow
the branch that goes towards my foot,
which is this top one. Let’s say this one
goes towards my foot, and this is going to be
now an even smaller artery. Let’s call it small artery. From there, we’re
actually going to get into what we call arterioles, so
it’s going to get even tinier. It’s going to branch. Now, these are very,
very tiny branches coming off my small artery. And let’s follow
this one right here, and this one is my arteriole. So these are all the different
branches I have to go through. And finally, I’m going to get
into tiny little branches. I’m going to have to draw
them very, very skinny just to convince you that we’re
getting smaller and smaller. Let me draw three of them. No. Let’s draw four just for fun. And this is actually
going to now get towards my little toe cells. So let me draw some
toes cells in here to convince you that I
actually have gotten there. Let’s say one, two over here,
and maybe one over here. These are my toes cells. And after the toe
cells have kind of taken out whatever they
need– maybe they need glucose or maybe they need some oxygen. Whatever they’ve
taken out, they’re also going to put
in their waste. So they have, of course,
some carbon dioxide waste that we need to drag back. This is now going to dump
into what we call a venule. And this venule is going
to basically then feed into many, many other venules. Maybe there’s a venule
down here coming in, and maybe a venule up
here coming in maybe from the second toe. And it’s going to basically
all kind of gather together, and again, to a
giant, giant set of veins. Maybe veins are dumping in
here now, maybe another vein dumping in here. And these veins are all going
to dump into an enormous vein that we call the
inferior vena cava. I’ll write that right
here, inferior vena cava. And this is the large vein
that brings back all the blood from the bottom
half of the body. There’s also another one over
here called the superior vena cava, and this is bringing back
blood from the arms and head. So these two veins, the superior
vena cava and the inferior vena cava, are dragging the
blood back to the heart. And generally
speaking, these are all considered, of course, veins. Let’s back up now and start with
the large and medium arteries. These guys together
are sometimes referred to as elastic arteries. And the reason they’re
called elastic arteries, one of the good reasons
why they’re called that is that they have a protein
in the walls of the blood vessel called elastin. They have a lot of
this elastin protein. And if you think about the word
elastin or elastic– obviously very similar words– you
might think of something like a rubber band or a balloon. And that’s probably the
easiest way to think about it. If you have a blood vessel,
one of these large arteries, for example, and let’s say
blood is under a lot of pressure because the heart is squeezing
out a lot of high pressure blood, this artery is
literally going to balloon out. And if you actually looked
at it from the outside, it would look like a
little sausage, something like this where it’s puffed out. So what’s happened there between
the first and second picture is that the pressure
energy– so the heart is squeezing out a lot
of pressurized blood. And, of course, there’s
energy in that blood. That pressure energy
has been converted over into elastic energy. It’s actually converting energy. We don’t really always
think about it that way, but that’s exactly
what’s happening. And when you convert
from pressure energy to elastic energy, what
you’re really then doing is you’re balancing out
those high pressures. So you’re balancing
out high pressures. And this is actually
very important, because the blood that’s
coming into our arteries is under, let’s not
forget, high pressure. So the arterial system we know
is a high-pressure system. So this makes perfect sense
that the first few arteries, those large arteries and even
those medium-sized arteries, are going to be able to deal
with the pressure really well. Now, let me draw a little line
here just to keep it straight. The small artery
and the arteriole, these two are actually sometimes
called the muscular arteries. And the reason,
again, if you just want to look at the wall of the
artery, you’ll get the answer. The wall of the artery is
actually very muscular. In fact, specifically,
it’s smooth muscle. So not the kind of muscle
you have in your heart or in your biceps, but
this is smooth muscle that’s in the wall
of the artery. And there’s lots of it. So again, if you have a
little blood vessel like this, if you imagine tons and tons of
smooth muscle on the outside– so let’s draw it like this,
little bands of smooth muscle. If those bands decide that
they want to contract down, that they want to
squeeze down, you’re going to get something that
looks like a little straw, because those muscles
are now tight. They’re tightly
wound, so you’re going to create like a little straw. And this process is
called vasoconstriction. Vaso just means blood vessel. And constriction is
kind of tightening down. So vasoconstriction, tightening
down of the blood vessel. And what that does is
it increases resistance. Just like if you’re
trying to blow through a tiny,
tiny little straw, there’s a lot of resistance. Well, it’s the same idea here. And actually, a lot of
that resistance and change in the vasoconstriction
is happening at the arteriole level. So that’s why
they’re very special and I want you to remember them. From there, blood is going to
go through the capillaries. I didn’t actually label
them the first time, but let me just write that here. Some, as they call
them, capillary beds. I’ll write that out. And then it’s going to go and
get collected in the venules and eventually into the veins. And the important
thing about the veins– I’m going to stop
right here and just talk about it very
briefly– is that they have these little valves. And these valves make sure
that the blood continues to flow in one direction. So one important thing
here is the valves. And remember, the
other important thing is that they are able to
deal with large volumes. So unlike the
arterial side where it was all about large pressure,
down here with the vein side, we have to think
about large volumes. Remember about 2/3 of your
blood at any point in time is sitting in some vein
or venule somewhere.

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