[BLANK_AUDIO].
Okay, now that you know the five
different types of blood vessels,
let's talk a little bit about the
structure of the blood vessel wall.
Because that is going to determine how the
blood vessels help to function in the body.
So, if we think about the structure of the blood vessel, what I'd like to do, first
of all, is look at the photo or
illustration that I have opened in front of you.
And
what you'll notice there is that all of the blood vessels in the body, except for
capillaries, have a wall that consist of three different tissue layers and
they call those layers the tunica externa, which is the outermost layer, the tunica
media, which is the middle layer, and
then most internally, we have the tunica intima.
So, if we think about how these different
layers are different in terms of their structure.
I'd like to focus first on the tunica intima.
The tunica intima is, a tissue, is made up of a tissue that is thin and smooth.
And it's continuous with the endocardium of the heart.
You remember the endocardium of the heart?
It lines the chamber of the heart.
And it covers the heart valves, right.
The endothelium, which lines the blood
vessels, is continuous with the endocardium.
And so, the endothelium also provides a surface that's smooth and low friction
so the blood can flow past it in a streamlined way as possible.
If you look at this photo, what you'll see is that
it's the tunica intima that is in direct contact with the blood.
The blood is flowing through the central opening in the blood vessel.
We call that central opening the lumen,
and the tunica intima directly surrounds the lumen.
Okay, so one of the other things we see from looking at this illustration
is that the capillary wall consists just of that endothelial layer.
But all of the other blood vessels in the body have
the three tunics in their walls, which I've talked about previously, right?
Okay, so the middle layer, what's going on
with the middle layer of the blood vessel wall?
The middle layer is the tunica media, and it's
made up mostly of elastic and and smooth muscle fibers.
Why do you think elastic fibers would be valuable in a blood vessel wall?
What would elastic fibers do?
>> So they can stretch and withstand different volumes of blood.
>> Yeah, so they're able to distend, but then after we
stretch the blood vessel and relive the blood volume a little bit.
What'll happen, Nia?
>> You also want to be able to constrict.
>> Yeah, the, well, not constrict but they can stretch or descend and then when it's
elastic fibers that cause the blood vessel to
do this we would call that a retraction.
So they descend, distend, and then retract.
We also have layers of smooth muscle in the tunica media.
And the smooth muscle is arranged in concentric rings, or concentric
layers around the wall, and the smooth muscle is able to.
>> Constrict.
>> Contract, and constrict the blood vessel.
Right, so smooth muscle contracts, and when that smooth
muscle, which is arranged in concentric layers around a blood vessel, when it
contracts it causes the blood vessel lumen to narrow.
And so, we call that, constriction.
And because it's happening in a blood
vessel, we can specifically call it vasoconstriction.
When the smooth muscle relaxes, it will allow the
blood vessel to, kind of increase the lumen diameter.
And we would call that state of the blood vessel vasodilation, right?
So, the combination of elastic fibers, and smooth muscle fibers are both valuable in
terms of help regulating how big the lumen diameter will be for the blood vessel.
This smooth muscle, that's in the tunica media, is innervated by
the sympathetic nervous system, and I know you've gotten the impression
that the sympathetic nervous system is this fight or flight nervous
system that gets really activated whenever we're in a high stress situation.
But the sympathetic nervous system is actually always a little but on, and
so it's always providing some baseline
innervation so the tunica media smooth muscle.
And it's always causing that smooth muscle to be
somewhat contracted so that the blood vessels are somewhat constricted.
Now, that's really important because we don't have
enough blood to fill all of the blood vessels.
If they were all maximally dilated all the
time, we wouldn't have enough blood to fill them.
And so we maintain some smooth muscle tone in the vascular smooth muscle,
and that helps us maintain blood pressure so that we can keep blood circulating.
What do you think will happen if
I increase sympathetic activation of the smooth muscle?
>> It'll cause more constriction?
>> It'll cause more constriction, because the smooth muscle
will contract more, and the vessel will constrict more.
Yes.
And the lumen diameter will get smaller.
All right, so the last of the tunics is the tunica externa.
The tunica externa is just made up of a, a layer of connective tissue.
It helps anchor the blood vessel in its place and helps protect the
outside of the blood vessel so that it's not damaged as we move around.
And that is, basically what's going on in the blood vessel walls.
Now, the thing is that not all blood vessels are alike, right?
We have arteries.
We have veins.
We have venules, we have arterioles.
They are not all alike and they are not all alike because exact
composition of the tunics varies from one type of blood vessel to another.
So, if I look specifically at the arteries.
I know that they are the the blood vessels that carry blood away from the heart.
There are actually several different kinds of arteries.
And the kinds of arteries that we have
closest to the heart are called elastic arteries.
They get that name because their tunica media contains relatively
more elastic fibers than we see in other kinds of arteries.
And those elastic fibers are important.
So, think about what happens in the large arteries that are close to the heart.
They are exposed.
Every time you go through ventricular systole and blood is
ejected into those large arteries, what happens to them, Steph?
>> They have the pressure from all that
blood on the arteries that, that are coming.
Like, the blood that's coming out to those arteries.
>> Yeah.
So the blood is gushing in.
And that's causing the arteries that are closer to the heart, to distend, right?
Because of that surge of blood that is pushed into the arteries.
Now, if we're going to keep a continuous flow of blood through the blood vessels.
We know that that blood that distended the elastic
arteries is not going to stay in one place, right?
It's going to gradually circulate through other blood vessels.
But the thing that makes it circulate is the surge of
blood has distended the blood vessels, the elastic fibers in the tunica
media will retract and keep that blood in the elastic arteries
under continuous pressure, forcing it off into other parts of the vascular system.
So, the heart can beat once per second, but because of those elastic fibers
being able to retract, we're able to have a continuous flow of blood.
That's kind of cool, isn't it?
I think that's pretty cool.
Okay, and our heart would have to do a lot more work if, if we didn't
store pressure in the elastic arteries and then have the
elastic arteries retract, and keep pushing the blood off into the circulation.
What do you think happens, when somebody has atherosclerosis
and the blood vessel wall gets stiffer, and can't distend like this?
What do you think is going to happen?
What do you think, Mia?
>> Well, it's going to be harder for the heart
to pump blood because that elastic isn't there to pull it.
>> That's exactly right, if somebody's elastic
arteries have stiffened, what will happen is,
when ventricular systole occurs, the elastic arteries
aren't going to want to go like this.
And so, the heart will have to work harder to force the blood in there, right?
Okay, alright, so we get a little bit beyond the elastic arteries.
And we get to the muscular arteries.
Now, the muscular arteries get their name because the
smooth muscle in the tunica media is more important.
In the muscular arteries, that smooth
muscle can basal constrict or basal, dilate.
And when it does that constriction, or dilation, it
helps to channel blood to different organs, or tissues.
And so, if you are exercising, and your skeletal muscles need an
extra blood flow, the muscular arteries that
supply the skeletal muscles would help to regulate
blood flow so that the skeletal muscle could get the blood supply that they need.
Finally, we get to the arterioles and the arterioles are
the smallest vessels that conduct blood away from the heart, right.
The thing that's distinctive about the arterioles is that that
they have a relatively large amount of smooth muscle in their
tunica media, and that smooth muscle is very responsive to signals from
the sympathetic nervous system in terms of vasoconstricting, or vasodilating.
Also, because we have so many arterioles, they, their
ability to vasoconstrict means that they can have a big impact on resistance in the
vascular system, so if they vasoconstrict, it's going to cause a
lot of resistance and what's that going to do to blood pressure, do you think?
>> It will increase blood pressure.
>> It's going to increase blood pressure, of course.
Okay.
So arterioles carry blood into capillary beds.
Capillary beds is where the gas exchange is going to
occur and then we have to get this blood back
to the heart by way of the venous system, the
smallest vessels that drain, the capillary beds are the venules.
The venules converge to form small veins.
Small veins converge to form large veins.
The thing that you should remember about the veins is that their walls are always
thinner than, than an arterial wall that is the same distance away from the heart.
So, in this picture we've been looking at, you can see that they've shown you a
microscope, a microscopic look at an artery and
a vein from some part of the body.
And what you can see is that the arterial wall is very, very thick.
There's much more smooth muscle.
The venous wall is very thin and, the veins,
because of their thin walls, have a tendency to collapse.
You can see that it's kind of flattened, right.
It could fill up with blood and distend a lot.
But generally they don't, they don't get totally filled with
blood because we're circulating the blood back to the heart.
The veins also have a larger lumen than the arterioles that they are close to and
that larger lumen means that when they are
holding blood, the blood will be under less pressure.
We are going to find out when we actually talk later about the
physiology of circulating blood that the veins maintaining a low
pressure on the blood helps to keep the circulation moving.
Alright, so I think that we've pretty much
covered the highlights of the vascular anatomy
and that will give you a good understanding for
thinking about how we keep the circulation
going and what creates blood pressure.
[BLANK_AUDIO]