00:01
Well, when we have a look at
an artery or a vein, we need
to understand something about the pressure
within those vessels. On the right hand side
of this slide, you can see a diagram illustrating
the main pressure within these vessels as
they pass from the aorta and then down to
a capillary bed then into small venules
and then into veins to be returned to the
heart. And when the heart pumps, the pressure
is called the systolic pressure, and the average
is around 120 millimeters of mercury.
00:44
And then when the heart goes to a resting phase or
a filling phase, pressure drops in these vessels
down to about 80. And that’s called the diastolic
pressure. And that changes the pressure
within the arteries. And as you see down the bottom,
there is a section of arteries that there
is classifications ranging from the aorta,
arteries, arterioles, etc to the capillary
beds. And what I want you to notice is that the
aorta has a very thin wall relative to the
size of its lumen. And I’ll explain why
that occurs or why that is later on in the
lecture. Within the arteries and small arterioles
have a thick wall, and they’re circular
in profile. That’s because they were standing
or at least they have the high pressure inside
them as they pass blood down to all the tissues
in the organs of the body. And once the blood
then gets into the capillary bed and beyond,
the pressure is very low. So one way in which
you can tell the difference between the artery
and the vein or small veins is that, bear
in mind, the pressure difference, because an
artery is acting under a fairly high pressure,
the lumen is going to be usually nice
and circular. As you see here in the image,
here is the section through the artery. It’s
got a nice circular profile because the
pressure of the blood inside it, passing down
through the tissues. If you look at the vein,
however, right next to it, the lumens collapse
down because it’s acting on a very low
pressure and only opens up and transmits the
blood forward when it’s needed to, to return
it back to the heart. Sometimes veins are
mere storage units, storing blood, and then
it’s returning that blood intermittently
back to the heart. But because their pressure
is very low, they normally collapsed.
In the diagram, have a look at the luminal
aspect of a vein. And usually, the lumen is larger
relative to the wall of its lining, and the
actual ratio or rather the comparison between
the relative thickness of the wall of the
aorta or an artery compared to the lumen is a
lot thicker. In a vein, the relative thickness
of the wall of the vein relative to the size
of the lumen of the tube, is a lot smaller.
03:42
In this section, there is also a section
through a nerve fascicle. This section is
taken through, perhaps, part of the limb of
the body, and you’re looking at a fairly
large artery, a fairly large vein, and a nerve bundle.
Those of you who do anatomy will understand
that usually, when you’re passing down through
the limbs, arteries, veins, and nerve bundles
accompany each other surrounded by connective
tissue, which is the green colored staining
component you see in this section. That happens
to be collagen. So let’s look now at the
layer of a typical artery and a vein. Here,
it’s easy to describe the wall of an artery,
because some of the components of the wall
are a little bit more obvious than you see
in a thinner wall of a vein. On the right-hand
side, is a diagram that you can use to then
try and work out or make sure you can consolidate
your understanding of the wall of a typical
blood vessel. Let’s have a look at the section
of the artery on the left-hand side.
05:02
It’s stained to show a number of different components
that I will point out. On the top left-hand
side of the section, is the lumen of the blood
vessel. You can only see a very tiny bit of
clear part of this lumen. Well, the layer of
the blood vessel closest to the lumen is
called the “tunica intima.” Tunica just
means a layer or a coat. Intima, it’s
the most intimate layer in relation to the
lumen of the blood vessel. The middle layer
is called the “tunica media.” The tunica media
has a little star next to it or an asterisk.
05:47
And that is to remind you that this is the
coat, this is the layer of the artery that
changes significantly. And it changes because of
the different roles this coat has in cardiovascular
function. This layer is contractile. It can
change dimensions. And that’s very important
particularly in arteries because you can distribute
blood to various parts of the body by opening
that wall or relaxing that wall to increase
the lumen diameter. If you get up and walk
around or run, the blood vessels, the arteries going
to your limbs, your lower limbs particularly,
are going to open up so that you can perfuse
your skeletal muscles with a lot more blood,
so this tunica media can relax. And therefore,
widen the lumen, allow more blood to flow
through. Conversely, when you want to diverge
blood away from certain parts of the body,
then that layer can contract, smaller lumen,
therefore, less flow of blood. And that contraction
and relaxation can be controlled by nerves of
the autonomic nervous system or other factors
that again, we’ll talk about in later lectures.
So it’s a very, very important layer.
07:18
On the outside, the third layer is the tunica
adventitia. It’s connective tissue.
07:24
Here, it’s fairly dense connective tissue, mostly
collagen. That’s an important layer for
a number of reasons. Most importantly, it
strengthens the wall of the artery. So on
the very high blood pressure, the artery doesn’t
rupture. It also blends the artery with surrounding
tissues. So often, the junction between what
we call the tunica adventitia and surrounding
connective tissue is often hard to find and
really not necessary. Think back at the slide
previously where I explained the neurovascular
bundle, the artery, the vein, and the nerve
wrapped up by connective tissue in a component
or perhaps a limb. It’s very hard there
to distinguish where the tunica adventitia
ends and where the connective tissue wrapping
around the artery, vein, and nerve begins. Now,
often inside large arteries in particular,
there is a little layer of elastic tissue
called the “internal elastic lamina.”
If you look carefully at this slide, you can
see this little wiggly line, clear wiggly
line running through. That’s the internal
elastic lamina. On the outside, there is also
an external elastic lamina that separates
the tunica media from the tunica adventitia.
09:00
And if you look very carefully in the tunica
adventitia, you can make out some little pink
dots. They represent elastic tissue. You can’t
see them very clearly, but there is elastic
tissue through most arteries. In one artery,
in particular the aorta, that elastic tissue
dominates for a reason I’ll explain later
on. Now, this is a good stain to show you
the components of an artery, and similarly,
a vein, because it shows you smooth muscle.
09:34
Smooth muscle is the brownish component you
see making up the wall of the tunica media.
09:41
And the other stain, the greeny colored stain
is the collagen. So the important thing to
understand here is that the tunica media is
made up of a combination of smooth muscle
and collagen. There is also elastic tissue
there, but in minimal amounts. And all these
three components help to enable the blood
vessel wall to relax or contract because of
the presence of smooth muscle that I explained
earlier, but the collagen strengthens the
blood vessel as well so it doesn’t rupture.
And the elastic tissue allows some aspect
of recoil or stretching. But this stain is
a good one to show that. One important point
to understand is that some of you may recall
or remember from your previous studies of
connective tissue, for instance, that fibroblast
lay down collagen and elastic tissue.
10:45
Well, in the case of an artery and a vein, those
connective tissue components, collagen and
elastic tissue, is actually made by the smooth
muscle cell. And as we’ll look at the other
lectures later on in this histology course,
you’ll realize that lots of other cells
too make collagen fibers and elastic fibers.
But here, importantly, it’s a smooth muscle
that has that job.