So, again, let’s take the next step and
talk about how blood vessels are structured.
I promised you before we would get some look
at them through the microscope so we can actually
see what the walls of these blood vessels
Each of the blood vessels, that is the veins
and the arteries – not the capillaries but
the arteries and the veins – have three
layers. Latin words are used as I’m sure
you know in anatomy. So the innermost layer
is called the tunica intima. That is the most
intimate layer with the blood. Then there’s
the middle layer, the tunica media. And then
there’s the outer layer, the tunica adventitia.
Tunica means coat.
Let’s talk first about the tunica intima.
It’s the innermost layer. It’s composed
of endothelial very simple squamous epithelial
cells and it’s very, very thin. It is actually
also an area for production of a number of
hormones that keep the underlying parts of
the vein and the artery functioning and healthy.
We may talk a little bit about that later.
The tunica media is the muscle layer of the
vessel wall, again both for the artery and
the vein. They are composed of circularly-arranged,
smooth-muscle cells and they are innervated
by the sympathetic nervous system, part of
the autonomic nervous system. When the sympathetic
nervous system signals and releases some noradrenaline
or norepinephrine from the endings of the
nerves, this contracts these smooth-muscle
cells so the vein contracts and the channel
narrows. The same thing happens on the arterial
side. When the sympathetic nervous system
sends stimulation, the arterioles contract,
resistance goes up, as we’ve talked about
before, and the amount of blood flowing into
the capillaries goes down. And, by the way,
blood pressure behind the arterioles rises
when they constrict. They are the resistance
vessels as we’ve talked about. Any time
that the smooth muscle contracts in the vein
or the artery, they constrict the channel
of either the artery or the vein. And any
time they relax, they increase the channel
of the artery or the vein.
The tunica adventitia is the outermost layer.
And you can see here in this diagram all three
layers are labelled. Level 1 – the first
arrow – is the tunica intima, the thin layer
of endothelium that touches on the lumen and
actually touches the blood. The second and
biggest layer – you can see all these wavy
fibres – they are the smooth muscle that
can contract or relax. And then number 3 is
the adventitial layer – the tunica adventitia
– that really just holds the vein or the
artery in place within the tissues so that
it doesn’t move around.
Now in this little diagram you can see a comparison
of the various layers, comparing artery and
vein and actually also capillary. Let’s
look first at the artery.
You’ll notice that in each of these three
vessels, the artery, the vein and the capillary,
the inner layer – the endothelium, the tunica
intima – is the same. It’s only one cell,
sometimes two cells thick and it’s about
the same in all three vessels. The difference
occurs in the amount of smooth-muscle tissue.
You’ll notice that there’s a lot of smooth
muscle – the tunica media – in the artery,
some but much less in the vein and none in
the capillary. So the capillary cannot contract
or dilate but both the artery and the vein
can contract and dilate. But you can see there’s
much more muscle in the artery so the artery
is much more capable of contraction because
it has much more smooth muscle compared to
And here we see an actual histological section.
You can see what is depicted here is an artery
right next to a vein. What the arrows show
is the thickness of the smooth-muscle layer.
Notice how much thicker the smooth-muscle
layer is in the artery compared to the vein.
And this is an actual human specimen and it
really reflects what we just showed you in
the diagram in the last slide.
And here we see another human specimen. Again
you’ll see the artery with its much-thicker
media with the smooth muscle, the vein with
its much-smaller smooth muscle. You’ll notice
that the arterial lumen or the channel is
even a little bit smaller than the vein. Remember,
60% of the blood is in the veins. They are
also the reservoir for the body. And by the
way, you can see two sympathetic nerves up
at the top of this histologic section. And
also you can see fat tissue – adipose tissue
– and adventitia, which are holding the
nerve, the artery and the vein in place.
And once more the diagram showing you a comparison
of the artery to the vein, showing you the
three layers: the very, very thin tunica intima
– one cell or at most two cells thick – the
thicker tunica media will all the smooth muscle
and you’ll notice, on the left, the artery
has a much thicker muscle layer than the vein
on the right. And then finally the tunica
adventitia which holds these vessels in place.
And you can see again the major difference
here is the thickness of the smooth muscle
Finally, let’s talk about which physical
principles influence the blood circulation.