blood back to the heart. Let’s look at the aorta
and the vena cava. They are very special
blood vessels. Here is a section of the aorta.
On the left-hand side, it’s sectioned and
it has been stained with normal hematoxylin
niacin. On the right-hand side, it’s a bit
collapse but that happened during the processing.
And the tunica media has been stained for
elastic tissue. You can see then that the
dominant fiber in the tunica media of the
aorta is elastic tissue. It has a very, very
important function. Here is a high magnification
picture of this elastic tissue, and you can
see the dark stained plates of elastic.
And if you look very very carefully in that elastic,
you can see that there are fenestrae or small
little gaps or pores between the elastic tissue,
the sheaths of elastic which are branching.
That’s important because it’s important to understand
that a blood vessel gets its nutrition directly
from the lumen, or in some cases, very large
vessels get their nutrition from vessels we
call vasa vasora, vessels that come from the
lumen or other vessels and they penetrate
the tunica adventitia, and they run along
the junction between the tunica media and
the tunica adventitia, and they supply the
wall of the blood vessels. So these fenestraes
are important because it allows the diffusion
of nutrients, oxygen, etc to go through
the depths of the wall of the blood vessel.
Blood vessels don’t have a direct blood
supply or capillary bed going into them. Now,
the role of this elastic in the aorta is very
very important. When the heart pumps, when
the ventricle contracts, the left ventricle
contracts and passes blood up into the aorta,
the aorta expands as it accepts that stroke
volume of blood, a hundred mils or so. It
expands because of the elasticity. That elastic
tissue expands just like a rubber band stretching.
And then when the heart goes through the resting
or filling phase, when the ventricle has stopped
contracting and is filling again as it resists
blood coming from the left atrium, the elastic
tissue inside the aorta then recoils just
like an elastic band contracting back to its
normal shape after you’ve stretched it.
And that recoil is very very important because
what that does is it maintains pressure
inside the aorta during the resting phase of
the left ventricle. And because it maintains
pressure during that resting phase or filling
phase, it maintains the flow of blood.
And as you get older, that elastic tissue starts
to disintegrate or be not so elastic,
similar to the elastic tissue in the dermis
of skin. When you look at the dermis of
skin, you often see just very dense collagenous
connective tissue. And that’s how the dermis
is described. But it does have elastic tissue
there as well. And like the aorta, the elastic
tissue in your skin, degenerates with age
as well. So your skin becomes wrinkling.
In the case of the aorta, when that elastic tissue
loses a lot of its recoil ability, then it
can create cardiovascular problems in the aged.
So that’s the importance of that elastic
tissue in the aorta. Now, here is a large
vein, the vena cava. You know it’s a vein