So, that's the sequence of blood flow through the heart.
Remind yourself of the physiology of the heart and the pumping of the heart,
the atria contracting together, and the ventricles contracting together.
Now, let's now look, again, at this diagram and understand that there are three different components to the wall of the heart.
First of all, the heart is lined on the outside by a thin capsule or thin membrane called the epicardium.
This epicardium is actually a visceral layer of the pericardium.
Because that epicardium reflects back on itself and creates a pericardial cavity or sac
in which the heart sits and in which the heart pumps.
And that cavity, that serous cavity is lined by fluid.
It's a very thin cavity, it's lined by fluid that reduces the friction during the beating of the heart.
So, the epicardium's on the outside. The thickest layer is the myocardium.
The myocardium is really full of cardiac muscle. That's the work part of the heart wall.
That's the part of the heart wall that does the contracting or the pumping, the myocardium.
And then, internally, the endocardium lines the entire inner surface of the heart.
It lines the heart valves as well and the septum between the atria
and also, both ventricles called the interatrial and interventricular septums.
So, now, let's look at those structures in a bit more detail.
Let's look at the histology of those structures.
First of all, the epicardium, the covering on the outside of the heart.
As you can see from the diagram and also, from the histological section,
it contains a lot of fatty tissue or adipose tissue and I'm sure those of you
who have been in a dissection lab always find that rather frustrating
when you're trying to locate some of the positions of the coronary arteries.
It's also composed of some connective tissue, collagen, elastic tissue,
all the sorts of normal connective tissue fibers you'd expect to find in some loose connective tissue.
And on the outside, it's lined by a squamous type epithelium called the mesothelium.
So, that mesothelium, underlying connective tissue, and adipose tissue
create the outside coating or covering of the heart, the epicardium.
The myocardium as I mentioned before, consists entirely of cardiac muscle
with some connective tissue skeletal components, and also, blood vessels.
But it's mostly cardiac muscle and as I've said before, that's the pump component of the heart.
It's lined on top, of course, by endocardium. It's lined on the top of the section here
but remember, it's lining the internal chambers of the heart.
So, have a look at the epithelium of the heart lining here, endothelium, and it consists of a squamous
epithelial cell lining as I was mentioned before and that squamous epithelium
sits on a subendothelial layer of connective tissue.
It's a very thin layer of supporting connective tissue, similar to a lamina propria in other organ systems.
And sometimes, there's also a third layer, the subendocardial layer, again, of connective tissue
and this separates the epithelial surface from the underlying cardiac muscle.
And in some way, protects the delicate endothelial surface of the heart from the rather harsh,
vigorous activity of the pumping cardiac muscle.
Well, one of the things you can't really appreciate when you look at histological sections of the heart,
is that it actually does have a very strong fibrous skeleton and that fibrous skeleton,
very dense connective tissue, dense collagen, helps to, well, it doesn't help
but is the insertion point for all the cardiac muscle, so it's a very important component.
On this slide, you could see a tiny little section taken through the myocardium of the atrium
and also, you can see that there's a septum of connective tissue between both atria here
and also, between the ventricles.
Here's a section here showing you a label of a piece of myocardium of the ventricle.
Notice though that in between this myocardium of the atrium
and myocardium of the ventricle, there is this clear component.
That clear component is part of the connective tissue, fibrous skeleton of the heart.
It's a membrane that separates the atria from the ventricle, and that's very important
because that then stops the wave of impulse, and therefore, the wave of contraction
going directly from the atrium muscle to the ventricle muscle.
So, it allows a delay, and, of course, that delay is very important
between the filling of the ventricle and the contraction of the atrium.
So, it's really an electrical isolation sort of tissue there. It's very important.