Layers of the heart Video Lecture - MBBS

So you're probably
feeling pretty
comfortable with the
diagram of the heart,
but let me just go ahead
and label a few things just
to make sure we're
all on the same page.
So blood flows from the right
atrium to the right ventricle
and then goes to the lungs
and then the left atrium
to the left ventricle.
So that's usually
the flow of blood.
And one of the things that
keeps the blood flowing
in the right direction,
we know, is the valves.
And two of the
valves I'm actually
going to give you new
names, something slightly
different from what we have
been referring to them by.
These are the
atrioventricular valves,
and you can take a guess as to
which ones I'm referring to.
Atrioventricular valves
are the two valves
between the atria
and the ventricles.
So one will be the
tricuspid valve,
and the other is
the mitral valve.
And just to orient us, this
is the tricuspid, the T.
And this is our mitral, or M.
And the atrioventricular
valves, these two valves,
if you look at them, they're
both facing downwards.
And one of the things that
you might be wondering
is, well, how is it
that they aren't just
flopping back and forth?
And these valves, in particular,
have a very interesting
strategy.
And that is that they actually
are tethered to the walls.
So they're held
down here like that,
and they have on the
other end of those
tethers a little muscle there.
Now, this makes perfect
sense if you think about it,
because the ventricles
are very strong.
We know the ventricles
are really, really strong.
And so if the ventricles
are squeezing,
there's a good chance
that that blood
is going to shoot up in
any direction it can go.
It's going to go back perhaps
through the mitral valve
if it can go there, or it'll
go through that tricuspid valve
if it can go there.
But the reason that it won't
is that these papillary muscles
are basically kind
of sending out
little lifelines, these chordae
tendineae lifelines, to keep
the valve from
flipping backwards.
So these chordae
tendineae, these cords,
are important for that reason.
They're keeping the valve
from flipping backwards.
So these are all
chordae tendineae,
and these are all the
papillary muscles.
And these are particularly
important, then, we can tell,
for when you're
trying to make sure
that the ventricles don't
screw up the valves.
And now let's say that by
accident our ventricle is just
too strong, too powerful.
Let's say that it broke
one of these cords.
Let's say it broke
this one right here.
And that's because our ventricle
was just forcing too much blood
back, and it just
snapped the cord.
What would happen?
Well, this would
basically kind of start
flipping back and forth.
It would flip this
way and this way.
And then on the next
heartbeat, blood
would start going
the wrong direction,
because this valve is not able
to keep that nice tight seal.
And so blood would basically
kind of go this way
when it wasn't supposed to.
And all of a sudden,
our flow of blood
is now going in the
wrong direction.
So the chordae tendineae
and the papillary muscles
do a really, really
important job
in preventing that
from happening.
So let's move our
attention to another area.
Let's focus on this
right here, which
is the interventricular septum.
And you can think of
septum as basically a wall,
interventricular septum.
In this interventricular
septum, the one thing
I want to point out, which
is maybe fairly obvious when
you look at it-- you
might think, well,
why did you even have to say it?
That's pretty obvious.
This area is really
thin, and this area
is really thick by comparison.
So the two areas are
not equal in size.
This is much thicker.
And the reason I
wanted to bring that up
is because this
first area in blue
is called the membranous part,
literally like a membrane.
And the bottom, the red
part, is the muscular part.
This is the strong
muscular part.
So you have two different
areas in that interventricular
septum, the wall
between the ventricles.
And one of the
interesting things
about the membranous
part, in particular,
is that a lot of babies
are born with holes
in that membranous part.
So when I say a lot, I don't
mean the majority of babies,
by any means.
But one of the most
common defects,
if there is going
to be a defect,
would be that you
would actually have
a communication
between these two
so that blood could actually,
again, flow from a place
that it's not supposed to
go, the left ventricle,
into a place it shouldn't be
going, the right ventricle.
So blood can actually
flow through those holes,
and that is a problem.
That is called a VSD.
And actually, you might hear
that term at some point.
So I just wanted to point
out where that happens.
And while I'm writing
VSD, you can take a stab
at guessing what
it might stand for.
Ventricular, and S is septal.
Again, septal just means wall.
And D is defect.
So a VSD is most common
in that membranous part,
more so than that muscular part.
Now, let's move on
again to one final thing
I want to point out, which is
I want to zoom in on the walls.
So here in a gray box I'm
going to kind of highlight
what's going on this wall
and how many layers there
are in this wall.
Let me draw out a
little rectangle
to correspond to
that little rectangle
I drew on the heart itself.
So there are three layers
to the heart muscle.
And actually, I'm going to
go through all three layers.
And we'll start from the
inside and work our way out.
So on the inside, you have
what's called the endocardium.
And I'm actually going
to draw the endocardium
all the way around here.
It goes all the way
around the valves,
so now you already learned
that the valves now
have endocardium.
It goes around the ventricle
and, as I showed you
in the beginning, also
around the atrium.
And it goes all the way up
and covers both the right
and left side.
The endocardium is
very, very similar
in many ways to the inner lining
of the blood vessels, actually.
So it's a really thin layer.
It's not a very thick layer.
It's the layer that
all the red blood
cells are kind of
bumping up against.
So when the red blood
cells are entering
the chambers of the heart, the
part that they're going to see
is going to be the endocardium.
So this is what it
looks like, and this
is that green layer all the
way around that I've drawn now.
So if I was to draw it kind
of in a blown-up version,
it might look like this.
Right?
And it's a few
cell layers thick.
And like I said, on the
inside you have some red blood
cells bumping along.
So maybe this is
one red blood cell,
and this is maybe another one.
And they would bump
into that endocardium.
Now, if you go a little bit
deeper to the endocardium, what
do you get to next?
Well, next you
get to myocardium.
And that would be, let's say,
the biggest chunk of our wall.
And that would look
something like this.
And that myocardium you can
kind of appreciate without even
having me point it
out, because it's
the most common part
of this entire thing.
So this is our
myocardium, and let
me go back and actually label
the endocardium as well.
And on the other
side-- and actually,
just notice that the words
are all pretty similar.
Myo means muscle.
And actually, while
I'm on myocardium,
let me just point
out one more thing.
The myocardium is where all
of the contractile muscle
is going to be, so that's
where a lot of the work
is being done.
And that's also where a lot of
the energy is being used up.
So when the heart needs oxygen,
it's usually the myocardium,
because that's the part
that's doing all of the work.
OK.
Now, on the other
side of myocardium,
what do we have on the outside?
Well, we have a layer
called the pericardium,
and let me try to
draw that in for you.
Pericardium is something like
this, kind of a thin layer.
And the interesting
thing about pericardium
is that there's actually
two layers to it.
So there's actually
something like this
where you have two layers, an
inner layer and an outer layer.
And between the two layers
you have literally a gap.
There's a gap right there.
And in that gap, you might
have a little bit of fluid.
But it's not actually cells.
I guess that's
the biggest point.
It's not actually cells.
It's more just a little bit
of fluid that hangs out there.
So this whole thing is
called the pericardium.
Now, you may be wondering
how in the world
do you get a layer that
has a gap within it.
So let me actually try to show
you what happens in a fetus.
So let's say you
have a little fetus
heart, a tiny little
heart like this,
and it gets a little
bit bigger like this.
And then it finally gets
into an adult heart,
something like that.
So this would be the
adult heart, right?
Well, at the same time
that the heart is growing,
you actually also have a sac,
almost like a little balloon.
And this balloon actually
begins to envelope the heart,
so this growing heart kind of
grows right into the balloon.
And so this balloon
kind of starts
going around it like that, and
you get something like this.
And then eventually, as
the heart gets really big,
you get something like this.
You basically have this
kind of inner layer
of the balloon
that's pancaked out
that doesn't even look
like a balloon anymore.
It's very flat, and then
it kind of folds back
on itself like that.
And it comes all the way around.
And now you can see why even
though it's continuous--
it's not like it breaks.
It is continuous here-- you can
see how if you actually just
were to look at one chunk
of it, like we're looking
at right here, you can
see how it would actually
look like a pancake.
And so on our heart
actually, it literally
would be something like
this, like a very thin kind
of pancake.
And I'm not doing a very, very
good job making it look thin,
but you can imagine
what it is that it could
look like if I was
to zoom in on it.
Basically, something
like that, where
you have two layers that
are basically just kind of
turned in on themselves.
And both layers put together
are called your pericardium.
Now, there are actually separate
names for the two layers.
So for example, the layer
that's kind of hugging up
against the heart, this layer
that I'm drawing right now,
this layer is called the
visceral pericardium.
So you call that the
visceral pericardium.
And the name visceral, this
right here, would be visceral.
And the reason it's
called visceral
is because viscera
refers to organs,
so that's called the
visceral pericardium.
And then this outer layer,
the one I'm drawing now,
is called the
parietal pericardium.
And that's the layer that
actually is on the outside,
so let me label that as well.
So that's this guy.
That would be the
parietal pericardium.
So now you can
actually see the layers
of the heart-- the endocardium,
myocardium, and pericardium.
And actually, just to throw
you a curve ball, because I'm
pretty sure you can handle
it, this visceral pericardium,
another name for it, just
because you might see it
sometime, is the epicardium.
Sometimes you might see
the name epicardium.
And don't get thrown off.
It's really just the
visceral pericardium.
It's just the outermost
layer of that heart
before you get to
the parietal layer.