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So you're probably feeling pretty comfortable with the diagram of the heart,

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but let me just go ahead and label a few things, 
just to make sure we're all on the same page.

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Blood flows from the right atrium to the right ventricle,

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then it goes to the lungs and from the left atrium to the left ventricle.

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That's usually the flow of blood.

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One of the things that keeps the blood flowing in the right direction is the valves.

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Two of the valves I'm gonna name, 
I'm giving them new names,

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something slightly different from what we've been referring to them by.

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These are the atrioventricular valves.

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You can take a guess as to which ones I'm referring to.

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Atrioventricular valves are the two valves between the atria and the ventricles.

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One will be the tricuspid valve and the other the mitral valve.

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Just to orientate us: 
this is the tricuspid (T) and this is our mitral (M).

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The atrioventricular valves, if you look at them, 
they're both kind of facing downwards.

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One of the things you might be wondering is
'How is it that they aren't just flopping back and forth?'

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These valves in particular have a very interesting strategy,

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they're actually tethered to the walls.

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They're held down here, like that.

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They have on the other end of those tethers a little muscle.

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This makes perfect sense if you think about it.

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The ventricles are very strong, right?
We know the ventricles are really really strong.

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If the ventricles are squeezing, there's a good chance the blood is gonna shoot up in any direction it can go.

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It's gonna go back perhaps through the mitral valve, it can go there, or it might go through the tricuspid valve.

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The reason that it won't, is that these papillary muscles are basically sending out little life lines, these chordae tendinae,

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to keep the valve from flipping backwards.

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So these chordae tendinae, 
these cords are important for that reason.

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They keep the valve from flipping backwards.

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These are all the chordae tendinae 
and these are the papillary muscles.

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These are particularly important for when you're trying to make sure that ventricles don't screw up the valves.

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Let's say that by accident our ventricle was just too strong, too powerful, let's say it broke one of these cords.

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Let's say it broke this one right here.

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That's because our ventricle was just forcing too much blood back and it just snapped the cord.

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What would happen?

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This would basically start flipping back and forth,
it would flip this way and this way.

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On the next heartbeat, blood would start going the wrong direction,

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because this valve is not able to maintain a nice tight seal.

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Blood would basically go this way when it wasn't supposed to.

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All of a sudden our flow of blood is going in the wrong direction.

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The chordae tendinae and the papillary muscles do a really important job in preventing that from happening.

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Let's move our attention to another area.

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Let's focus on this right here, 
which is the interventricular septum.

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You can think of a septum as a wall.

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Interventricular septum.

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This interventricular septum, the one thing I want to point out, which is maybe fairly obvious,

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when you look at it, you might think, you didn't have to say it, it's pretty obvious,

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this area is really thin and this area is really thick by comparison, right?

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The two areas are not equal in size, this is much thicker.

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The reason I bring that up is because the first area, in blue, is called the membranous part.

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It's literally like a membrane.

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The bottom, the red part, is the muscular part.
This is the strong muscular part.

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You have two different areas in that interventricular septum,
the wall between the ventricles.

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One of the interesting things about the membranous part in particular,

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is that a lot of babies are born with holes in that membranous part.

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When I say a lot I don't mean the majority of babies by any means.

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But one of the most common defects would be that you would have a communication between these two.

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So blood would flow from the left ventricle, into a place it shouldn't be going, the right ventricle.

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Blood could flow through those holes and that is a problem.

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That is called a VSD. You might hear that term at some point, so I just wanna point out where that happens.

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While I'm writing this, you can take a guess at what this might stand for.

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Ventricular Septal (septal just means wall) Defect.

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A VSD is most common in the membranous part, 
more so than the muscular part.

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Let's move on again to one final thing I wanna point out, 
I wanna zoom in on the wall.

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Here in this grey box I'm gonna highlight what's going on in this wall, how many layers there are in this wall.

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I'm gonna draw a rectangle to correspond to the little rectangle I drew on the heart itself.

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There are 3 layers to the heart muscle.

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I'm gonna go through all three layers,
we'll start on the inside and work our way out.

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On the inside you have what's called the endocardium.

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I'm gonna draw the endocardium all the way around here,
it goes all the way around the valves.

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So you've already learned that the valves got endocardium.

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It goes around the ventricle and, as I showed you in the beginning, also around the atrium.

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It goes all the way up and covers both the left and right side.

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The endocardium is very similar in many ways to the inner lining of the blood vessels.

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It's a really thin layer, not a very thick layer.

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It's the layer all the red blood cells are bumping up against.

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When the red blood cells are entering the chambers of the heart, the part they're gonna see is the endocardium.

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This is what it looks like, this is that green layer all the way around that I've drawn now.

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If I was to draw a blown up version, it might look like this.

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It's a few cell layers thick.

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On the inside you have some red blood cells bumping along.

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This is one RBC,this is another one and they would bump into that endocardium.

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If you go a little bit deeper than the endocardium, 
what do you get to next?

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Next is the myocardium.

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That would be the biggest chunk of our wall.

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It would look something like this.

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The myocardium you can kind of see the shape, 
just without me pointing it out,

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because it's the most common part of this entire thing, right?

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This is our myocardium.

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Let me go back and actually label the endocardium as well.

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Just notice that these words are all pretty similar. 
Myo- means muscle.

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While I'm on the myocardium, 
let me just point out one more thing.

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The myocardium is where all of the contractile muscle is gonna be.

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That's where a lot of the work is being done.

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It's also where a lot of energy is being used up.

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When the heart needs oxygen, it's usually the myocardium,
because that's the part that's doing all of the work.

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Now on the other side of the myocardium,
what do we have on the outside?

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We have a layer called the pericardium.

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Let me try to draw that for you.

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The pericardium is something like this,
kind of a thin layer.

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The interesting thing about pericardium is 
that there are actually two layers to it.

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It's actually something like this.

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Where you have two layers, an inner and an outer layer and between the two layers you literally have a gap.

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There's a gap right there.

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In that gap you might have a little bit of fluid, 
but it's not actually cells.

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I guess that's the biggest point,
it's not cells, it's just a little bit of fluid.

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So this whole thing is called the pericardium.

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You might be wondering
'How do you get a layer that has a gap within it?'

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Let me try to show you what happens in a fetus.

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Let's say you have a little fetus heart, a tiny little heart like this.

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It gets a little bit bigger, like this.

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And finally it gets into an adult heart, something like that.

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This would be the adult heart, right?

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Well, at the same time the heart is growing, 
you also have a sac, almost like a balloon.

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This balloon begins to envelope the heart.

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This growing heart grows right into the balloon.

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This balloon starts going around it, like that.

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You'll get something like this.

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Eventually, as the heart gets really big,
you get something like this.

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You basically have this inner layer of the balloon that's not even looking like a balloon anymore, it's very flat.

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Then it folds back on itself, like that, and it comes all the way around.

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Now you can see why even though it's continuous, 
I mean, it's not like it breaks, it is continuous here,

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you can see how, if you were just looking at at chunk of it, like we're looking at right here,

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you can see how it would look like a pancake.

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On our heart it literally would be something like this,
a very thin kind of pancake.

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I'm not doing a very good job making it look thin,
but you can imagine what it is,

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what it could look like if I was to zoom in on it,
basically something like that.

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You have two layers that are just turned in on themselves.
Both layers together are called the pericardium.

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There are actually separate names for the two layers.

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For example, the layer that's kind of hugging up against the heart, this layer that I'm drawing now,

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this layer is called the visceral pericardium.

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So you call that the visceral pericardium.

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The name visceral, this right here would be visceral, the reason it's called visceral is because viscera means organs.

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So that's called the visceral pericardium.

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This outer layer, the one I'm drawing now,
is called the parietal pericardium.

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That's the layer on the outside.

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Let me label that as well as this guy,
that would be the parietal pericardium.

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Now you can actually see the layers of the heart, 
the endocardium, myocardium and pericardium.

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And to throw you a curveball, 
as I'm pretty sure you can handle it,

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this visceral pericardium, another name for it, 
because you might see it sometime, is the epicardium.

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Sometimes you might see it, the epicardium.

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Don't get thrown of, it's really just the visceral pericardium.

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It's just the outermost layer of the heart before you get to the parietal pericardium.