Layers of the heart Take a closer look at the heart, explore some of its interesting features and get to know the three layers that make up the heart. Rishi is a pediatric infectious disease physician and works at Khan Academy.
Layers of the heart
- 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.
- Blood flows from the right atrium to the right ventricle,
- then it goes to the lungs and from the left atrium to the left ventricle.
- That's usually the flow of blood.
- One of the things that keeps the blood flowing in the right direction is the valves.
- Two of the valves I'm gonna name, I'm giving them new names,
- something slightly different from what we've been referring to them by.
- These are the atrioventricular valves.
- 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.
- One will be the tricuspid valve and the other the mitral valve.
- Just to orientate us: this is the tricuspid (T) and this is our mitral (M).
- The atrioventricular valves, if you look at them, they're both kind of facing downwards.
- One of the things you might be wondering is 'How is it that they aren't just flopping back and forth?'
- These valves in particular have a very interesting strategy,
- they're actually tethered to the walls.
- They're held down here, like that.
- They have on the other end of those tethers a little muscle.
- This makes perfect sense if you think about it.
- The ventricles are very strong, right? We know the ventricles are really really strong.
- If the ventricles are squeezing, there's a good chance the blood is gonna shoot up in any direction it can go.
- It's gonna go back perhaps through the mitral valve, it can go there, or it might go through the tricuspid valve.
- The reason that it won't, is that these papillary muscles are basically sending out little life lines, these chordae tendinae,
- to keep the valve from flipping backwards.
- So these chordae tendinae, these cords are important for that reason.
- They keep the valve from flipping backwards.
- These are all the chordae tendinae and these are the papillary muscles.
- These are particularly important for when you're trying to make sure that ventricles don't screw up the valves.
- Let's say that by accident our ventricle was just too strong, too powerful, let's say it broke one of these cords.
- Let's say it broke this one right here.
- That's because our ventricle was just forcing too much blood back and it just snapped the cord.
- What would happen?
- This would basically start flipping back and forth, it would flip this way and this way.
- On the next heartbeat, blood would start going the wrong direction,
- because this valve is not able to maintain a nice tight seal.
- Blood would basically go this way when it wasn't supposed to.
- All of a sudden our flow of blood is going in the wrong direction.
- The chordae tendinae and the papillary muscles do a really important job in preventing that from happening.
- Let's move our attention to another area.
- Let's focus on this right here, which is the interventricular septum.
- You can think of a septum as a wall.
- Interventricular septum.
- This interventricular septum, the one thing I want to point out, which is maybe fairly obvious,
- when you look at it, you might think, you didn't have to say it, it's pretty obvious,
- this area is really thin and this area is really thick by comparison, right?
- The two areas are not equal in size, this is much thicker.
- The reason I bring that up is because the first area, in blue, is called the membranous part.
- It's literally like a membrane.
- The bottom, the red part, is the muscular part. This is the strong muscular part.
- You have two different areas in that interventricular septum, the wall between the ventricles.
- 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.
- When I say a lot I don't mean the majority of babies by any means.
- But one of the most common defects would be that you would have a communication between these two.
- So blood would flow from the left ventricle, into a place it shouldn't be going, the right ventricle.
- Blood could flow through those holes and that is a problem.
- That is called a VSD. You might hear that term at some point, so I just wanna point out where that happens.
- While I'm writing this, you can take a guess at what this might stand for.
- Ventricular Septal (septal just means wall) Defect.
- A VSD is most common in the membranous part, more so than the muscular part.
- Let's move on again to one final thing I wanna point out, I wanna zoom in on the wall.
- Here in this grey box I'm gonna highlight what's going on in this wall, how many layers there are in this wall.
- I'm gonna draw a rectangle to correspond to the little rectangle I drew on the heart itself.
- There are 3 layers to the heart muscle.
- I'm gonna go through all three layers, we'll start on the inside and work our way out.
- On the inside you have what's called the endocardium.
- I'm gonna draw the endocardium all the way around here, it goes all the way around the valves.
- So you've already learned that the valves got endocardium.
- It goes around the ventricle and, as I showed you in the beginning, also around the atrium.
- It goes all the way up and covers both the left and right side.
- The endocardium is very similar in many ways to the inner lining of the blood vessels.
- It's a really thin layer, not a very thick layer.
- It's the layer all the red blood cells are bumping up against.
- When the red blood cells are entering the chambers of the heart, the part they're gonna see is the endocardium.
- This is what it looks like, this is that green layer all the way around that I've drawn now.
- If I was to draw a blown up version, it might look like this.
- It's a few cell layers thick.
- On the inside you have some red blood cells bumping along.
- This is one RBC,this is another one and they would bump into that endocardium.
- If you go a little bit deeper than the endocardium, what do you get to next?
- Next is the myocardium.
- That would be the biggest chunk of our wall.
- It would look something like this.
- The myocardium you can kind of see the shape, just without me pointing it out,
- because it's the most common part of this entire thing, right?
- This is our myocardium.
- Let me go back and actually label the endocardium as well.
- Just notice that these words are all pretty similar. Myo- means muscle.
- While I'm on the myocardium, let me just point out one more thing.
- The myocardium is where all of the contractile muscle is gonna be.
- That's where a lot of the work is being done.
- It's also where a lot of energy is being used up.
- When the heart needs oxygen, it's usually the myocardium, because that's the part that's doing all of the work.
- Now on the other side of the myocardium, what do we have on the outside?
- We have a layer called the pericardium.
- Let me try to draw that for you.
- The pericardium is something like this, kind of a thin layer.
- The interesting thing about pericardium is that there are actually two layers to it.
- It's actually something like this.
- Where you have two layers, an inner and an outer layer and between the two layers you literally have a gap.
- There's a gap right there.
- 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 cells, it's just a little bit of fluid.
- So this whole thing is called the pericardium.
- You might be wondering 'How do you get a layer that has a gap within it?'
- Let me try to show you what happens in a fetus.
- Let's say you have a little fetus heart, a tiny little heart like this.
- It gets a little bit bigger, like this.
- And finally it gets into an adult heart, something like that.
- This would be the adult heart, right?
- Well, at the same time the heart is growing, you also have a sac, almost like a balloon.
- This balloon begins to envelope the heart.
- This growing heart grows right into the balloon.
- This balloon starts going around it, like that.
- You'll get something like this.
- Eventually, as the heart gets really big, you get something like this.
- You basically have this inner layer of the balloon that's not even looking like a balloon anymore, it's very flat.
- Then it folds back on itself, like that, and it comes all the way around.
- Now you can see why even though it's continuous, I mean, it's not like it breaks, it is continuous here,
- you can see how, if you were just looking at at chunk of it, like we're looking at right here,
- you can see how it would look like a pancake.
- On our heart it literally would be something like this, a very thin kind of pancake.
- I'm not doing a very good job making it look thin, but you can imagine what it is,
- what it could look like if I was to zoom in on it, basically something like that.
- You have two layers that are just turned in on themselves. Both layers together are called the pericardium.
- There are actually separate names for the two layers.
- For example, the layer that's kind of hugging up against the heart, this layer that I'm drawing now,
- this layer is called the visceral pericardium.
- So you call that the visceral pericardium.
- The name visceral, this right here would be visceral, the reason it's called visceral is because viscera means organs.
- So that's called the visceral pericardium.
- This outer layer, the one I'm drawing now, is called the parietal pericardium.
- That's the layer on the outside.
- Let me label that as well as this guy, that would be the parietal pericardium.
- Now you can actually see the layers of the heart, the endocardium, myocardium and pericardium.
- And to throw you a curveball, as I'm pretty sure you can handle it,
- this visceral pericardium, another name for it, because you might see it sometime, is the epicardium.
- Sometimes you might see it, the epicardium.
- Don't get thrown of, it's really just the visceral pericardium.
- It's just the outermost layer of the heart before you get to the parietal pericardium.
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