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
⇐ Use this menu to view and help create subtitles for this video in many different languages. You'll probably want to hide YouTube's captions if using these subtitles.
- 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.
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
Have something that's not a question about this content?
This discussion area is not meant for answering homework questions.
Share a tip
When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
Have something that's not a tip or feedback about this content?
This discussion area is not meant for answering homework questions.
Discuss the site
For general discussions about Khan Academy, visit our Reddit discussion page.
Flag inappropriate posts
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
- disrespectful or offensive
- an advertisement
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
- a tip or feedback in Questions
- a question in Tips & Feedback
- an answer that should be its own question
about the site