Articles, Blog

Layers of the heart | Human anatomy and physiology | Health & Medicine | Khan Academy

February 14, 2020

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.


  • Reply Amiya R October 24, 2012 at 7:15 pm

    Hi thanks for your videos they re so useful 🙂 ..I have a question is the mitral valve,the sae as the bicuspid valve if not what does that do? thanks

  • Reply khanacademymedicine October 25, 2012 at 2:43 pm

    Yes, they are the same thing. Mitral valve = Bicuspid valve.

  • Reply phdan123 January 22, 2013 at 12:25 am

    Nope, the mitral valves are between the atria and ventricles. The semilunar valves (there are two) separate the right ventricle and pulmonary artery, and the left ventricle and aorta. Hope this helps

  • Reply phdan123 January 25, 2013 at 3:20 pm

    My bad. Still, they're not called the semilunar valves. We just refer to them at the left and right AV, so sometimes I fuck up the naming

  • Reply Sainath Krishnamurthy January 29, 2013 at 12:42 pm

    Thank You, Rishi.

  • Reply Ryan Holding February 13, 2013 at 7:41 pm

    So is it possible for the Cordae Tendinae to break? And if so, if the blood flow goes the wrong directions, then what would happen?

  • Reply khanacademymedicine February 14, 2013 at 4:55 am

    Great question. This can result in a valve that doesn't work properly and flops back and forth instead of providing a good seal. Blood can go the wrong way in this situation and lead to less efficient pumping of blood to the body.

  • Reply Stq89 February 20, 2013 at 3:20 pm

    hi could you please make a lecture on arrhythmias and the different types

  • Reply Riann Zabaleta February 23, 2013 at 1:04 am

    Standing ovation!

  • Reply Magiccat179 March 22, 2013 at 9:59 pm

    You are a LIFE SAVER! I have the worst A&P II professor in the entire world. You explain so clearly. Where have you been all my life?

  • Reply Urine tiger April 9, 2013 at 7:26 pm

    and in that gap you MIGHT HAVE A LITTLE BIT OF FLUID!!!!!!..but it, uh

  • Reply princesszm April 26, 2013 at 12:29 pm


    Atrio-Ventricular valves = Tricuspid and Bicuspid (a.k.a Mitral) Valves

    Semi-Lunar valves = Pulmonary and Aortic Valves

  • Reply Karam Jeet May 19, 2013 at 6:47 pm

    Rishi, you are a gifted teacher. Are you considering more advanced medical presentations for those of us with the background already?

  • Reply jackline padilla June 18, 2013 at 11:29 pm

    wow I am a nursing student and I am enjoying all these videos!!! Amazing learning tool!

  • Reply Lisa Harrison June 23, 2013 at 11:42 pm

    Nursing Student…question… Cardiac Tamponade…does the blood fill in the gap of the Perimetrium? Thx Khan…I used your videos for Chemistry 101 also.

  • Reply Riffat Ahmed July 17, 2013 at 2:04 pm

    I AM FROM Bangladesh,you better come and visit us and check it out how you are helping us.thank you very much.

  • Reply Zaakaryeh August 18, 2013 at 12:15 pm

    Is there a name given to this, and what's the typical prognosis?

  • Reply DB August 18, 2013 at 11:29 pm

    "Don't screw up the valves"

  • Reply Phillipe Nover September 1, 2013 at 1:45 am

    I'm watching all of your video for my RCIS exam in two weeks. You have the best video on YouTube!

  • Reply Famerly Love September 23, 2013 at 6:40 pm

    Thank you so much! I am a paramedic student and was wondering if you have a video on simplifying EKG's. You are a great teacher and really thank you for taking your time to post these!

  • Reply flarefiragax October 14, 2013 at 12:30 pm

    This video is so much clearer than the lecture given by the professors at my university. They try to sound overly formal, which only gets in the way of what they're trying to communicate. Thank you so much!

  • Reply HaxorHzn October 27, 2013 at 2:46 pm

    Mitral Regurgitation

  • Reply PFBCheaterReporter November 6, 2013 at 6:32 pm

    Hey guys. Great vid.

    My dad was formerly a fatty. He revitalized his body from 293lbs of fat into 212 lbs of massive muscle. I couldn't believe it! I just subscribed myself because I wanna strengthen. He made use of the Muscle Building Bible (Google it)…

  • Reply fizah khan January 5, 2014 at 11:42 am

    (y) thank u ^-^

  • Reply Orla Cullen January 28, 2014 at 8:24 pm

    really good at explaining !thanks !

  • Reply Yvette Amanda Castro January 28, 2014 at 8:35 pm

    Thank you for doing these videos. They help me in better understanding the heart.

  • Reply ahmed elmarhomy March 4, 2014 at 12:25 am

    Nice and Helpful …… Thank you 

  • Reply Abby nakaddu March 17, 2014 at 5:41 am

    Thank you so much for this video. You explained the layers of the heart and the intraventicular septum better than my teacher. Thanks

  • Reply dancer111798 March 26, 2014 at 11:59 pm

    Isnt it epicardium? Not endocardium?

  • Reply You reeh naw March 28, 2014 at 7:07 am

    I just hit a gold mine finding your channel. You will help me conquer my human anatomy course. Thank you!

  • Reply Linda Soto April 13, 2014 at 9:26 pm

    i can not say it enough! your channel has changed my life! you helped conquer physiology  and now my anatomy class! 

  • Reply Herleen Pabla April 19, 2014 at 3:11 pm

    What about fibrous pericardium? Is it another layer outside the parietal pericardium?

  • Reply MrPuukjeens June 23, 2014 at 11:30 am

    Is not in ventriculus dextra three papillares muscles?

  • Reply jennifer bernard July 10, 2014 at 11:58 pm

    Excellent video!! I think all Khan Academy videos are great!! They are getting me through A&P!! Thank You

  • Reply LowLa Lve September 23, 2014 at 11:02 pm

    why aren't u my teacher??

  • Reply andrew rawlings October 2, 2014 at 9:16 am

    You are an excellent teacher.

  • Reply Natasha Jarrett October 9, 2014 at 3:49 am

    Good way of teaching..I understand better now…

  • Reply Nikolas Martinez October 15, 2014 at 10:23 pm

    How thick is each layer? Is it on a cellular level because I remember hearing you say the red blood cells bump the endocardium.

  • Reply Cclunda October 17, 2014 at 4:24 am

    Khan Academy RULES!

  • Reply Musheera Mohammed October 22, 2014 at 4:54 am

    thank you i really really thank you from the deep of my heart …. you do know how much this vedio help me out…… we really appreciate what you are doing….

  • Reply Alvina November 3, 2014 at 2:50 pm

    very helpfull! thank you

  • Reply Yasmine Yasmine December 6, 2014 at 10:56 am

    U r the best thing that happened to my life thanks

  • Reply Ronnie C. January 28, 2015 at 4:02 pm

    This all makes sense to me now. Thank you!

  • Reply Rich K February 27, 2015 at 9:16 pm

    perfect explaination

  • Reply Ricardo Dawkins March 18, 2015 at 3:29 pm

    thanks for this it helped 

  • Reply Ruth Friesen March 28, 2015 at 9:11 pm

    Can we all take a moment to be wow'd at his writing skills with his mouse?! I don't know about you people, but my mouse writing is not nearly that legible 😛

  • Reply HowieTheHightower April 5, 2015 at 4:17 pm

    The paricardial sac that I have heard of before is just in between the two layers of the pericardium?

  • Reply Lyndsay Vines May 9, 2015 at 11:53 am

    Aw man this makes so much more sense now!  Thank you!

  • Reply Bogdan June 21, 2015 at 10:26 am

    The structure that he labeled as pericardium is actually only a part of it, called the serous pericardium. There is one more layer of dense connective tissue on the outside, called the fibrous pericardium that has been omitted. Please upvote and/or comment to this so that others can see it.

  • Reply Gustav Barrera June 22, 2015 at 3:14 am

    This is a great explanation! Greetings from Mexico, man!

  • Reply Maya Katanchi August 29, 2015 at 1:09 am

    Great explanation,, Thank You

  • Reply Riyaz Manjiyani September 7, 2015 at 11:11 pm


    I noticed at time (7:16–7:20), right after you finished talking about the endocardium layer, you mentioned that you go deeper into the myocardium.

    I think that was a mistake.

    From superficial to deep, the tissues should go as follows: Pericardium, Myocardium, and Endocardium.

    Therefore, myocardium is NOT deeper than endocardium; it is more superficial.

  • Reply Yonna Dalangin September 13, 2015 at 11:46 pm

    Oh you forgot the fibrous pericardium, the outermost layer.

  • Reply Michael Nissim September 14, 2015 at 7:17 pm

    Isn't there a layer called the the Epi – Cardium?

  • Reply Amanda Dela October 16, 2015 at 8:42 am

    Thank u . But i was hoping a little bit advanced explanation

  • Reply Aloka K October 28, 2015 at 7:45 pm

    Thank u..! But there is two type of pericardium they are 1_fibrous pericardium (which contain strong fibrous ) and the second one is a 2_serous pericardium (the inner portion of pericardium which composed two layers A_visceral layer (which contact with myocardium ) B_parietal layer (which contact with fibrous pericardium ).

  • Reply Hélène Staley November 7, 2015 at 12:06 pm

    I like this video. Great presentation.

  • Reply Michael Li December 4, 2015 at 2:46 am

    You're amazing at explaining

  • Reply Blewusi George Selali December 5, 2015 at 7:27 am

    this week all am doing is watching all your 10 circulatory system videos, u re that good

  • Reply 0 jarin January 16, 2016 at 12:18 am

    woooah sal i pretty watch this video forever in this life

  • Reply Emily Miller February 4, 2016 at 1:18 am

    well done, thank you!! 🙂

  • Reply Noa Krale February 5, 2016 at 7:36 pm

    life saver

  • Reply jamaicancollegegirl March 21, 2016 at 11:58 pm


  • Reply AllergicRants April 6, 2016 at 6:26 pm

    I am a Spaniard going to school in Germany to become a nurse and you have no idea how much your videos are helping me. I get so lost in translation while in class sometimes…

  • Reply Trost June 25, 2016 at 10:17 am

    An ear rape at 8:52 plz fix.

  • Reply Joseph DiMarco July 15, 2016 at 3:57 am

    Rishi is easily the best Khan Academy instructor

  • Reply Farrin Hinds October 27, 2016 at 4:09 pm

    Thank you best explanation EVER !!!!!!!

  • Reply Miki Rayner June 4, 2017 at 9:30 pm

    Pulmonary Vein?

  • Reply ejazkhanak47 June 16, 2017 at 11:00 am

    Thank you!

  • Reply Millie R September 22, 2017 at 10:09 pm

    Thank you thank you thank you.!!!!!!!… great video!

  • Reply Radwan رضوان Abu-Issa ابوعيسى November 2, 2017 at 7:01 am

    there is no such balloon, the epicardium "Visceral pericardium" is a different structure from the pericardium "parietal pericardium", the epicardium forms from the splanchnic mesoderm of the sinus venosus (not from Spetum Trasversum) where the pericardium is formed, instead, from the pleural (peritoneal) membrane!

  • Reply fact things November 2, 2017 at 10:42 pm

    Kindly request to you do you make your videos on animation.

  • Reply صقر وايل January 26, 2018 at 2:27 pm

    epicardium then pericardium which consist of two layer

  • Reply Sam Beard February 16, 2018 at 5:59 am

    I love you

  • Reply Kyle Barrios March 6, 2018 at 1:54 pm

    i need to learn this so thank you

  • Reply Jia Er June 1, 2018 at 4:00 am

    Hi so may I ask what is the difference between endocardium and visceral pericardium?

  • Reply Webble Pebble 121 June 14, 2018 at 9:12 pm

    Correct me if I am wrong, but isn't their a serous membrane known as the epicardium that surrounds the innermost layer of the pericardium, I am eleven years old and read this in a book and I am thinking this is correct

  • Reply عبده ماهر June 16, 2018 at 5:35 am

    Want arabic traslation please???????????????????????!!!!!!!!???????????!!!???

  • Reply Spazip September 29, 2018 at 1:46 pm

    Dude I love your videos. I have huge concentration issues so getting through biology and fysiology in school is a trouble for me. I don’t understand or remember any of the material they give me, but SOMEHOW i understand and remember it when I watch your videos. You are a great teacher! Thank you!

  • Reply Soraya Askari November 24, 2018 at 4:39 am

  • Reply Thuraya Hassanin March 24, 2019 at 8:37 am

    Thank u 🌸👍

  • Reply wa'ad al awamleh April 14, 2019 at 6:07 am

    The time i broke my headphones: 8:45 And in that gap you might have A LITTLE BIT OF FLUID

  • Reply Ricardo Costa September 21, 2019 at 10:01 am

    And in that gap, you might have A LITTLE BIT OF FLUID but its not actually cells.

  • Reply Andre Saliba October 6, 2019 at 3:20 pm

    "and in that gap, you might have A LITTLE BIT OF FLUID." rip headphones users

  • Reply zahraa 313 October 14, 2019 at 4:41 pm

    thank you

  • Reply ANKIT mathematician February 4, 2020 at 1:28 pm

    What is the function of pericadium

  • Leave a Reply