Lub Dub | Circulatory system physiology | NCLEX-RN | Khan Academy


If you take a good long
listen to your heart, you’ll actually notice
that it makes sounds. And those sounds are
usually described as lub dub, lub dub, lub dub. And if you actually
try to figure out what that would spell
out like, usually it’s something L- U- B, D- U- B. And
it just repeats over and over and over. And to sort of figure out
where those sounds come from, what I did is I took
that diagram of the heart that we’ve been using
and actually exaggerated the valves, made
them really, really clear to see in this picture. And we’ll use those
valves to kind of talk through where those
sounds are coming from. So let’s start by
labeling our heart. So we’ve got at the top, blood
is coming into the right atrium and going to the
right ventricle. It goes off to the
lungs, comes back into the left atrium and
then the left ventricle. So these are the
chambers of our heart. Now, keep your
eye on the valves. And we’ll actually
talk about them as the blood moves through. So let’s start with blood going
from the right atrium this way into the right ventricle. Now, at the same moment
that blood is actually going from the right atrium
to the right ventricle, blood is actually also
going from the left atrium to the left ventricle. Now, you might think,
well, how’s that possible? How can blood be in
two places at one time? But now remember that
blood is constantly moving through the heart. So in a previous
cycle, you actually had some blood that was
coming back from the lungs, and that’s what’s dumping
into the left ventricle. And in a new cycle,
you have a bit of blood that’s going from the right
atrium to the right ventricle. So you have simultaneously
two chambers that are full of blood– the
right and left ventricle. Now, to get the blood
into those ventricles, the valves had to open. And specifically, let’s
label all the valves now. So here you have
our tricuspid valve, and I’m going to label
that as just a T. And then up here, you
have the pulmonary valve, and this’ll be just a P.
And on the other side, you’ve got the mitral valve,
which separates the left atrium from the left ventricle. And you’ve got the aortic valve. So these are the four
valves of the heart. And as the blood is
now in the ventricles, you can see that the tricuspid
and the mitral valve are open. So far, so good. Now, I’ve actually drawn the
pulmonary valve as being open. But is that really the case? And the answer is no,
because what happens is that as blood is moving
down from the right atrium to the right ventricle,
let’s say that– and I’m going to
draw it in black. Black arrows represent the bad
or the wrong direction of flow. So let’s say some
blood is actually trying to go that way, which is
not the way it should be going. What happens is that
these two valves, they, based on their
shape, are actually not– they’re going to jam up. They’re going to basically
just jam up like this, and they’re not going to
let the blood pass through. So this is what happens
as that valve closes down. And the same thing
happens on this side. Let’s imagine you have some
backwards flow of blood by accident, meaning that it’s
going in the wrong direction. Well, then these valves
are going to close down. So the white arrows represent
the correct flow of blood, and the black arrows represent
the incorrect flow of blood. So these valves
shut down like that. So now you can see
how the valves, the aortic and pulmonary
valve, are actually closed when the mitral and
tricuspid valve are open. So what happens after this? So now our ventricles
are full of blood, right? They’re full of blood. And let’s say they squeeze down,
and they jettison all the blood into those arteries. Well, now you’re going to
have– this is actually going to close down. Let’s say this
arrow flips around. These arrows become white,
because the direction of flow is going to be in the
direction we want it. It’s going to go
this way and this way And to allow that,
of course, I need to show you that these open up. And they allow the blood to go
the way that we want it to go, so now blood is going to flow
through those two valves. But similar to before, you
could have some backflow here. You could have backflow here. And you can have backflow here. So you can imagine
now, let’s say you have a little
bit of backflow that wants to go this way, which
is the wrong direction. Right? Well, then these valves
are going to close up. They’re going to say, no,
you can’t go that way. They’re going to close
right up, and they’re going to not allow
blood to go that way. So this is going to happen on
both sides, both ventricles. And the valves shut. And so basically the
backflow of blood is not allowed, because
the valves keep shutting. And when the valves snap shut–
so for example, right now the tricuspid valve and the
mitral valve snapped shut. Well, that makes a noise. So when T and M snap shut, that
makes a noise that we call lub. That’s that first noise,
that first heart sound. In fact, sometimes people
don’t even call it lub dub. They say, well, it’s
the first heart sound. And to make that even shorter,
sometimes people call that S1. So if you hear S1,
you know they’re talking about that
same exact thing. And this dub is called
the second heart sound. And, no surprise, just
as before, if that’s S1, this is S2. So you’ll hear S1 when the
tricuspid and mitral valve snap shut. So far, so good. But you also know that if
that’s what’s making noise, you can kind of guess– and
it’s a very smart guess– that at the same time, the pulmonic
valve and the aortic valve just opened. So if the other valves snap
shut, these just opened. Right? You can kind of assume
that, although the noise you’re hearing is
actually from here. So what’s happening with dub? Well, the opposite. And what I mean by
that is– let me now show you what happens
a moment later. Well, after the ventricles
are done squeezing, then we get to a
point where you might have a little bit of flow
that way and that way, just as I drew before. And these valves
snap shut as well. So now these snap shut. And as these snap
shut– because they don’t want to allow
backflow, right? They’re going to
snap shut like that. They make noise. And so when you have
dub, you actually have noise coming from the
pulmonic and aortic valve snapping shut. And that must mean that then
the other two valves just opened up– the tricuspid and
mitral just opened. You can assume that, right? And I didn’t draw
that in the picture. Let me update my picture
now to show that. So now these two have
opened up, and blood is coming into the
ventricles again. So it’s actually a nice little
rhythm that you get going. And every time these valves go
open and shut, you hear noise. So you can kind of figure out
what’s happening based on– and these actually–
let me erase that. And now you have white
arrows going this way. And we’ve returned to
where we started from. So you basically
have a full cycle, and between these two– so
let’s say from lub to dub, because there’s a little
bit of space there. If you were to follow
it over time, over time, this is what it might look like
if this is a little timeline. You might hear lub here,
or the first heart sound. I’ll just call it S1. And you might hear S2 here,
the second heart sound. And then you’ll hear S1
again over here and S2. And what’s happening between
the two– so between these two, this time lag here– is
that blood is actually squeezing out, because the
pulmonary and aortic valves just opened. It is squeezing out and
going out to the whole body. So this is when blood
is going to the body, and sometimes we
call that systole. And between dub and
the next lub– so in this area right here–
well, at that point, blood is kind of
refilling from the atriums into the ventricles, and
we call that diastole. So now you can actually
listen to your heart. And you can actually
figure out, well, if you’re listening to the
sound between lub and dub or the space in time
between lub and dub, that’s when you’re
having systole. And if you’re listening to
or waiting for the sound to start up again–
so you just heard dub, and you’re waiting for lub
again– then that space in time is diastole.

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