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Health and medicine
Course: Health and medicine > Unit 2
Lesson 9: Pressure volume loops- Pressure in the left heart - part 1
- Pressure in the left heart - part 2
- Pressure in the left heart - part 3
- Left ventricular pressure vs. time
- Left ventricular volume vs. time
- Drawing a pressure-volume loop
- Understanding the pressure-volume loop
- End diastolic pressure-volume relationship (EDPVR)
- End systolic pressure-volume relationship (ESPVR)
- Reimagine the pressure volume relationship
- What is preload?
- Why doesn't the heart rip?
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Pressure in the left heart - part 2
Watch the pressure in the left heart go up and down with every heart beat! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Want to join the conversation?
- If the aorta is compliant and stretches, wouldn't this reduce the pressure instead of increasing it? PV = nRT so pressure is inversely related to volume.. So if the aorta is being stretched out, then that should reduce the pressure in the aorta? Any help would be great! Thanks!(6 votes)
- Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels. The aorta stretches because of the pressure from the blood. You think well, but that is different, the blood pressure will decrease if the volume goes up only if the walls aren't stretched (that is vasodilation, widening of blood vessels because of relaxation of the smooth muscle cells within the vessel walls).(7 votes)
- Does the aorta always have three branches? Is this just a standardization for teaching?(4 votes)
- Yes, the aortic arch (or transverse aorta) branches off into 3 main arteries that go to different parts of the body and include the brachiocephalic trunk, left common carotid artery, and left subclavian artery.(8 votes)
- Do you guys discuss the left atrial pressure? I am trying to figure out why the left atrial pressure rises slightly in the wigger's diagram. Is it because the AV and semilunar valves are both closed and the back flow of blood causes pressure?(2 votes)
Video transcript
So we've talked
about the pressure in the left ventricle
and the aorta. And I thought it would be
fun to sketch it out again. But I wanted to go
into some details, because when we did it last
time, I brought up some issues but quickly glossed over them. And you may be wondering
about them now. So I thought we would get into
some of the details and finer points of the pressure in
the heart, the left side of the heart. So you know we usually
think of pressure in millimeters of mercury. So that's why I want to just
write that out so it's clear. And I'm going to
sketch out the pressure tracing we said happens with the
aorta and the left ventricle. So first, the left ventricle. We said that the
pressure rises slowly, and then eventually,
the left ventricle is going to contract
really hard. And that's the real cause of
why the pressure rises abruptly, and then it relaxes. And eventually as it
relaxes more, it falls. And then blood falls into that
chamber from the left atrium, and that's why the pressure
goes back up again. So that's a very
quick way of thinking about the left ventricle. And then of course,
you have the aorta. And I'm actually going to do the
aorta a little bit more slowly. The aorta, of
course, there's blood leaving the aorta, going to
different parts of the body, and then let's
stop it right there and now talk about
exactly what happens when the pressure of the aorta
and the left ventricle cross. So let me draw
for you, I'm going to draw in the side what
this looks like in the body. So you've got the
left atrium here. And then you've got the
left ventricle coming off, and it is enormous. So just draw it that way. And then there's the
aorta, also a large vessel, coming off of the
left ventricle. And the aorta wraps
around and goes down, but it leaves a few branches. One there, one there, and
let's draw a third one. So classically, three branches
coming off of the arch. We call that the
arch of the aorta. And then that's the aorta there. So let me label this stuff. This is my aorta, this
is my left atrium, and my left ventricle. So what is happening? I'm going to draw a little blue
arrow then to start us off. What is happening right there? Well at that point, we know
that we have a couple valves. This is, let's say,
our mitral valve here. And I'll label that mitral. And while I'm at it, I'll
draw in the aortic valve here. And this is my aortic valve. So these are my two valves. The mitral valve is closed. And remember, it closed
actually a long time ago. I shouldn't say a long time
ago, because not a lot of time has passed between
these two points. But it closed over there. So if I'm going to
show it closing, I'm going to have to block it
off, just to make it very clear that it's closed. Really, no blood can
pass through that gate. So, we're in the
middle of contraction. And I'm going to draw
pressure with yellow arrows. So you've got a lot
of pressure coming in here because
of the contraction of the left ventricle. A lot of contraction
there, a lot of pressure. And the question is, is
the aortic valve opened? Well, if I'm talking about
my blue arrow on my figure, well, not really. It's still hasn't
gotten the same pressure that the aorta has. So the valve is still closed. So really, both
valves are closed, and pressure is building
in the left ventricle. Now, as the left ventricle
pressure increases even more and rises, let's say
we get to this point. Well, all of a sudden,
now the pressure is greater than the aorta. So let's actually show that by
drawing a little opening there. So now, that valve opens. And you actually have
blood flowing this way. And actually, of
course, it flows into all of these
different places. So it starts flowing through the
aorta, through all the vessels. But one thing that
often is forgotten is that the aorta is
actually compliant. Let's actually write
that out, because that's the big key for
understanding how the aortic pressure
tracing looks. So compliance. And what does compliance mean? Basically, I think of
it as stretchability. So if you can actually stretch
these vessels, and you can, then you can begin
thinking about them almost like you would think
about a balloon. They're actually stretchable,
the vessels themselves. And so while there's
blood rushing around, it's also pushing
off into the walls. The walls themselves
are going to stretch, because I said it's a
little bit like a balloon. And really, what you
get is you actually get these walls are no longer
going to be as they look now. They're going to stretch out. You get something like this. You basically get stretched out
vessels, something like that. So this is actually
quite interesting because now it begins
to look balloon-like. And what's actually
stretching it out? Well, blood is
stretching it out, right? It's not like you're using
a puff of air or something, like you would for a balloon. Literally, blood is
pushing into the walls and stretching them out. So you've got blood in here. That's what's causing
the stretching. So while you're
stretching the walls, you actually have increase
of aortic pressure. So the aortic pressure
starts to rise. And that's what the
tracing looks like. So it rises to that point. So that's the most
pressure that's in the aorta at any given time. So this is the point when
the walls are actually maximally stretched. So this is when there's maximal
stretching of the walls, right here. And then, you can see the
left ventricle pressure starts falling. And as it falls, so does
the aortic pressure, because it's all a
continuous space. But as this is falling,
as this starts to fall, what's happening? This is my question. What's happening to the walls? Well, at that moment when
it begins to fall again, the walls, remember,
they're like rubber bands. The moment that you put
less pressure on them, they're going to
start to recoil. Immediately, they
start to recoil. So they start doing this. They're recoiling. And so you've got
two things happening. As these yellow arrows in
the aorta are being created, and those yellow arrows
represent recoil, the yellow arrows in the
ventricle are disappearing. Remember, that's contraction. So you have this
passive process. We think of it as a
passive process here. Passive recoil. Remember, that is
this elastic energy. And I say passive
because it's elastic and it's not chemical energy. And in the ventricle, you've
got this active contraction. So I just want to
contrast the two. Active contraction is
using chemical energy. When the ventricle
contracted, we were basically burning
up a bunch of ATP. That's how we got that. So remember, that's ATP. When I say chemical,
I'm talking about ATP. Whereas when I say
passive recoil, it's passive because it's
not using chemical energy, but it is using elastic energy. And that was actually
stored up, remember? That was stored up energy. And when was it stored up? It was stored up when
the left ventricle first forced blood into the aorta. So you stored up some energy,
and now those rubber bands are recoiling. And so it's a very natural
process, it makes sense. And so, while we're on the
downhill of our pressure slope, the bands are recoiling. And now eventually
what's happening is that pressure here is rising
just slightly from the fact that you've got some
recoil, and pressure here is falling quickly,
because basically, now this process of
contraction has gone away. On the down swing of the
left ventricular pressure, these arrows go away. So at this point now, you've
got slight increase in pressure in the aorta, slight decrease in
pressure in the left ventricle. So what's going to happen? Well, at some point, the
pressure in the aorta is actually going
to slightly exceed the pressure in
the left ventricle. So again, this point where you
actually have more pressure in the aorta than
the left ventricle. You might be like well,
how can that ever be? Aren't they continuous? Right, they are continuous. But a different process is
happening here than here. Here, you've got less pressure
in the left ventricle, less pressure because the
contraction has stopped. And in the aorta, you've
got more pressure, because you've got
recoil of the walls. So as the pressure
gradient switches, now you've got more pressure
in the aorta, what happens? Well, you've got a little bit of
blood that pushes out this way and pushes out this way,
because of course, the walls are coming back in. And the moment that
there is slightly more pressure on the aortic
side, this valve snaps shut. Aortic valve snaps shut. So now that valves snaps shut,
and that's very interesting. And that's actually a
very critical moment Let me actually make
some space here. And this is all
happening, of course, within fractions of a second. That valve snaps shut. And there's blood that
literally wanted to go in, and basically gets the
door slammed in its face. And it actually kind of
recoils and goes back around. So it wanted to go towards
the left ventricle. The aortic valve
snapped shut and pushed that blood back into
the aortic space. And of course, as all
these blood snaps back, what's it going to do? It's going to push again. It's going to push
back, just like it did the first time,
out on those walls. So these pressure
arrows reverse. And you actually
get pressure going the other way because
of this snap back. So you might think, well, that's
such a small amount of blood. How can it make such
a big difference? But it does. That tiny amount of blood that
hits off of the aortic valve pushes back into the aorta
and actually increases the pressure in the aorta. So, let me actually write
that out very formally here. You actually have a snap back. That's just my own terminology
of blood bouncing off the aortic valve. OK, so now it's
really clear where that pressure's coming from. It's coming from the
snap back of blood. And as a result, you
actually have a slight pick up of pressure in the aorta. So now, blood is going to
reenter the walls here. And now finally, you've got
your aortic valve closed, and you've got blood in the
walls and blood leaving, as it always has, through
all these different routes. Leaving through all
the vessels, going to the different
parts of the body. So at the end of this,
what's going to happen? Well eventually, all this blood
is going to want to leave. And the walls will
basically empty out. So all this blood will
finally leave, empty out, and the walls go back to
their original shapes. So let me just show them
in their original shape, and the whole process
can start over again. So, these walls go back to
where they were when I first drew them, something like that. And this is, of
course, due to the fact that the rubber bands
are basically back in their natural position. And as that's happening,
blood is draining off to the different
parts of the body, and you basically get
something like that. So, this point and this
point are the same. So you really come full circle. And what you get here,
what it looks like, is a notch, because
pressure goes up here and it comes down here. So it basically looks
like a little notch. Now before, I drew it
just more straight. I didn't really draw
that notch, because it was a little bit clearer
to show the simpler way, but truly this is
actually what happens. And this notch is called
the dicrotic notch. So I'm going to draw
this again for you with the dicrotic
notch, and you'll see how it differs from
the way we drew it before. The way we drew it before,
this is the inaccurate way, was something like this. I said well, it
goes down like that. The more accurate way would
be to say it goes down, but then there's actually a
notch that comes in there. So this increase,
this difference, is because of the
compliance of the aorta.