So you're pretty familiar
with this image of the heart. We have the four chambers. And I'm just going to
start by labeling them-- the right atrium and right
ventricle on this side and the left atrium and
left ventricle on this side. And the question is this. What happens if we
choose two spots? I'm going to choose
this one right here, with the little x in left
ventricle and some other spot over here in the aorta,
let's say, with the purple x. And this is, of course,
our aorta up here. What happens if I follow the
pressure at those two spots? So of course, I was
going to say maybe if I'm the red blood
cell sitting there, but we know red blood
cells move around. But let's say I'm just following
the pressure at those two spots, those
locations, over time. So this will be time over here. What does the pressure
look like at that location? And let's say I'm following
for, let's say, one second. So, if my heart
rate was somewhere like 60 beats a minute,
that would basically mean one second
would be one beat. This is 60 beats a minute. And just keep in mind,
60 beats a minute is actually pretty low. So that would be like
if I'm reading a book or sitting around relaxing. And on this side,
let's do pressure. And I'll do 0 to,
let's say, 100 up here. And remember, the
units of pressure are millimeters of mercury. Now actually,
before I start, let me even jump into
some naming, just so we don't have to stop later. These are the valves. This is the mitral valve. And below it, I've drawn
the tricuspid valve. And put together, we would
call these the AtrioVentricular valves. This is the
AtrioVentricular valves. And AtrioVentricular,
I've capitalized A and V because sometimes you'll
see the word AV valves. AtrioVentricular valves are
also known as AV valves. And the other two
valves, here, this is the pulmonary
valve down here. And on the other side of it
right here is the aortic valve. Put together, we would call
these the semilunar valves. And actually, semilunar,
you might think of it as like a half moon. But there's no shorthand
for that, usually. So people just call them
the semilunar valves. So what does the
pressure tracing look like at my yellow x? Let's go back to that question. So I'll start out
here on the axis. So it starts out really low. Left ventricular pressure
is surprisingly low most of the time. And you'll actually see
that now when I draw it out. You know, I always think of
it as this chamber that's cranking and pushing
and high pressure. And that's true. There is some of the
points along the way when the pressure gets very high. But for most of the time,
it's actually pretty low. And it creeps along,
goes up very slowly. Well, why is it going up at all? That's the first question. Why doesn't it just stay steady? Whatever pressure it
is, why is it going up? Well, it's going up
because there's actually flow here through
the mitral valve. So blood is going into the
left ventricle initially, and when there's more blood
in that chamber, over time, the pressure slowly builds up. Just like if I'm pouring
water into a water balloon, over time, every little bit
more water I put in there, the pressure in the
balloon goes up. So, that's why it's creeping up. And then you get
to a point where all of the sudden, there's
a muscle contraction. So you have a depolarization
wave that comes through, and all of this heart muscle
is cranking, just pushing in. And of course, simultaneously,
the right ventricle is doing the exact same thing. So all of the things that I'm
saying for the left ventricle, for the most part apply
also on the other side. So they're contracting. And the left ventricle
contracts hard, and the pressure begins to rise. Now just right there, just
right where it begins to rise, you might say, well,
what did you even do? And at that moment where
I drew a slight increase, a tiny little increase--
you can see it if you squint your eyes-- at
that moment, this valve closes. Why? Because at that moment,
at that very moment, there is a little bit
of push back here. And the slightest bit of push
back makes the valve shut. So once the pressure on
the left ventricle side is greater than the pressure
on the left atrium side, the valve shuts. And so the valve
closes at that point. And then you have a
rise, a rapid rise, in the left
ventricular pressure. And it goes up,
up, up, like that. So let me just write that
down since I just said it. The AV valves close. And that's a new thing,
because they were open. So let me box that. AV valves close. And the semilunar valves, the
other ones, they stay open. Semilunar stays open--
nothing new there. Actually, sorry, I said open,
and I'm even writing open. But I mean close. Sorry. So let me not confuse you and
just change that right now. Semilunar valves stay shut. And to drive home that message,
I'll even put a block there. So they're still shut. So all the valves at this
point are now closed. And that might be news to you. You might have thought, well,
I thought at some point, some valves were always open. And that's not true. At this point, both
valves are closed. And the left ventricle
is basically squeezing. But the blood has
really nowhere to go. It's just sitting
in a trapped room, and the pressure is
rising and rising fast. And then at some point,
it gets to a spot where an interesting
thing occurs. So let me actually
draw that here. Let me erase this slightly. Let's draw the aortic pressure. So the aortic pressure, let's
say, is something like this. And it's drifting down. Well, why is aortic
pressure drifting down? Well, it's because in the
aorta, blood is rushing away. Remember, from the
previous squeeze, you've got blood rushing
away, out all these vessels. It's going away. And as blood rushes
away, of course, pressure is going to fall. Remember, there
is a relationship between having more blood
volume and pressure. So as the blood volume
in the aorta drifts away, the pressure drifts down. And this aortic
pressure is rising. And at that spot-- it's
hard to see, again. I'm going to just circle it
to draw your attention to it. But at that spot, you'll see
the aortic pressure is slightly below left ventricular pressure. And because the pressure
is slightly below, all of the sudden, this
is no longer blocked. Now you've got a free path here. This valve opens up,
and blood can come in. So new blood can come in
from the left ventricle. So blood starts rushing in. And of course, the left
ventricle is still contracting, so it still continues to rise. And it gets to about there. And the aortic pressure
is going to rise. And so let me draw that. Actually, I might have switched
colors, but you'll forgive me. It starts to rise. And it follows the course
of the left ventricle. So basically, it's rising with
the left ventricular pressure because there's a
continuous space there. There's no valve
between the two. So what happened at
this spot exactly? Let's just recap it. Well, the AV valve
is still closed. Nothing has changed there. But the semilunar valve opened. And that's the
interesting new thing. That's the cool, new thing
that allowed the blood to go from the left
ventricle out into the aorta. And now what happens? Well at some point, all this
contraction I've drawn relaxes. It finally goes away. It goes away on both sides. And all these black
arrows, I'm going to erase. And this muscle now,
instead of depolarizing, begins to re-polarize. Now, why didn't I erase
that last black arrow? Because again, there is some
pressure in the left ventricle. And in fact,
looking at my graph, you can see there's
not just some, but there's a lot of
pressure still in there. So all that's changed is that
the muscle is now relaxing. And if the muscle is relaxing,
then this yellow line begins to drift down. And if the left ventricular
pressure drifts down, so does the aortic pressure. That drifts down too. Well at some point,
what will happen? Well, the aortic
pressure is still high. And think about this. This is actually a tricky point. It's still high because
there's resistance. Remember, there's resistance
from all of the blood vessels. There's resistance here. All these blood vessels are
offering lots of resistance. So, with all this resistance
from the blood vessels, the aortic pressure stays high. And the left ventricular
pressure is relaxing. It's drifting back down. And so this arrow, instead
of blood just going one way, there's a little bit of pressure
coming back the other way. There's a little bit
of pressure going this way, this
way, and this way. And there is some
pressure this way still because the left ventricle
still has some pressure. So they're matched. There's pressure
coming from the aorta and also pressure coming
from the left ventricle. Initially, the
left ventricle just overwhelmed the amount
of pressure in the aorta. But as it's drifting back
down, now that aortic pressure is matching the left
ventricular pressure. And at this particular moment,
that left ventricular pressure is going to be lower than the
aortic pressure, something like that-- an
interesting cross-over. And that's, again, because
the aortic pressure stays high because of all
that resistance, but the left
ventricular pressure continues to drift down
because it's relaxing. And the moment that the
pressure in the aorta is greater than the pressure
in the left ventricle, this shuts down again. So that valve slams shut. And so at that point,
what would we say? Well we say, well, the
semilunar valve closed, and the AV valve opened. Sorry, I keep saying
that, and I apologize. The AV valve is still closed. I didn't mention anything
with the AV valve. That's still closed, as it has
been for the last two points on our graph. In fact, let me just label
this point one, point two, point three. So at this point,
what's the next thing that's going to happen? Well, the left ventricle
continues to relax, and it goes all the way
down through my word, semilunar closed, all
the way down almost to 0. And before it gets there,
before it gets all the way to 0, the pressure's so
darn low in here that now, left atrial
pressure is actually higher than left
ventricular pressure. And blood can flow back through. So if blood can
flow back through, then we know that as blood
fills up a ventricle, the pressure continues
to slowly rise over time. And it gets to
about there, which is the same spot
that we began at. And meanwhile, the
aorta continues to drift down because,
just as in the beginning, we said well, when time
passes, blood drifts down through to all the vessels. So blood is now drifting
away, as it did before, into all the vessels. And as it drifts away,
the pressure in the aorta drifts back down again. And it goes something like that. Actually I guess further,
because my second is not there-- something like that. And you have to assume,
based on my drawing, that these two
points are the same. And if they're not, then I
haven't drawn it correctly. So that's what the
aorta does at the end. So let me just label
this now, this last point here, this point four. Sorry, before I get to
point four, point three, the thing that was interesting
and new-- I should just box it. I'm just trying
to box the things I want to draw your
attention to-- is that the semilunar valve closed. So what is happening
at point four? Now at point four--
a couple things. The AV valve now finally opens. I said that prematurely before. And that's the new thing. The AV valve opened. And the semilunar
valve is still closed. So that did not change. So this is exactly what happens
at the four stages, the four important points. So, just to recap-- and
actually, as I recap, let me mention something new
that's interesting as well. Remember that when valves
close, they usually make noise. So it's like a
slamming of the door. So when a valve
shuts, it makes noise. And so you have to look
on these four points, when does a valve close
for the first time? Well, the AV valve
closes right here. This is the point
where it first closed. So when the AV valve closes,
it makes a loud thwack, a loud noise. In fact, sometimes
people call it a lub. You hear that lub noise if
you listen to your heart. And that's the first heart sound
because that door just shut. And you can see, based on
the pressure differences, it's really closing because
the left ventricular pressure is going up so fast. So when you hear that
first lub-dub, when you hear that lub,
that's also the beginning of the high pressure,
the contraction of the left ventricle. Now point two, does
any door close? Does any valve shut? Not really. The semilunar valve opens,
but opening of a valve doesn't really cause any noise. At point three, the
semilunar valve closes. So that's where the aortic valve
and pulmonary valve slam shut. So again, you get
some noise up here. This causes noise. So at point three, if I was to
draw a straight line down here, you'd get some noise. I'm going to cover up my
number four, but that's OK. You know where it is. And this is the second noise. And it's coming from the
closing of the semilunar valve. And then, point four, the
AV valve opening, well, opening doesn't create noise,
and the semilunar valve is still shut. So really, the two sounds,
the heart sounds, S1, S2, come from the closing
of the AV valves initially, the
mitral and tricuspid. And then the dub
comes from the closing of the aortic and
pulmonary valves.