By this point,
you may be getting kind of sick of these
pressure volume loops. But I assure you they're
very, very worthwhile getting to know. In fact, let's learn
one more thing out of these pressure volume loops. Let's try to squeeze
all the knowledge we can out of these things. So I'm going to draw one
for you here very quickly. And on this side,
we've got pressure in millimeters of mercury. And on this side we've
got milliliters, right? We're just going to use
the same units we've kind of gotten
very familiar with. And we're going to do 50,
and 100, and 125 or so on this axis. That's a pretty normal
set of terms or numbers. And then on this
side, we've got 0, 50, and I'm going to go up
to 120 or so up here. So this would be a
pressure volume loop. And I like to kind of start
out where the pressures are low and the volumes are
really, really high, so somewhere around here. I'm going to imagine that this
is kind of the end of diastole, where my left ventricle
is full of blood, right? It's going to start
rising in pressure slowly. And as it rises,
it's going to get to let's say about this point. And then the blood is going
to start entering the aorta. And as it enters
the aorta, I've got to get to a nice high pressure. I know that that's the
target or thereabouts. And so the pressure kind
of rises even higher. And then the volume
starts to fall as volume enters
the aorta, and so it leaves the left ventricle. And then the left
ventricle continues to lose its blood to the aorta. And the blood in the left
ventricle that's left is around 50. And then you start
having relaxation. So you relax all the way
down and the pressure falls to about that point. And then it continues
falling, but now there's a little bit of blood
kind of entering into the left ventricle. So it's starting
to fall in pressure but continues to now
start rising in volume. And it continues
to rise until it's ready to do the whole thing
all over again, right? And now if this is kind of
our overall pressure volume loop, what I want to
do is kind of focus in on one particular point. Let's focus in on
this point right here. And this is the end
of systole, right? We talked about the end of
systole being right here. This is where it begins
to start relaxing. And when I say "it," I
mean the muscle cells. So you've got a
muscle cell over here. And I like to draw them
kind of branched just to remind you that
they're muscle cells. And you've got now this cell
completely contracted down, right? So it looks like this with the
actin and myosin completely overlapping, right? Because that's what
you expect to happen at the end of systole. And, of course, I'm
drawing it this way really just to remind you what's
going on inside of the cell, although you know
that, of course, a cell has many, many, many
sarcomeres, not just one. And of course, this one
I've drawn just having one. But you get the
idea that there's a lot of overlap between
the actin and the myosin. In fact, that's what these
little red lines represent, just the major proteins
instead of heart cells. So these are my heart
cells full of protein, and they're completely
contracted, right? So sometimes they like to
relax, and sometimes they like to contract. And at the end of systole, where
I've drawn that orange arrow, these muscle cell are
completely contracted down. I'm going to write "contracted"
just to kind of remind you that that's what's going on. And what are they waiting for? I mean, what's next? What are they hoping
will happen next? Well, they are hoping
that they can now get rid of all that calcium,
and so they can relax. So you've got a lot of
calcium in this space. And they're kind of hoping
that the calcium will go away and they can relax. And so imagine now
that the calcium-- I'm going to draw calcium as
a white circle, right? And I'm going to fill it in. This is my calcium, right? And imagine that I've
got calcium in here. I'm just going to draw
little white circles in here. And instead of allowing
my heart cell to relax, I'm going to kind of do
something interesting. And I remember calling
it a trick last time. I guess I can call this a trick. And the trick is I'm
going to trick my heart cell into not relaxing. I will basically not
allow it to relax, because I load up this cell--
imagine I can somehow do this-- with lots and lots of calcium. And so I just fill
it with calcium. It's chock full
of calcium, and it has no way to really
get rid of it. So it is going to continue
to be contracted, right? It's just going to continue to
be contracted if I can somehow fill this with calcium and not
allow it to go into relaxation. So my heart cell's
completely contracted. That's the key, right? I have done this. And so as a result of
doing this now, of course, this won't happen,
this relaxation bit. This won't happen and
neither will the next bit. So basically I'm kind of
forcing myself to continue to remain contracted, and
all this kind of disappears. And so what happens is
that now my heart is basically full of blood, right? I've got a heart full of blood. I'm going to draw it over here. I'm going to just kind of
ignore for the time being the left atrium and the aorta. But this is my left
ventricle full of blood. So this is chock full of blood. And what I'm going
to do is I'm going to take a little
needle-- watch this. I'm going to take
a little needle, and I'm going to try to
add a little bit of blood or take a little
bit of blood away. And I guess I'm going to start
by taking a little bit of blood away just to see what
will happen, right? So I take a little
bit of blood away just to see what will happen. And let me choose
a different color. Let's choose a green color. And at this point now, my heart
is-- again, it's full of blood, right? And I'm going to
now take a needle, and I'm going to take some
blood off of the heart. I'm going to actually just
pull it off like that. So now I've got
blood in my syringe. And as a result, what
have I really done? Well, I've lowered
the volume, right? I've lowered the volume. Actually, let me
switch to a green color and show you that I've
lowered the volume. And if I lower the volume,
basically, what will happen? Well, if I lower the
volume, the pressure will start to fall, right? The pressure goes down. So it goes something like that. And I can do this
again, and I could see if the pressure falls. And, oh, it does. And I could do this again. In fact, I could take
all the blood out of the left
ventricle, and I could see that basically my
pressure will go down to zero. So I basically have
now a few dots. I can connect my dots. And I can see that I create
this basically kind of a line, right? And so this line is what
you would get if you just keep reducing, and
reducing, and reducing the volume in the heart. Now, what if I did the opposite? What if I-- instead
of reducing the blood, what if I actually added blood? And of course, it
might be kind of tricky to think
about adding blood. But just remember you can
always add air to a balloon if you try hard enough. And similarly,
you could actually push blood into a
full left ventricle if you have enough
pressure pushing down. So I could actually do this. I could actually try to do this. In fact, I'm going to add, let's
say, a little bit of volume here, right? And I'm going to notice
that the pressure goes up. In fact, it goes up even more
than it ever did before, right? It actually rises above the
line that I had drawn in blue. And in fact, I might
even do it again. I could say, well, let's
just add some more volume. Let's just see what happens. And the pressure
goes up even higher. So I could connect these lines. I could say, OK, well, let's
see what these lines look like. And basically, it's forming
a nice straight line. In fact, to see it
a little bit easier, let me just get rid
of this blue stuff. Let's get ride of
all this stuff. And you can see that you get
this nice straight line that relates volume to pressure. And so this relationship between
volume and pressure or pressure and volume is happening with
the muscle cells contracted. Remember, all this
time my muscle cells are bathing in calcium. They're completely
contracted down. They have not been
able to relax. They're contracted. And so you could
even say, well, this is basically the relationship
between pressure and volume at the end of systole. So you could say this is the
end systole pressure volume relationship. And this is kind of the
long way of saying it. People actually shorten
all of this down. They don't say all these words. They usually say ESPVR--
End Systolic Pressure Volume Relationship. And all it means is that if
you could get a situation where your heart cells are
completely contracted, completely contracted
down-- and that's why I kind of made up the
bit about filling it up with calcium, because I guess
that's one way to do it, but completely contracted,
and you just kind of added volume or took it away,
what would the pressure do? How would it change? And this line tells you that. In fact, one final point
I want to make about this is if you remember,
there's a relationship now between elastance--
remember the term elastance-- and
pressure and volume. In fact, if you take
pressure divided by volume, that gives you elastance. So with this line
that we've drawn, you can actually say, well,
there's a slope to this line, right? I tried to draw a
straight line so imagine there's a straight
line between all these points, right? Well, the slope of that line--
slope is elastance, right? That is the elastance. In fact, you might even see the
term-- wherever the pressure volume loop falls on
this ESPVR, sometimes you even see the line
actually labeled, and you see it called E0. In fact, you'll often see that. So E0 refers to the
slope of the line that is formed at the end of systole.