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# Drawing a pressure-volume loop

Video transcript

So what do you see here? You see two figures, right? On the top figure, you've
got pressure and time, and this is actually
the pressure of the left ventricle
over some period of time. In this case, I guess
that's about 1.2 seconds. And on the bottom, you've
got volume over time. And again, I've drawn
it also over 1.2 seconds so you can kind of see
exactly what's happening in the left ventricle
as time passes. So let's actually
do it step by step. We'll kind of go through
this very, very carefully and figure out for
any given point, like this point right
here, what is the pressure and what is the volume. So here, the pressure
looks like it's about, I would say, I don't know,
around 10 millimeters of mercury. I'm just going to
write a circled 10. And at that same point in
time, if I was to kind of just go down that same
point in time, I would find that the volume
is about 125 millimeters, milliliters. So that's the volume,
and that's the pressure at that particular
time point, right. Now let's do another one. Let's do this one up here. This is about 80
millimeters of mercury, and if I was to
drop that line down, that gets me to about here. And of course that
is the same volume. Really nothing has
changed in the volume even though the
pressure has shot up. So those two are
really easy, but let's do a slightly tougher one. Let's do something up here
where the pressure is about 120. Pretty high pressure. In fact, it looks on this
graph like the highest pressure it ever goes to. And dropping the line down,
I'm going to get to, let's say, here. And I'm going to say
that x is 75 milliliters. So at the peak pressure of 120,
the volume is 75 milliliters. And I'm just going
to keep going. This is, let's say about
100, pressure of 100. And you'll see how we can
actually use this information to make another graph. So we're just going
to kind of collect information from these
two graphs using volume and pressure, and we're going to
make another graph of our own. This I'm going to say it's about
20, and 20 drops down to here. And this looks like it's
about 50 milliliters. Right? And you can kind of start
getting the idea here. This is actually probably
the lowest pressure. Here the pressure is about
five, and dropping it down, it looks like it
gets to about here. I'm just going to say 75 again
just to kind of round off. And then here, you have kind of
a slightly elevated pressure. This is about 12,
I'm going to say. And then this last
little bit right here is 10, back
where we started. And if you drop those
down, the pressure right before it kind of maxes
out on the volume, let's say it's about 123,
slightly less than completely full. And then that final x is
going to be back at 125. Right? So this kind of how the
volume changes over time and how the pressure
changes over time. And actually I'm going to
take now our two graphs, and I'm going to try
to merge them together. I guess you can kind of
think of it as a super graph. So this one is
going to have volume down here, using the
same units as before. We usually measure
volume in milliliters. And we'll do 50
over here, and I'm going to estimate that's
about 125 over there. And on this side we're
going to do pressure. So pressure we measure
usually in millimeters of mercury, although
of course you could use pounds per square
inch, or something else. But we're going to do
millimeters of mercury because that's
what we usually do. And I'm going to say this
is, just kind of estimating, I'm going to say
that's about 120. So these are the two new
axes we're going to use, and we've got to pick
some point to begin at. And I'm going to assume that
the first point I started at, this one over here, is a
good place to begin again. So we can start there. And the pressure
there was about 10, and the volume was about 125. So I'm going to make
a little red X there. That's our starting
point on our new graph. And from there, it went
up to a pressure of 80. Well, 80 is about over here,
and the volume did not change. So I'm going to do a
little red X there, and I'm going to connect the two
lines like this, basically just kind of show the two
connecting like that. And we know that over time,
changes are happening, changes happening between
the first and second X. And it takes a
little bit of time, and how much time exactly
I'm going to write out. Remember it was
about 0.05 seconds. That's about how much time it
took to go from the first spot to the second spot. And let's just
keep track of that. So if you go forward in
time you should go up on our graph, at least
in the beginning. So what happens after that? Well, then you get to a point
where the pressure is really high, 120, and the
volume is about 75. So I'm going to draw
a little red X there, somewhere there, right. And that's when the
blood is actually leaving the left ventricle
and entering the aorta. So this is going to be
the next spot, right. And then the
pressure falls again. It goes down to about 100. And the volume is
still about 50. So volume has gone down. Pressure has gone down too. So it goes back down like that. So this yellow chunk, that
I've divided into two just to kind of make sure I include
the very, very high pressure of 120, but that entire yellow
chunk takes time, of course. And how much time does it take? Let me mark it. So this whole bit
takes some time. And we said that
whole bit combined takes about 0.25 seconds. So about a quarter of a
second to do that bit. So it's interesting, right,
because the first bit happened really, really fast, 0.05
seconds, and the second bit takes five times as
long, but on our graph it's not like you
see a line that's five times longer, right? Because again, this is
pressure and volume, and these graphs do
not actually show you the time, which is why I have
to separately show it to you, just to convince you that
some segments do in fact take longer than others. Now the next part, the
volume stays the same, but the pressure
falls, and we said it goes down to a
pressure of about 20. So let's say 20 is
about there on my graph. So it's going to fall. And I'm going to try to
keep the colors consistent. So we've got green now. This is when the left
ventricle is relaxing. So the pressure falls
at that point, right. And how long does that take? Well, if you remember
this bit right here takes about 0.15 seconds. So, again, compare that to the
contraction, the first part, which took only a
third of the time, but the two lines in many
ways look very similar, right? One's going up,
one's going down, but the time is different. Now, let's continue and
see how the pressure falls really low, right, about five. And we said the volume at
that point is about 75, so the pressure falls down
to somewhere like this. And then it starts
to rise again. So let me show that. So you've got kind of a
decline here in pressure, and then you've got a rise. So then it starts rising again. And remember, there's a
point where it hits 12. So it's going to do
something like that, rise, and then it's going to go
up just slightly at the end and then go like that. So that would be our
pressure-volume loop. Now, most times when
you see it drawn, you'll see it drawn
something like this, but almost never do you
see this last little bump. So I'm just going to
draw it, even though you know that it's there and
I know that it's there, just to kind of show the
way that most people draw it in books is like that. So that's what the
pressure-volume loop looks like in most places. And actually, let me put
the last little bit of time in there. So let's do this
little bit right here. And this takes how long? This takes about 0.55 seconds. So the majority of
the time is spent on that last bottom
part of the loop. And so in sum, this is
our pressure-volume loop.