Reimagine the pressure volume relationship | Circulatory system physiology | NCLEX-RN | Khan Academy

I want to talk a little bit
about the idea of pressure

and volume.

And we're actually
a clear up some

misconceptions I think
I may have caused.

I apologize for them.

But I think this is
a good chance for us

to reflect on the things
that we've learned

and also build up a
couple of new ideas.

So let's draw volume going
that way and pressure going up.

And one of things I
want to start with

was the end systolic
pressure volume relationship.

We drew it something like this.

So we said this is
the relationship

at the end of systole
between the two,

between pressure and volume.

And one thing that I wanted
to bring up immediately

was the idea of
increasing volume.

So as I go up to this yellow
line, I'm increasing volume.

And sometimes the way I've
drawn that-- actually maybe I'll

make a little bit of
space on this canvas.

And sometimes the way I
draw an increase in volume

can be a little bit misleading.

So I've drawn, for
example, in the past,

I've drawn a left
ventricle like this.

And I said well as blood
goes into my left ventricle,

it basically does this.

You have more and more
blood filling up the heart.

And I've drawn this
sort of a picture,

and it really does tell
you about a couple things.

It tells you that you
have filling happening.

And that part, I'm OK with.

But the part that
I'm not OK with

is the idea that
it basically seems

like you have a fixed volume.

It looks like a fixed
volume on the heart,

or the left ventricle anyway.

And it almost makes
it look like you're

filling up a glass of water.

Basically, that's kind of
what it looks like a glass.

And really the correct way to
think about left ventricular

filling is a little
bit more like this.

You should be thinking of
it more along the lines

of a picture like this
where you basically

have a smaller volumed the
left ventricle filling up

with blood.

And over time it kind
of feels in completely.

So it starts out like that.

And then you add more blood
and it becomes like that.

And then you finally
fill it up like that.

So that would be the
more accurate way

of showing what's going
on in the left ventricle.

And, of course,
all three of these,

then, are the left ventricle
at different points in time.

So this second picture also
tells you about filling,

so you get the idea that
blood is filling in the heart.

But it does a better
job of showing you

that the volume changes.

The volume of the left ventricle
changes, so it's not fixed.

And that's correct.

This is the better
way of looking at it.

And kind of an analogy
might be a balloon,

you might think of a balloon
for this filling process.

So I want to be very clear
that the left ventricle is not

like a glass.

It's like a balloon.

And that it's not
a fixed volume.

It actually changes.

So this is probably the more
accurate way of thinking about.

And I apologize for doing
this sort of a drawing.

Truthfully, I didn't
mean to confuse anyone,

but I just want to
demonstrate filling.

And I probably just do it
in a quick and hurried way.

And so I want to
clarify that point now.

So this is what it
would look like.

And actually I could
take this a step further

and say well what
if I was to do this?

What if I was to take a
cross section like that,

cut it with a blade at
these three points in time?

Wouldn't you agree
that you would actually

get an interesting cross
section view of it,

if I was to take
it like that and I

was to erase these top bits out.

And you were to now
look down at the heart,

you'd basically see kind
of an interesting view.

And I'll actually try to
draw that view out for you.

And it's helpful actually
to do it that way.

And I'll tell you why So
this one would basically

look like this.

And this one would
look like this.

And this one might
look much larger

than the other two
something like this.

And again this is just
looking at a cross section,

so it's nothing
different at all.

It's just looking at
the cut surface of it.

And all three you'd
expect to be full.

So this is how I'm going
to use our diagram.

I'm actually going to
use these kinds of images

now to show what filling of
the left ventricle looks like,

so we can actually get
a real sense for it.

And you'll see an interesting
problem that comes up.

So let's do that.

Let's draw a couple
of circles here.

I'm going to draw, let's
say, a big circle here

where it's really large.

And then let's say the volume
is little small at this point.

So let's draw
something like that.

And the volume is
really, really tiny.

Let's draw something
like that over here.

Now if you have these three
volumes, you might say,

well, OK you've colored them in.

Well, one thing
you'd have to admit

and you'd realize pretty
soon is that at the bottom

of this curve, you
have a small volume,

but it does take a little bit
of blood to fill that volume in.

When you have zero
volume-- like right

here there's zero
blood in there--

it would be an empty
left ventricle.

And then you'd actually add
a little bit of blood to it.

Let's say you fill it up.

Let's say halfway.

And now you've got a
half full ventricle.

And then you keep doing it
and you have a full ventricle.

So you basically are going
this way along the curve.

But until you have
a full ventricle,

and this is the point, until
you have a full ventricle

you actually don't have
any increase in pressure.

So previously when I drew
out the end systolic pressure

volume relationship
with that yellow line,

I drew it the way
you see it now.

But now I'm telling you that
the truth is that it actually

looks a little bit
different, especially

at the bottom end of this curve.

So I'm going to erase this
and draw it in properly.

And this is the more accurate
way of drawing it in.

You basically have
almost no-- or really no

increase in pressure.

I shouldn't say almost no.

And then once you get
to a full ventricle,

now you start seeing an
increase in pressure.

And really the way that an
increase in pressure looks

is that you have
a larger volume.

And that's what you're
starting to see.

You're start to see
that larger volume.

So even a tiny
bit of pressure is

going to push out on
the left ventricle.

And you'd actually notice that
because now it gets larger.

So the left ventricle actually
doesn't change in size

initially.

And, finally, when the pressure
starts actually mounting up

it starts changing in size.

So you can start
appreciating why

I am saying that this first
yellow line is incorrect.

Let me erase it completely
so it doesn't distract you.

So I've drawn out the end
systolic pressure volume

relationship, but what I
want to do is now add to it

our end diastolic pressure
volume relationship.

We know it goes
something like that.

And let me just label
it in a yellow color

just to be parallel.

So this is our end diastolic
pressure volume relationship.

Now if I was to say, well, what
would the cross section look

like?

Now let's just kind of
choose a couple points

to say this is this point.

This is this point like that.

And if I said what would
the same volume look like

on the other curve, I
would have to actually

just draw a line
down and say, OK.

This is this volume right here.

And this is this
volume down here.

And along those
points-- let me ask

you just mark it
on my other curve.

Those points would
be right there.

And I actually could just
similarly draw them out.

I could say well
this is about that.

And then the other one looks
maybe a little bit larger.

It would be something like that.

So these are my two curves.

Right?

Now I'm trying to make them
look as similar as possible

to the other ones.

And I'll fill them in.

So that's what the volumes
would look like at these points.

So really when you
look at the volumes,

they look about the same.

They don't look any
different at all.

And so you're left wondering
well how in the world is it--

and this is actually very,
very confusing to think

about for folks-- how
in the world is it

that the pressure is so darn
high on the end systolic curve

whereas it's low on the end
diastolic curve, when they look

the same?

They don't look any different.

And to figure this out-- I think
one easy trick I've been using

is to just imagine what's
happening at the muscle level.

So the muscle cells are kind
of contracting and pulling

in those z-disks.

At the end of systole, we've got
tons of contraction happening.

And it's happening here too.

In fact, it's happening at
every part of this curve.

And if I was to try to simplify
this, instead of drawing

hundreds of arrows
like this, I could

do this for every single point.

So instead of drawing
hundreds of arrows,

you could imagine
that I can actually

connect all these
arrows like this

and that I would have a
similar effect if I just

drew it like this.

I could simply draw almost
like a rope or a band--

imagine a band or rope
that's pulling and tugging

this way and this way.

If I was actually to
draw the band like that,

you could imagine then, it
would be the same effect

as the hundreds of little
muscles that are contracting.

And to take it a
step further, you

could actually even imagine
people yanking on that band.

So this is how I
picture it, just

people yanking on that band.

These are like two
little workers,

let's say, yanking on
the band and pulling it

in opposite directions.

And if they were pulling
it in opposite directions,

you basically have what we
think of as contraction.

You could have
little workers that

are basically yanking on all
these things, yanking away.

And by yanking away, what
you basically end up with

is a force of contraction.

So this is basically how
I imagine contraction,

having workers yanking in
two different directions.

And if you had them going
all around the heart

in every direction you
could possibly imagine,

that is what a contracted
ventricle is like.

And because they're
yanking so darn hard,

because they're pulling
so hard on this thing,

you basically have
a lot of increase

in pressure building up on the
inside of these ventricles.

And you really don't have that
happening on the other side

because on the end
diastolic curve--

I guess the question is
do we have any workers?

Are they yanking?

And the answer is no.

The muscle cells are
completely relaxed.

They're relaxed.

They're just hanging
out and taking a nap.

You can imagine your workers
are really not yanking at all.

And as a result,
you don't have any

of that increase in pressure.

You have just a very,
very low pressure.

And so that's the
reason you can imagine

there's a difference, even
though the volumes are

the same, that there's a
difference in pressure.

So final question that plagues
a lot of people-- and I'm

actually going to make
a little bit of space

to answer it-- is so why is
their blood in the ventricles

at the end of systole?

I mean isn't that the point
where all of the blood

has exited the ventricles
and gone into the aorta?

Why is there any
blood in there anyway?

Shouldn't it be empty?

And to answer this--
to think about this,

we can actually draw a
pressure volume loop.

I'm just going to
draw it in purple just

to create a little
difference in color.

And let's say that
I have contraction

right here where I
have a big purple dot.

That's where I begin
my contraction.

So I'm going to draw going
up from there like that.

And let's say now my
ejection is happening.

And let's say, just rides
over my picture of the worker

like that.

Let me actually draw
one final volume piece,

and that would be what
is the volume here.

Because we know that the
volume is not changing there,

it's constant volume.

And at this point,
you begin ejections.

So this is all ejection.

I'm going to write ejection on
the curvy part of the curve.

So this is ejection
happening right here

over the hump like that.

So ejection is happening
between my two white lines.

And here in the vertical part, I
could draw a picture like this.

I could say, well, my
heart will be really full.

So it'll look-- in fact,
let me make it bigger.

My heart is going to
be really, really full.

Let me try to
illustrate that nicely.

So I could have
something like this.

I could have
something like that.

And if my ventricle is that
big, if it's that large-- let

me actually just
color it in now.

Then what's actually happening
when I have ejection?

Well, I'm going to cut
and paste this little guy,

and show you on the top
what it would look like.

So let me just drag this
little fella over here.

And now this, if this
is how I start out,

then when I eject
blood, you're basically

going to have
something like this.

You're going to have an
amount that goes away,

and an amount
that's left behind.

So the amount that's left
behind is, of course, the amount

that I showed you on the side.

And I'm cutting it out.

And this doughnut hole
shape that's left,

this is actually
our stroke volume.

This is our stroke volume.

So you actually do have a lot of
blood that goes into the aorta.

Of course, that's important.

And you have a little
chunk that's left.

So now you can see that at
the beginning of contraction,

you end up with having
a lot of blood here.

This is where you start.

And then you lose
a lot of blood.

This is our stroke
volume that you lose.

And then you are left with
a little bit of blood here.

And that's at the end of
systole So at the end of systole

you do have some blood left, but
you don't have nearly as much

as you had when
you began systole.