00:02
In this lecture, we’re going to
deal with pulmonary blood flow.
00:05
We have a number of learning
goals that we want to obtain.
00:08
The first of these is
is after this lecture,
you’ll be able you understand the
mechanism of pulmonary blood flow,
both increases and decreases.
00:18
After this lecture, you will
also be able to identify the
factors that affect
pulmonary blood flow
especially pulmonary artery pressure,
lung volumes, hypoxia and gravity.
00:30
Pulmonary blood flow can
probably be best understood if
we use a little bit of a
compare and contrast approach.
00:37
You are very familiar with
the arterial circulation.
00:40
And so in this, this is what everyone’s
circulation is on the arterial side
of the circuit, that’s
from the left ventricle,
all the way around
to the capillaries.
00:48
This is characterized by very high
blood pressure, usually 120/80.
00:53
There’s high resistance in
this particular circuit.
00:56
And the nice thing is, it’s
carrying a lot of oxygen,
having a high PaO2 and
a very low PaCO2.
01:04
After you go through a capillary bed,
you’re going to extract
O2 and give up CO2.
01:11
So on the venous side
of the circulation,
the pressures are lower,
the resistances are lower.
01:17
You also have lower O2 and higher CO2.
01:21
So that’s the systemic
side of the circulation.
01:24
If we compare now this to
the pulmonary circulation,
the pulmonary artery still
has very low blood pressure.
01:33
It has very low
resistance, low O2.
01:36
Why still low O2?
It’s because it’s going
from the right ventricle
to the pulmonary vasculature
to undergo gas exchange.
01:44
It hasn’t gone through gas
exchange yet, so O2 is still low.
01:49
CO2 is high.
01:51
But once you’ve gone through the
lungs, on the pulmonary vein side,
you have still low
pressure, low resistance,
but now you have oxygenation.
02:01
So O2 is high and CO2 is low.
02:05
So this is a nice way to think
about pulmonary blood flow.
02:10
Hopefully, you’ll recognize by looking
at the two different blood flows
that pressures are very low
on both sides of the lung.
02:17
Resistances are low on
both sides of the lung.
02:20
So there are certain factors that
affect blood on the pulmonary side
that don’t affect blood
flow on the systemic side.
02:28
So let’s go through how blood flow
changes through the pulmonary circuit.
02:35
This is a graph that looks at
pulmonary vascular resistance,
which is the resistance of blood
flow through the tubes of the lung.
02:42
And then we’re comparing that to
mean pulmonary artery pressure.
02:47
So this is the pressure pushing
the blood through those tubes.
02:51
And this forms a very
unique relationship
in which at low pulmonary artery
pressures, resistance is high.
02:59
But as pulmonary artery
pressure increases,
you actually get a decrease in
pulmonary vascular resistance.
03:06
And this is fairly unique.
03:08
So as pressure goes up, you have
lowering of the resistance.
03:13
What does this do for blood
flow is as pulmonary blood flow
increases as pulmonary
artery pressure increases.
03:22
So then they are more linearly
related in a positive manner.
03:26
This is a very
interesting process.
03:29
It means that it’s not governed in the
same way as your systemic vasculature,
that will be covered in the
cardiovascular section.
03:37
So this brings up a very
unique point of view
and that is we have from
the previous slide,
an increase in pulmonary
artery pressure.
03:46
And as pressure increases, the pulmonary
vascular resistance decreases.
03:52
So what that meant was as
pulmonary artery increased,
blood flow also increased.
03:58
How does this happen?
Well, it seems to occur
via two mechanisms.
04:04
And let me go through
what those are.
04:06
The first is going to be a recruitment
and the second is a distension issue.
04:10
But let’s kind of walk
through these step by step.
04:13
In normal pulmonary vasculature,
some of the blood vessels are collapsed
even during the rested state.
04:21
There are some of the blood
vessels that are not collapsed,
but yet the blood is not
moving very rapidly
through those
particular capillaries.
04:32
So both of those items have
low blood flow through them.
04:39
There are some of the capillaries
though, just as you would expect,
are open and do conduct blood.
04:45
So there’s a little bit
of a notion here that you
only get blood flow through
some of the capillaries.
04:51
Other parts of the
capillaries are collapsed
and some have just
low blood flow.
04:57
As pressure though increases,
what you get is an opening
of these blood vessels.
05:04
First the ones that were
previously collapsed start to open
and the ones that were not
conducting flow through very rapidly
as the pressure increases, those are
pushed through to a much greater extent.
05:19
The ones that had had
normal blood flow,
widen.
05:24
And so that is the distension component
versus the recruitment component.
05:30
So we have recruitment that occurs through
the previously collapsed blood vessels.
05:36
We had distension that occurred
through the blood vessels
that were already open
that became wider.
05:43
Therefore, we have an end result
of an increase in perfusion or blood
flow through the entire lung.
05:50
So this is how the process of
increasing pulmonary artery pressure
increases pulmonary blood flow through both
a recruitment and a distension mechanism.
06:02
Now the other factor that
dramatically affects blood flow
through the lungs is the
lung volume itself.
06:11
So lung volume can be thought
of as how inflated the lung is
versus how much you’ve
been able to breathe out.
06:19
So if we plot pulmonary vascular
resistance over lung volume.
06:24
With low lung volume being on
the left hand side of the graph
and high lung volume being on
the ride hand side of graph
and those are denoted as RV or
residual volume at low lung volume.
06:36
And TLC or total lung
capacity at high lung volume.
06:41
There is an effect of
compressing the capillaries.
06:47
So as you compress capillaries,
you get an increase in
resistance at high lung volumes.
06:54
Oppositely, in terms of the
blood vessels themselves,
as you go to a low lung volume,
there is an increase in
pulmonary vascular resistance.
07:08
So we have two different factors
that are affecting here.
07:13
One having high resistances at low
lung volume and then coming down
and the other is having high
resistance at high lung volumes.
07:25
This yields a total pulmonary
vascular resistance
in this kind of a
U shaped curve.
07:33
That means that at a
moderate lung volume,
the resistance through the lungs, in
terms of the blood flow, is lowest.
07:43
While you’re at a low lung volume,
you have high resistance.
07:46
And at high lung volume, you have high
resistance to pulmonary blood flow.
07:51
Okay. So how in the
world do these happen?
Let’s first take the compression
effect and discuss that.
07:57
And then we'll discuss the mechanism
behind the supply vessel dilation effect.
08:03
So if we look at the compression effect
and that’s denoted here as low
lung volumes, low resistance,
high lung volumes,
high resistance.
08:13
You can see on this graph in the slide here
where we have capillaries denoted as red.
08:20
And the alveoli as a kind
of hexagon white shape.
08:25
As you inflate those alveoli,
you can notice that they will
squeeze out some of the red
and this is compressing these
pulmonary capillaries.
08:38
You might think that
seems very odd that the
capillaries can be compressed
such as like this.
08:44
However, you should
think of there is low
blood pressure in the
pulmonary vasculature.
08:51
So factors such as inflating these air sacs
can actually increase the
resistance of flow through that
as there’s no longer as
large of a luminal diameter.
09:03
So that is the
compression effect.
09:05
Inflate the air sacs,
you have more resistance than
you did at low lung volumes.
09:12
Now, if you look at the
vessel dilation effect, for this,
we need to remember that the
blood flow supply is surrounded
by not only alveoli, but also
other connective tissue.
09:24
And what you can do is pull
on this connective tissue
by creating a negative pressure.
09:30
And we call that a negative
pleural pressure or P sub PL,
At this point, you have all the air sacs
pulling on that particular blood vessel.
09:42
And what happens is it makes
that blood vessel wider
or have a larger luminal diameter.
09:49
And in that case, it would
decrease its pressure.
09:53
And as you decrease the
pressure then, you’d have more
flow through that area
because of lower resistance.
09:59
And so that denotes the low lung
volume, having higher resistance,
and high lung volumes, having lower
resistance to pulmonary blood flow.
10:09
Putting all these together,
it gives us that U shape
of pulmonary blood flow to
pulmonary vascular resistance
in terms of the
lung volume effect.
10:22
Now, the last factor that
really affects pulmonary blood
flow is the amount of oxygen
that is in each air sac.
10:30
And this is a difference
that occurs in the
pulmonary vasculature versus
systemic vasculature.
10:37
So low PO2s cause
pulmonary blood vessels
to vasoconstrict only within
a very, very small area.
10:45
And this is different
from systemic vasculature
and it’s usually a low PO2
vasodilates the blood vessel.
10:54
So this is an opposite
kind of effect.
10:57
Now why this is important.
10:59
This can actually be a beneficial
effect as you can decrease
the blood flow to alveoli that
have a low oxygen concentration
and preferentially shunt
that blood flow to alveoli
that have a high
oxygen concentration.
11:16
So in fact, it allows
you to oxygenate blood
even to a greater extent than would occur
if you perfuse all the different alveoli.
11:26
Okay.
11:27
That seems a little bit complex, I know,
but I have to add one more factor to this.
11:32
And that factor is that
if you have condition
in which PO2 decreases
across all the alveoli,
you get a vasoconstriction
throughout the lung.
11:44
And so in this particular case,
it overrides particularly
that local effect
and you get vasoconstriction
across the whole lung.
11:52
So you have both a local effect and a
kind of systemic effect of low PO2,
both of which cause
vasoconstriction in the lungs.
12:01
If you vasoconstrict
these blood vessels,
you increase the amount of resistance
which decreases pulmonary blood flow.
12:08
Okay.
12:09
The last kind of thing that
affects blood flow is gravity.
12:14
So this is really only going to affect
you when you’re in the upright position.
12:18
So for example, even if you’re sitting
now in front of your monitor at home,
what you have is an effect of
being upright on the lung.
12:27
Gravity is naturally pulling blood
flow to the lower aspects of the lung.
12:34
It seems a little bit complex.
12:36
So let’s go through
these step by step.
12:38
We have broken the lung
up into three zones.
12:41
So I’m going to first talk
you through zone number 1.
12:44
It has the lowest
amount of blood flow.
12:47
In fact, it may have very little
or at all much blood flow in it.
12:51
And the reason is is because P, small A,
which is the pressure within
the pulmonary arterial,
is lower than what is in P, capital A,
and that P, capital A is what’s the
pressure within the alveoli itself.
13:10
And if the pressure in the alveoli is
greater than the pressure in the arterial
that’s moving through,
you have very low flow because the
arterial is compressing the capillary.
13:23
Okay?
In zone 2, this is a little
bit easier process to think of.
13:29
There’s higher amounts
of blood flow
and the reason why there’s a
higher amount of blood flow
is because the pressure within the
arterial side of the capillary
is large enough to get some blood
flow past the alveolar pressure.
13:47
And so you have a moderate
amount of blood flow.
13:50
You have the highest amount of
blood flow, when P, small A,
which is the partial pressure –
Sorry. The pressure within the
arterial side of the capillary
is greater than the pressure,
which is in the alveoli.
14:04
If that’s larger, there’s no impedance
of blood flow through the circuit.
14:10
So zone 1, zone 2, and
zone 3 are all based upon
what is the pressure within the
arterial side of the capillary
versus the pressure that’s
inside the alveoli or air sac.
14:23
If you have higher pressures on the
arterial side of the capillaries,
you will get more blood flow
past the air sac or alveoli.
14:33
The more that that alveoli
has a higher pressure,
than what’s on the arterial side
that will impede blood flow.
14:40
And that happens as a
lowering of zone effect.
14:47
And the reason for that is because
there’s a hydrostatic effect
on the blood that
affects its pressure.
14:55
So you can think of this as a
process in which at heart level,
you have a certain pressure.
15:01
Anytime you go below the level of the
heart, the pressure will increase.
15:05
And you’ve probably all
experienced this if you have,
let’s say, take your socks off and
look down at the tops of your feet.
15:14
If you’re standing upright,
you’re blood vessels in the tops
of your feet may be distended
because there’s a higher amount of pressure
as you are below the level of the heart.
15:25
If you raise your feet
up to heart level,
the pressure within those
vessels goes down and
they will no longer be
distended or harder to see.
15:33
So this is simply an effect
of gravity on blood pressure.
15:39
Why does it affect pulmonary
blood flow so much
but doesn’t affect
systemic blood flow?
It’s because pulmonary blood
flow is so low to start with.
15:48
Therefore, only small effects such as
things like gravity, lung inflation,
the amount of hypoxia
that you might have
affect the pulmonary vasculature so much
more because pressure started off so low.
16:06
We’ve covered a lot of the important
topics in this particular lecture.
16:09
Of these, you’ll need to make sure
you understand the mechanisms
of how pulmonary blood flow
both increases and decreases.
16:18
Also, you’re able to understand and
be able to identify the factors
of pulmonary blood flow such
as pulmonary artery pressure,
lung volumes, hypoxia
and gravity.