Now, what involves the kind
of resistance to airflow?
Airflow resistance is based
upon a specific law of tubes.
In this Poiseuille law, which is resistance
is eight times the length of the tube
times a factor, which accounts
for the air viscosity
divided by pi times radius
raised to the fourth power.
This law of rigid tubes allows
us to calculate a resistance.
This is a theoretical equation so it’s not
one we’re going to plug numbers into.
But let’s kind of tease through it so we
know what are our most important factors.
If you look at this
kind of graph here,
where you have airway
generation on the X-axis
and you have resistance
along the Y-axis.
Hopefully, you can appreciate
that there is a change
in the amount of resistance as you
increase the number of airways.
This is an important process.
This means that as you add
airways to the system,
you decrease it’s
Another factor that affects
is pulling on the air
So as you pull on an air
sac, you make it what?
As you make it larger, you’re
increasing the radius.
And therefore, you’re decreasing
the amount of resistance.
So the effective radial traction
as you pull on something
allows an airway to dilate
so it has lower resistance.
So parallel circuits, radial traction,
both decrease resistance in the airways.
The other factor that we
need to think about is that
the diameter or the radius
of the airway is what?
The most important aspect in this
particular law of rigid tubes.
If you decrease the diameter,
you increase the resistance.
And therefore less air will be
able to move into the lungs.
This can be observed in this
classical pulmonary function test.
So this is someone’s normal
pulmonary function test.
So I will walk you through or actually
you through this particular diagram.
We have time on the X-axis and
we have volume on the Y-axis.
So what the green line is showing you
are inhalations and exhalations.
So in the example,
it’s inhale, exhale.
Maximal deep breath in
and then a maximal breath out.
And you keep breathing out until you
get all the air out of your lungs.
And what we’re looking for here
is a volume change overtime.
If you can get air out rapidly, then you
have less resistance in your airways.
Let’s contrast this to someone who
has an obstructive lung disease.
So in this case, you still have time
on the axis and volume on the Y.
And you’re going to do this.
Normal breath in,
normal breath out.
Normal in, normal out.
Normal in, normal out.
Maximal deep breath in.
And now when you breathe out,
it doesn’t go out as fast.
And I’ll do that until I get as
much air as I possibly can out.
So they’re both trying at the same
amount of effort to breathe the air out.
It just doesn’t come
out as rapidly.
And that is because there’s more
resistance in those individuals
with an obstructive
Let me give you some
An example of this particular response
and you can see the two
pulmonary function tests,
is let’s say you have a foreign
object trapped in your airway.
Asthma is a great example of
an increase in resistance.
Because what happens there is there
is a bronchial constriction.
In cystic fibrosis, there’s
increase mucus production
and therefore, the airway is a
lower diameter or lesser diameter.
Chronic bronchitis and emphysema, you
also can get increases in resistance.
But this, instead of having
something within the airway,
it occurs oftentimes because
in emphysema’s case,
because you have dynamic
compression of the airways.
So with dynamic compression
of the airways,
we have a normal lung
over here on the left.
This is going to be during a forced
expiration type of maneuver.
So you’re generating a
lot of pleural pressure.
In this case, it goes all the way
up to 20 centimeters of water.
Now, you’ll notice that the value within
the alveoli or this air sac is a +35.
The reason that occurs is because
there is both the ability for this 20
to press in on the airway as well as
that airway naturally wants to recoil.
And you add those two together
and you get this 35 cm of water.
This yields a 15 differential between
what’s in the alveolar space
and what is in the
If there’s more pressure in the airway or
the alveolus, it will still stay inflate.
As you travel up the tube to
zero, which is the mouth,
you can see that there’s
drop in pressure.
And this simply occurs because as
you’re travelling through a tube,
you’ll lose energy
because of resistance.
But you’ll notice in this example,
you still have a +5 differential
between airway pressure
and pleural pressure.
So the airway stays open.
We’ll contrast that to
someone who has emphysema.
In emphysema, there is destruction
of some of the elastic fibers.
Meaning that the alveoli no longer
wants to collapse in the same way.
This then, if a person does the same
expiratory manuever of a +20 cm of water,
you only are going to
add 5 to that instead
of 15 like you did in
the normal condition.
It still maintains that
the alveoli will be open
because it has a higher pressure within the
alveolus than it does in the pleural space.
But now, as you travel
down that tube,
you’re still going to be losing energy
because of resistance of the tube.
So if you drop to somewhere
like 15 millimeters of mercury,
now you’re at a spot in which pleural
pressure is greater than airway pressure.
And this causes a
compression of the airways.
And as you decrease the diameter of the
airway, you increase its resistance.
And so this is a great example
of how emphysema increases
airway resistance through the
dynamic compression of airways.