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now what involves the kind of resistance
to airflow air flow resistance is based
upon a specific law of tubes in this
Pasillas law which is resistance is 8
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
and this is an important process this
means that as you add air waves to the
system you decrease its overall
resistance another factor that affects
airway resistance is pulling on the air
sacs themselves so as you pull on an air
sac you make it what larger as you make
it larger you're increasing the radius
and therefore you're decreasing the
amount of resistance so the effect of
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 light 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 these classical
pulmonary function tests so this is
someone normal pulmonary function test
so I will walk you through or actually
breathe 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 our
inhalations and exhalations so in the
example it's inhale exhale inhale exhale
inhale exhale
máximo 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 over time
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 x-axis
and volume on the Y and you're gonna 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 they'll do that until they get as
much air as they possibly can out so
they are 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 disorder
Airways let me give you some clinical
examples an example of this particular
response 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's a bronchoconstriction in
cystic fibrosis there's increased 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 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
millimeters or centimeters of water now
you notice that the value within the
alveoli or this air sac is a plus 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
centimeters of water this yields a 15
differential between what's in the
alveolar space and what is in the
pleural space if there's more pressure
in the airway or the alveolus it will
stay inflated
as you travel up the tube to 0 which is
the mouth you can see that there's a
drop in pressure and this simply occurs
because as you're traveling through a
tube you lose energy because of
resistance but you notice in this
example you still have a positive v
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 ovule I no
longer want to collapse in the same way
this then if the person does the same
expert ory maneuver of a + 20
centimeters 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 portal 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
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