00:02
So there are three
physical factors
that are going to influence
our pulmonary ventilation.
00:09
These factors are going to
influence how easy it is
for air passage in and out
of our respiratory system.
00:18
The first is our
airway resistance.
00:21
Second is going to be
alveolar surface tension,
and third is going to
be lung compliance.
00:28
So let's take a look
at each of these three.
00:32
So starting with
airway resistance,
friction is a major non-elastic
source of resistance to gas flow
and friction does
occur in our airways.
00:45
The relationship between
air flow pressure
and resistance can be described
in the following
mathematical equation.
00:54
Flow is equal to the
change in pressure
divided by the resistance.
01:01
Because the resistance
is in the denominator
the higher the resistance
the lower the air flow.
01:09
So in this equation
our change in pressure
is going to be the
pressure gradient
between the atmosphere
and the alveoli.
01:19
And this change is usually
about 2 millimeters of mercury,
But even this very
small difference
is sufficient enough to move in
about 500 milliliters of air.
01:32
Also gas flow is going
to be inversely related
to resistance again,
because the higher
the resistance
the lower the hair airflow
and this kind of makes
sense the more you resist
that ability for air to flow
then the less flow
there will be.
01:51
So resistance in the respiratory
tree is usually insignificant
for two major reasons.
01:58
The first reason
is that the diameters
of the airways
in the first part of
the conducting zone
are pretty big
they're pretty large
and there's just not
a lot of friction
or resistance.
02:09
Secondly,
because of the progressive
branching of the airways
as they get smaller even
though they're getting smaller
there's still an increase in
the total cross-sectional area
are places where the air can go.
02:26
So any resistance is
usually going to occur
in the medium sized bronchi,
but this resistance is
going to start to disappear
at the terminal bronchioles
where diffusion is what
drives gas movement
instead of the actual air flow.
02:44
So in these terminal
bronchioles,
the resistance is actually
going to be controlled more so
byour autonomic nervous
system and the smooth muscles
which are going to allow for
constriction and dilation,
so it's not really
being controlled
by the airflow
but more so by the our autonomic
nervous system instead.
03:06
All right.
03:07
So if we look at
the relationship
of resistance as
the airwaves branch,
we find that most
of the resistance
that we have in our airways
will beat the medium
sized bronchi.
03:19
But this is quickly dissipated
as more branching occurs.
03:24
But once we reach
the respiratory zone,
resistance is going to be zero
because again now
it is our autonomic
nervous system
and smooth muscles
that are controlling
resistance or airflow
and not necessarily the
air coming in from outside.