Besides this notion of compliance,
the other thing that
we have to take into
account is something known
as surface tension.
Surface tension is an
interesting physics principle
in which a water molecule
in air will form a droplet
and it does because water
adheres to each other
and forms this surface
tension around it.
So molecules at the surface
of the drop adhere together
and therefore create a drawing
away from the center,
which then cause there to be surface
tension on the air to water interface.
This can also be seen in a place
where water is simply around
the surface of a particular object
such as an air sac or alveoli.
There is water on the
inside of that air space.
So the same thing will happen,
the lining component will
generate surface tension
simply because there is an
air to water interface.
Luckily, we have two
types of pneumocytes.
The second, type II pneuomocyte,
which is in a lower prevalence,
only 5 to 10% of the alveolar space
is covered by type II pneumocytes.
They produce a substance
A surfactant is a very
It’s primarily a lipid, phospholipid, but
has a little bit of protein in it as well.
But what’s very important about it is
it has both a polar and non-polar side.
So what happens, if you think
about this as having normal
surface tension of an alveolus
or you have water molecules,
a surfactant molecule can
be placed on this surface
and what it does is reduce the
amount of surface tension
that two water molecules will produce
when placed next to each other.
So the more amount of surfactant that’s on
this barrier between the air and water,
the lower the surface tension.
So why is this important in
pulmonary and respiratory medicine?
And that is because there is a
law known as the law of LaPlace
in which you will have --
Certain sized air sacs will have a
different amount of surface tension.
So here, we have two air sacs.
The larger the air sac, the
easier it is for it to inflate.
The smaller the air sac, the more
surface tension that it has.
What surfactant does is reduces the
need to have different-sized air sacs.
So if pressure comes into an
alveoli, with surfactant present,
it will inflate it no matter if it’s
small in size or larger in size.
So surfactant allows us to take out
this principle of surface tension
because a surfactant
molecule helps reduce it.
However, there are some clinical conditions
such as a premature infant being born,
who may not have produced
enough surfactant yet.
And in that case, they’re
going to operate under
this principle of having
too much surface tension.
And if you were mechanically
ventilating a patient such as this,
you have the propensity to
overinflate certain air sacs
that have reduced surface tension
such as those large alveoli.