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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 causes
there to be surface tension on the air
to water interface this can also be seen
in a note a place where water is simply
around the surface of a particular
object such as an air sac or alveolus
there's water on the inside of that
airspace so the same thing will happen
the lining component will generate
surface tension simply because there's
an air to water interface luckily we
have two types of Numa sites the second
new type type to Numa site which is in a
lower prevalence only five to ten
percent of the alveolar space is covered
by type-2 Numa sites they produce a
substance called surfactant surfactant
is a very important molecule it's
primarily a lipid phospholipid but has a
little bit of protein in 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 being
having normal surface tension of a navvy
OS where 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 this is 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 over
inflate certain air sacs that have
reduced surface tension such as those
large a vo line
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