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in this particular lecture we're going
to cover ventilation we have a number of
learning goals to achieve and the first
will be to describe what the respiratory
cycle is the next will be to apply the
various methods for us to understand
ventilation and these two methods are
minute ventilation and ovular
ventilation then we will also talk about
the resistance to ventilation
we will also understand how the work of
breathing works and how compliance and
surface tension effect work of breathing
finally we will predict how a lung
diseases such as a restrictive lung
disease or an obstructive lung disease
affect these various components of work
of breathing okay let's get right into
ventilation to breathe we have a
specific respiratory cycle and we're
going to use pressure differentials to
allow air to flow from outside the mouth
into the air sacs or alveoli what is
kind of interesting to think about is
that you always start off with a
negative or sub-atmospheric pleural
pressure so pleural pressure in our
example in this diagram is minus five
centimeters of water so what this means
is there's always a negative pressure in
a certain portion the pleural space of
your lungs the reason for that is that
the chest wall wants to expand and the
lungs want to collapse and they are
pulling on each other and that creates a
sub atmospheric or negative pressure in
this case it's at five when a person
breathed this in negative pressure in
the plural space increases and what this
will do or how this works is places like
the diaphragm pull down and as you pull
the diaphragm down you stretch on the
pleural membranes and create a greater
negative pressure this then allows the
lungs to expand and air to rush in
so let's take this now into more of a
graphic format from kind of our cartoon
when someone breathe is in you're going
to have an increase volume in the lung
and that can be seen here as a volume
increasing and then during exhalation
volume decreases Interpol oral pressure
usually looks almost opposite of that as
you breathe in pleural pressure
decreases and upon inhalation or sorry
exhalation it returns back to normal but
notice that it's still negative it just
becomes more negative during inspiration
and less negative during exhalation how
does this work for the pressure within
the air sacs themselves if you go from A
to B you see that there is during
inspiration lower pressure within the
alveoli this is very important because
air flow only occurs by a pressure
differential so to get air into the air
sacs you need to have a lower pressure
than what mouths pressure is and if you
get a lower pressure air will then want
to travel in from outside the body into
that air sac during the midpoint of your
inspiratory cycle that is the lowest
amount of negative pressure that you
have within the ovular space as you
return to a resting position whereas
denoted here at C ovular pressure wave
back to zero when you try to get air out
of the air sacs pressure has to be
positive compared to mouth pressure and
positive pressure will create a
differential from the ovular sacs to the
mouth and air will flow out so it's all
about creating a pressure differential
between the air sacs and atmospheric
pressure or what's outside the mouth if
there's a negative pressure in the air
sac volume will flow in if it's a
positive
sir volume will flow out
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