In this particular lecture, we’re
going to cover ventilation.
And 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 alveolar ventilation.
Then, we will also talk about
the resistance to ventilation.
You will also understand how
the work of breathing works
and how compliance and surface
tension affect work of breathing.
Finally, we will predict how lung
diseases such as restrictive lung disease
or an obstructive lung disease affect these
various components of work of breathing.
Okay, let’s get right
To breathe, we have a
specific respiratory cycle.
And we’re going to use
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 subatmospheric
So pleural pressure in our example
in this diagram is -5 cm of water.
So what this means is there’s
always a negative pressure
in a certain portion of the
pleural space of your lungs.
And 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 subatmospheric
or negative pressure.
In this case, it’s at 5.
When a person breathes in, negative
pressure in the pleural space increases.
And what this will do or how this works is
places like the diaphragm pull down.
And as you pull the
you stretch on the pleural membrane 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 breathes 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,
Intrapleural pressure usually
looks almost opposite of that.
As you breathe in, pleural
And upon exhalation, it
returns back to normal,
but notice that it’s still negative.
It just becomes more
negative during inspiration
and less negative
How does this work for the pressure
within the air sacs themselves?
If you got from A to B,
you see that there is, during inspiration,
lower pressure within the alveoli.
This is very important because airflow
only occurs via pressure differential.
So to get air into the air sacs,
you need to have a lower pressure
than what mouth 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
pressure that you have
within the alveolar space.
As you return to a resting position
whereas denoted here as C,
will be back to 0.
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 alveolar sac
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 pressure,
volume will flow out.