00:01
In this particular lecture, we’re
going to cover ventilation.
00:05
And we have a number of
learning goals to achieve.
00:08
And the first will be to describe
what the respiratory cycle is.
00:13
The next will be to apply the various
methods for us to understand ventilation
and these two methods are minute
ventilation and alveolar ventilation.
00:23
Then, we will also talk about
the resistance to ventilation.
00:28
You will also understand how
the work of breathing works
and how compliance and surface
tension affect work of breathing.
00:36
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.
00:47
Okay, let’s get right
into ventilation.
00:51
To breathe, we have a
specific respiratory cycle.
00:55
And we’re going to use
pressure differentials
to allow air to flow from outside the
mouth into the air sacs or alveoli.
01:04
What is kind of interesting to think
about is that you always start off with
a negative or subatmospheric
pleural pressure.
01:14
So pleural pressure in our example
in this diagram is -5 cm of water.
01:20
So what this means is there’s
always a negative pressure
in a certain portion of the
pleural space of your lungs.
01:27
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.
01:40
In this case, it’s at 5.
01:44
When a person breathes in, negative
pressure in the pleural space increases.
01:50
And what this will do or how this works is
places like the diaphragm pull down.
01:56
And as you pull the
diaphragm down,
you stretch on the pleural membrane and
create a greater negative pressure.
02:05
This then allows the lungs to
expand and air to rush in.
02:12
So let’s take this now into more of a
graphic format from kind of our cartoon.
02:19
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,
volume decreases.
02:35
Intrapleural pressure usually
looks almost opposite of that.
02:40
As you breathe in, pleural
pressure decreases.
02:44
And upon exhalation, it
returns back to normal,
but notice that it’s still negative.
02:52
It just becomes more
negative during inspiration
and less negative
during exhalation.
03:00
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.
03:14
This is very important because airflow
only occurs via pressure differential.
03:21
So to get air into the air sacs,
you need to have a lower pressure
than what mouth pressure is.
03:28
And if you get a lower pressure,
air will then want to
travel in from outside the
body, into that air sac.
03:36
During the midpoint of your inspiratory
cycle, that is the lowest amount of
pressure that you have
within the alveolar space.
03:46
As you return to a resting position
whereas denoted here as C,
alveolar pressure
will be back to 0.
03:54
When you try to get air
out of the air sacs,
pressure has to be positive
compared to mouth pressure.
04:01
And positive pressure will create a
differential from the alveolar sac
to the mouth and
air will flow out.
04:08
So it’s all about creating a pressure
differential between the air sacs
and atmospheric pressure or
what’s outside the mouth.
04:17
If there’s a negative pressure in
the air sac, volume will flow in.
04:23
If it’s a positive pressure,
volume will flow out.