00:01
propria and the underlying submucosa. When you
go down from the trachea towards the lung,
the trachea is going to branch prior to going
to the left and right lung inter bronchi and
those bronchi divide further into a number of
bronchopulmonary segments. They go to different
lobes of the lung. And then within those bronchopulmonary
segments, there are further divisions and
these divisions are defined anatomically.
And those of you do the anatomy of the lung will
know these divisions. Here I just want you
to appreciate that as we move down from the
trachea into the lungs, these bronchi are
still supported by cartilage. And they
pass down to the deep levels of the lung
getting smaller and smaller, but they are
still highly supported by the cartilage. The
cartilage in these structures, as well as
the trachea, stops these conduits from actually
closing, from collapsing during the breathing
process. So they are very important to be
supporting the open conduit. And again if you
look very carefully in these sections, you
can make out some very tiny pieces of cartilage
or some more prominent pieces of cartilage.
And that is because as you move from the trachea
down into these bronchi, then you no longer
have the horseshoe-shaped ring of cartilage.
01:31
The cartilage just becomes isolated plates as
you move further and further down the bronchi
tree. Smooth muscle is also prominent. It starts
to appear in these bronchi and it becomes
increasingly dominant as the bronchi move
down to become smaller and smaller and smaller
until they finally become bronchioles where the
only main components supporting the wall of
these bronchioles is smooth muscle. All the
cartilage is absent. So the difference between
a bronchus or bronchi and a number of these
conduits and a bronchiole is the fact that
the bronchiole does no longer have cartilage
supporting it, it just has smooth muscle.
02:21
And this can be a problem, that smooth muscle
can contract under various stimuli. Smooth
muscle does not need necessarily just an innervation
to contract. Various chemicals can make smooth
muscle contract and this is sometimes deleterious.
The asthmatic suffers because sometimes these
smooth muscle contracts and therefore, closes
the airway or the passage of air down these little
airways, the bronchioles. Let us look at one
of these bronchioles in more detail.
02:59
Bronchioles actually, originating from the bronchi form
firstly terminal bronchioles. A terminal bronchiole
then proceeds to branch into further bronchioles
called respiratory bronchioles. And when one
terminal bronchiole divides into a number
of respiratory bronchioles that defines what
we call the pulmonary lobule. It is because
the lung originated from an outgrowth of the
gut and develops like a gland. That is why
we refer to the term lobule and in a moment
acinus. And sometimes it is not surprising in the epithelium
all the way through the respiratory
tract they find enteroendocrine cells. We are
not sure what they do, but they're present probably
because of this fact that the lung developed
as an outgrowth from the gut and developed
as now a gland. One thing you notice on the
slide is that the smooth muscle is arranged
in certain orientations, in a circular manner
and also in a spiral type of manner. And this
helps to open and close effectively the airway
so that air can then pass down into the alveoli
below for exchange. There is also elastic
fibers all the way through down to the very
fine components of the alveoli. They help,
when the lung expands during inhalation
of air, and then breathe out expiration. Those
elastic fibers help to recoil the structure,
recoil these bronchioles and the alveolar
and therefore, help to force the air out
during expiration. They are absent or at least
minimal in people who suffer from emphysema
and therefore, that makes breathing and the
exchange of gases very difficult in
those individuals. The respiratory bronchioles branch
into alveolar ducts and finally alveoli, that
we will see in a moment. But one thing happens to
the epithelium. Up until the terminal bronchiole,
the epithelium has remained pseudostratified
ciliated columnar, typical respiratory tract
epithelium. Now the epithelium is going to
change. It is going to lose the cilia, become
more cuboidal and then finally it is going
to become more squamous. Because now they are
changing from being a conducting conduit, warmer,
moisten and clean the air to being a conduit
purely designed for the exchange of gases
across the air sacs into the blood stream.
05:54
Sometimes those ciliated epithelial cells
will be looking at in the respiratory tract
at epithelium, they can change. They can go
through a transformation. They can often become
cuboidal or even stratified squamous in areas
where there happens to be abrasion. The vocal
cords, for instance, are lined by stratified
squamous epithelium because that resists the
abrasion of the air passing very quickly pass
them. Other places around the larynx, the
epithelium is pseudostratified epithelium.
But in certain places there can be this change
or transformation of the epithelium, for instance,
a smoker. The smoke can change this epithelium.
06:40
Epithelial cells can lose their cilia and
mucous secretion can seize and therefore the
smoker has the typical smoker's cough because
they do not have the mucous or the cilia to
beat the debris back into the parts of the
body where we can swallow or get rid of it.
07:00
Luckily the epithelium is rather versatile,
it can renew those lost epithelial cells when
they are given a chance to actually heal.
One thing I want to point out on this slide
before we move on is that the alveoli actually
have little tiny pores, pores of Kohn.
07:19
They allow air to pass from one alveolus to another
if there happens to be an obstruction in the
alveolar ducts, those very small airways that deliver
air to individual alveoli. Here is a diagram
on the left hand side showing you the very
top part, a bronchiole moving into a terminal
bronchiole, then into respiratory bronchiole
and then to the alveolar ducts and finally
the alveolar sacs. And on the right hand side
is the description of the epithelial changes.
07:58
The epithelium changes from the pseudostratified
ciliar epithelium down to these cuboidal type
epithelial cells and finally they are going
to form very very thin squamous cells. Sometimes
if you look at the respiratory bronchioles,
and can just make out a very small slip of smooth
muscle as that smooth muscle finally disappears.
Now there are clara cells. Cells we call
clara cells sitting in the epithelium and these
are present also higher up in the pathways
of the air. These clara cells secrete the
surface active agent that stops the surface
of these bronchioles collapsing, coming together,
sticking together because there they moisten
surfaces and that would not get any good.
We would not want our airways to stick together
and therefore, prevent air from passing down
there. So these Clara cells secrete that agent
that reduces the surface tension between the
two surfaces. Well, now we go from the bronchiole
into the alveolar sacs and the alveolar ducts
all the passage ways where the exchange occurs.
09:14
And the bottom image you see there labels
basically these terminal bronchioles and a
few clara cells which are hard to see, opening
into respiratory bronchioles, into passage
ways and sometimes as I've mentioned before, very
small slips of smooth muscle can be seen.
09:36
Those respiratory bronchioles then pass into
the alveolar ducts. There are long corridors
of very thin epithelial surfaces. The air
passes along those corridors into all the
alveolar sacs that branch out from them, or
out pocketings of these alveolar sacs and
this is where all the exchange occurs. In the
alveolar ducts, the alveolar sacs and the
individual alveolus or alveoli. Let us have
a look at the surface of these alveoli.