Now, let’s take a look at the trachea. And
what you’ll want to remember here will be
the vertebral levels where the trachea begins
and where it bifurcates.
The trachea is beginning at this point right
below the cricoid cartilage of the larynx,
which is here. And so, this is the initial
point of the trachea. It has a vertebral level
of C6. It then will travel distally and then
it’s bifurcating at this level. And this
will have a vertebral relationship between
T4 and T5. The distance travelled from C6
to T4 or T5 is only about 10 to 11 centimetres.
We’re only looking at a respiratory passageway
that is about 4 inches in overall length.
Not as long as we might think.
Along its length, you’ll see various cartilaginous
rings that are incomplete posteriorly and
the number of these cartilaginous rings is
anywhere from 16 upwards to 20.
Where the trachea bifurcates at its T4-T5
relationship, we can see a keel-like projection
at that level bifurcation. And that is shown
right in through here on the illustration.
That keel-like projection is called the carina.
And then you can see the right bronchus here
and you can see the left bronchus on the left
side of the carina.
What we’ll want to understand here is which
one of the bronchi is wider than the other
and which bronchus is more vertical than the
other. And in this case the answer to both
questions is the right bronchus.
The right bronchus is wider in diameter than
the left. The right bronchus is more vertical
than the left. The left bronchus comes off
at a more acute angle. We can utilize our
understanding of the characteristics of the
right bronchus when we think about aspiration
of a foreign object and the likelihood of
the bronchus then that will receive it. And
if you did follow a foreign object that’s
small enough, it has a greater likelihood
of entering the right bronchus.
The wall of the trachea is made up of layers
or strata. And those layers include the innermost
layer of the mucosa. External to that is the
submucosa. The third layer that’s even more
external is the cartilaginous/fibromuscular
layer. And then the outer layer or the coat
of the trachea is going to be the adventitia.
All four of those layers are depicted in this
slide. Here we have the mucosa and the portion
of the mucosa in direct contact with the air
moving though the lumen will have the epithelium
that lines the mucosa.
The submucosa is going to be characterized
by the presence of numerous glands that we see here.
The cartilaginous fibromuscular layer is demonstrated
here where we have the hyaline cartilage.
And then you can see posteriorly that the
rings of hyaline cartilage are incomplete
and the ends here of the rings are bridged
by fibromuscular components. So, here is the
collagenous component and then in red we have
the trachealis or muscular contribution to this region.
The outermost layer is your adventitia. That
is simply a connective-tissue coat.
There are various types of bronchi. So, let’s
describe the various divisions of the bronchi.
The first bronchi that form will be those
bronchi that divide where the trachea ends.
And these are primary bronchi: a right one
and a left one.
So, we have one primary bronchus for each lung.
Shortly after, each primary bronchus will
divide into secondary bronchi. And we can
see some secondary bronchi here and here and
here, for example. There are three secondary
bronchi to the right lung, two secondary bronchi
to the left lung. And secondary bronchi are
also known as lobar bronchi. The right lung,
as we’ll see shortly, has three lobes hence
it has to have three secondary or three lobar
bronchi. The left lung, normally, only has
two lobes and that is why we only have two
secondary or two lobar bronchi
associated with the left lung.
Each secondary bronchus will then divide into
tertiary segments. And we see numerous tertiary
segments in each one of these branching patterns.
Tertiary bronchi are also known as segmental
bronchi because tertiary bronchi are going
to supply each bronchopulmonary segment. Tertiary
bronchi will then undergo further successive
divisions. And there are many of these divisions
before they’ll finally lead to our system
of bronchioles. And the largest bronchiole
is referred to as a primary bronchiole.