00:01
All right, now that we've seen
the bones and muscles
that surround the thoracic cavity,
let's look at the
thoracic cavity itself.
00:08
The first thing we'll notice is
the diaphragm at the inferior edge.
00:14
The diaphragm is going
to be pretty important
because it's the primary
muscle of inhalation.
00:19
And it also separates
the thoracic and abdominal
cavities from each other.
00:24
In fact, there was at one time,
just a single large body cavity,
and the diaphragm is
actually what separated
the thorax from the abdomen.
00:33
The next thing we'll see,
coming down from the neck
into the thorax is the trachea.
00:38
And it's going to branch into
bronchi that supply the lungs,
which are going to make up
the majority of the volume
of the thoracic cavity.
00:46
But there is this gap
in between
the two lungs in the middle
called the mediastinum.
00:52
That's going to hold some
pretty important structures to
especially the heart.
00:57
If we turn our attention
to this blue membrane,
surrounding and lining
the thoracic cavity,
that's going to be
the parietal pleura.
01:07
And if we trace it
all the way to where
the trachea and bronchi
start to enter the lung,
we see that it jumps off
that area and turns green
and becomes the visceral pleura.
01:19
Parietal pleura, means the
pleura surrounding a cavity
because parietal means wall.
01:25
So this is essentially the
wall of the thoracic cavity.
01:29
Visceral refers to an organ.
01:31
So visceral means it's on
the surface of the organ itself.
01:35
And these are the same terms
you'll see in the abdomen
when you talk about a different
membrane called the peritoneum.
01:42
But otherwise, parietal and
visceral mean the same thing
whether you're talking about
pleura, or peritoneum,
in the thorax, or the abdomen.
01:49
In between the parietal
and visceral pleura
is a little space called
the pleural cavity.
01:56
Normally, it's very small,
but it's exaggerated here.
01:59
So you could actually appreciate
that there is a space.
02:02
Normally, there's a very small
amount of fluid called pleural fluid
that exists in this cavity,
which helps to reduce the friction
between the two layers of pleura
as the lungs expand to contract
during breathing.
02:16
And then there's this
area of the lung here
little indentation where structures
enter an exit called the hilum.
02:23
And that's a term you might hear
in other organs
where structures enter and exit.
02:27
For example, the kidney or the
spleen, they both have a hilum.
02:32
Now, the visceral pleura,
is largely insensitive to pain.
02:37
It doesn't have the
right kind of innovation,
but the parietal pleura does have
what we call somatic innervation
or innervation by
regular spinal nerves.
02:44
So they are able to produce
a very sharp type of pain
if there's trauma or
inflammation in that area.
02:53
And so different innervations
affect different parts
of the parietal pleura.
02:58
If we start at the area where the
superior thoracic aperture is,
it's relatively close to the neck,
so we call that the cervical pleura.
03:07
And it's innervated by the
nearby intercostal nerves.
03:11
Similarly, the parietal pleura
underneath the ribs,
it's called the costal pleura,
and it's innervated by the
surrounding intercostal nerves.
03:21
The pleura that lines the
superior surface of the diaphragm
is called the diaphragmatic pleura,
and that's innervated by
phrenic nerves.
03:29
Phrenic means diaphragm,
first a diaphragm,
and it's the same nerve
that's going to provide
the motor innervation
to the diaphragm.
03:37
And then the portion of parietal
pleura in the mediastinum
is called the mediastinal pleura,
and it's also innervated
by the phrenic nerves.
03:44
And that's because we'll see
that the phrenic nerves
actually take a route
through the mediastinum
in order to reach the diaphragm.
03:51
Finally, you'll notice that
there are a couple portions
where the lungs don't quite fill
the pleural cavity very well
in the pleural almost forms
a little double fold on itself.
04:02
And we call that a pleural recess.
04:07
So, if we take the lungs out
in order to see the parietal pleura
a little bit better,
we can see how that cervical
pleura is really high up.
04:17
It's right at the superior
thoracic aperture.
04:20
We also see the costal pleura,
as the name implies being
right in the area of the ribs.
04:26
Diaphragmatic pleura,
sitting over the superior surface
of the diaphragm,
and just were expected
the mediastinal pleura
right at the level
of the mediastinum.
04:35
And here we see a little gap there.
04:38
That gap is where the
parietal meets the visceral
because that's where the
lungs starting to make contact
with the pleura here.
04:45
If we zoom in,
we see that that gap
is where something called
the root of the lung
is going to pass through.
04:51
And that stuff like the bronchus,
the pulmonary artery,
and the pulmonary veins.
04:55
All collectively called the
root going through this gap.
05:00
And the parietal pleura in this
area forms this little double fold
that extends inferiorly to
form this anatomic landmark
called the pulmonary ligament.
05:11
And if the lung was there,
it would line up perfectly
with this opening with its hilum,
because that's the indentation
or depression in the lung
where these structures are
going to enter an exit.
05:24
So let's put some of those things in
so you can actually see
the root of the lung
a little bit better.
05:29
First, we see the trachea.
05:31
Coming down before branching into
the right and left mainstem bronchi.
05:36
We have the pulmonary trunk
coming from the heart,
divided into the right and left
pulmonary arteries.
05:44
And coming back from
the lungs to the heart,
we have the right and left
pulmonary veins.
05:51
And after it's wrapped around
this route of the lung,
the pleura here forms
that double fold
inferiorly that we call
the pulmonary ligament.
06:01
So, there's a few other
quirks that are worth
talking about when it comes
to the pleura,
especially when we're looking
at the cervical pleura.
06:09
So, the cervical pleura
is relatively exposed
at the superior thoracic aperture.
06:16
It's relatively
unprotected by bone here.
06:19
So, it's covered by a
thick connective tissue
called the suprapleural membrane
to give a little more
added protection.
06:27
Another term you might
hear is a reflections.
06:30
So a pleural reflection
is where the junctions of
these different pleura meet up.
06:36
For example, we have
the sternal line of reflection,
where the mediastinum and
costal meet anteriorly.
06:44
We have the costal line
of pleural reflection,
where the costal pleura meets the
diaphragmatic pleura inferiorly.
06:54
And then we have the
vertebral line of reflection
where the costal meets
the mediastinal posteriorly.
07:02
We mentioned gaps where the lungs
don't completely fill
as pleural recesses.
07:07
So let's look at a couple
of these pleural recesses.
07:11
One prominent one is out laterally
where it forms a very sharp angle
called the costodiaphragmatic
or costophrenic recess.
07:20
And if you have too
much pleural fluid,
something called a
pleural effusion,
and you're sitting or standing
upright and you get an x ray
fluids going to
accumulate in this recess.
07:31
In fact, you'll hear the
term from a radiologist
very commonly called blunting
of the costal phrenic angle,
and decoding that is
saying, anatomically,
that recess is filled
with excess pleural fluid.
07:45
There's another one more mediately,
the costomediastinal recess,
but it's quite a bit smaller and
a lot less clinically relevant.
07:54
Finally, let's talk about
those nerves briefly.
07:57
The phrenic nerves
are pretty interesting
and have a pretty interesting
embryologic development.
08:03
And you can kind of see that
in action by how long they are.
08:07
They actually originate
up in the cervical area
coming from cervical spine
C3, 4, 5,
and then descend through the
mediastinum to reach the diaphragm.
08:18
So while their major job is motor
innervation of the diaphragm,
they actually innervate a
lot of things along the way
to provide sensory information.
08:26
And we already seen that
with the mediastinum
and diaphragmatic pleura.