00:00
So that's the muscles and the skeletal system.
00:02
Now, we'll talk about the respiratory tract
in more detail. We'll talk about the airways
to start with. And the airways start, essentially,
at the nose, but I'm going to talk about the
lower airways only, which start at the larynx.
And this starts with the trachea, which is
a tube that runs from the larynx down to the
division at the carinae into the right and
the left main bronchi. And the order of the
airways are: trachea, bronchi, these divide
into bronchioles, terminal bronchioles, finally
reaching the alveoli at the very bottom, at
the very distal end of the lung. The airways
are there to conduct air to the alveoli, and
in the alveoli, the oxygen in the air gets
into the blood, and the carbon dioxide in
the blood gets into the alveoli. So the business
end of the lungs is the alveoli, and the airways
are all… are really there… just there
to assure that air is supplied to the alveoli.
01:01
The trachea is a D-shape in cross-section,
so posteriorly, there's a flat, tendinous
bit. Anteriorly, there are cartilage curves
with muscle and tendinous insertions... tendinous
layers in between. It's about 11 cm long,
a couple of centimeters in diameter. It run
from the larynx right down to about the level
of that 4th or 5th thoracic vertebra, where
it divides into the right and to the left
main bronchi, and that division is called
the carina. The important thing about the
trachea is that because it's running through
the mediastinum—and the mediastinum is…
are the structures between the right and the
left lung—it's running next to blood vessels,
thyroid gland, mediastinal lymph nodes, the
esophagus, a variety of nerves, and that makes
it at risk of any problems that are affecting
those also affecting the trachea. So, for
example, it's not uncommon for a very large
mediastinal nodes to cause obstruction partially
of the trachea.
02:02
After the trachea, you have the bronchi dividing
into the right and the left lung. And the
lungs themselves are very elastic, spongelike
material consisting of very large numbers
of alveoli. The right lung is slightly larger
than the left lung. The contents of the lung
are the bronchial tree, the alveoli, the blood
vessels, the lymphatics draining the lung.
02:25
And importantly, they're surrounded by a thin
layer of visceral pleura. All the vessels
and all the air… the bronchi (the right
and the left main bronchi) enter the lung
through what we call are the hila, and these
are gaps in the visceral pleurae which are
very medial in the middle part of each lung.
So the bronchial tree arises from the trachea.
02:52
It conducts air into each lung, and it also
has incomplete cartilage rings just like the
trachea does, although it's not a D shape
now; they're more… they're round instead.
03:03
The bronchi branch successively to form smaller
and smaller bronchi, and eventually, you end
up with the alveoli. The main divisions, initially,
of the bronchi are the right and left main
bronchi. The left main bronchi then divides
into a left upper bronchus and a left lower
bronchus. The right side will divide into
a right upper bronchus, and then there's an
intermediate bronchus. And then the intermediate
bronchus will divide into a right middle bronchus
and a right lower-lobe bronchus.
Each of those bronchi will feed air to a specific
lobe. So on the right-hand side, there will
be three main lobes: the right upper lobe,
the right lower lobe, and the right middle
lobe. And on the left-hand side, there would
be a left upper lobe and a left lower lobe.
The functional equivalent of the right middle
lobe on the left side is called the lingula,
and that is part of the left upper lobe. And
these lobes are divided by invaginations of
the visceral pleurae into the lung, and they
form fissures—the main one being the oblique
fissure, which is between the upper lobes
and the lower lobe, with on the right-hand
side being an additional fissure called the
horizontal fissure, which divides the upper
lobe from the middle lobe.
04:13
The bronchi, once they reach one lobe of the
lung, will then divide, and there are segments
for each lobe. And I'm not going to go into
these in detail, because it's… we don't
need to know precisely each segment. But here's
a diagram showing each of the segments are
present. And as you can see, they vary in
number, depending on which lobe we're talking
about. The smaller lobes, such as the right
middle lobe, only having two segments, and
the larger lobes, such as the lower lobe,
having five or... four or five segments. And
each of those segments has an individual bronchus
feeding into it. And accompanying that bronchus,
there's an individual pulmonary artery, pulmonary
vein, and there'll be draining lymph nodes
as well.
The bronchi divide further and further in
each segment, becoming tertiary bronchi and
then small conducting vessels called
terminal bronchioles, and these eventually become respiratory
bronchioles. And one respiratory bronchiole will
feed one gas exchange unit called
an acinus. And that gas exchange unit will
consist of alveoli, which is like small balloon-like
objects opening off the terminal bronchiole
and the alveolar ducts to form a cluster of
air-filled sacs at the end of each bronchiole.
05:23
This is the business end of the lung. This
is where gas exchange occurs. The important
thing about the alveoli is that they are closely
opposed to pulmonary capillaries, so each
alveolus will have a lot of blood vessels—capillary
blood vessels—surrounding it. And therefore,
oxygen can move into the blood vessels from
the alveoli relatively easily. So, for example,
alveoli, about 7% of their surface is covered
in pulmonary capillary. The numbers of alveoli
are enormous. There's about 500 million in
the average adult, and because of that, there's
an incredible surface area of the lungs, allowing
gas exchange to occur over a very large area.
06:03
And the average area is about 75 m2. Alveoli
open off individual alveolar ducts, but there
are the occasional connections between alveoli
called the pores of Kohn.
06:17
If you look at the airway divisions—if you
start at the trachea and then move down to
the alveoli—there's about between 23 and
25 divisions before you get to an alveolus.
06:26
And the actual area of the lung increases
exponentially as you go through those divisions,
with the cent… with the trachea being 2
cm across, but then there's the 500 million
alveoli with a surface area of 75 m2, and
this is shown in this diagram. And you can
see the exponential increase is mainly occurring
in the lower airways.
06:44
This is a table giving the numbers of each
individual air unit—air conductor unit—including
alveoli, their rough diameter, and then, essentially,
their cross-sectional area. And you can…
and this basically gives you the data that
the previous graph displayed, with the exponential
increase in the surface area occurring when
you get down to terminal respiratory bronchioles,
the alveolar ducts, and the alveoli themselves.