00:01
So what's the histology of the lung? If you
get a microscope and look at the epithelium
of the conducting airways, what does it look
like? Well, largely, the conducting airways
will have columnar epithelium, so that's a
single layer of cells based on a basement
membrane. There are oblong in shape. The nucleus
tends to be at the base on the basement membrane,
and then there'll be cilia projecting out
the surface of the epithelial cell. And the
cilia are protein structures which allow mucus
to move back up the airways and out of the
lungs, and that is a primary immune mechanism
for defending the lung against anything that
is inhaled that should not be there.
00:44
Underneath this layer of squamous epithelium,
there will be various submucosal structures:
a little bit of muscle around the main bronchi,
some fibrocartilaginous layers, connective
tissue, etc. Interspersed in the mucosal layer,
there will be the occasional goblet cell,
which produces mucus, and there are also submucosal
glands which produce mucus, which are shown
here in the middle part of this diagram, with
an opening coming out into the epithelium
from which mucus is secreted by the cells
in the submucosa. As the airway goes… as
you go down the airway towards the bronchioles,
the amount of submucosa thins out, and the
height of the columnar epithelium becomes
smaller, and eventually, you'll lose your
cilia as well.
If you were to look at the alveolar epithelium
in more detail, it'd look like this. There
would be, as we described just now, an alveolus
containing air. There's a type 1 pneumocyte
there, you can see, which is forming a very
large surface area of the alveolus. There's
the occasional type 2 pneumocyte, with its
synthetic function, and then in between alveoli,
you can see, there's a small capillary with
red cells in the middle and a filial cell
forming a very thin capillary wall. And this
is important, this structure, because of the
small gap between the alveolus and the red
cell in the capillary allows diffusion of
oxygen from the alveoli into the red cells
much more rapidly than it would be if there
was a larger gap. The interstitium, which
is in the middle, really is very thin in the
lung and should remain so for good gas exchange.
02:31
I've mentioned already the visceral pleurae.
So the pleurae are two layers which are consist
of flattened squamous cell mesothelial cells—a
single layer, a monolayer, which covers the
underside of the thoracic cavity, so the ribs
and the muscles of the parietal cavity of
the parietal pleurae and the surface of the
lung, the visceral pleurae, with a small gap
between the two. Now that gap has a little
bit of fluid in it in normal people, but it's
a potential gap that can be filled up with
air or filled up with fluid during disease
situations. The visceral and the parietal
pleurae merge at the hila, where there's a
hole in the visceral pleurae created by this
merging with the parietal pleurae that allows
entry of the blood vessels and the right and
the left main bronchi into the lungs. The
nerve supply to the pleurae is from the intercostal
nerves that we described earlier as lying
underneath each rib. The nerve supply to the
pleurae, which overlies the diaphragm, is
from the phrenic nerve.
So there is a pleural space, but it contains,
in normal people, a minimal amount of fluid,
acts as a sort of oil so that when you breathe,
things can move nice and smoothly. The fluid
forms from the bronchial intercostal circulation,
so you get fluid oozing out from the lungs
from the bronchial circulation and from the
intercostal circulation but then drains down
the lymphatics. And in general, that fluid
formation and drainage is in balance, so the
pleural space does not expand. But when you
get disease that increases the amount of fluid
coming out from the bronchial intercostal
circulation (such as heart failure) or impairs
the lymphatic drainage (if you've got abnormal
pleurae, for example), then you may end up
with pleural fluid forming in the pleural
space and accumulating there to cause what
we call a pleural effusion.
04:27
I've mentioned the hila already. This is a
diagram showing them. So the gray part of
the lung is covered in visceral pleura, and
the gap where the visceral and the parietal
pleurae fuse is right there in the middle
of the medial aspect of each lung. And you
can see the variety of blood vessels and airways
that enter the lung and nerves at that point
on either side.