Growing up in the 90s, the only time I ever saw lung anatomy was in the context of “smoking
bad here’s a black lung”.
This image accomplishes that pretty well, but like any curious anatomist, I want to
go deeper than surface level.
So in today’s video we’ll take a look at the histology of the respiratory system
— all the cells of the trachea, bronchioles, alveoli, and all the gooey stuff in between.
[intro]
If you’re new to the channel, welcome, my name is Patrick and this channel is all about
anatomy and how we learn about it.
As always, I have the accompanying notes for this video linked in the description if you
want to check those out.
Otherwise, let’s get started.
The whole purpose of the respiratory system is to handle gases — both breathing in oxygen
and excreting carbon dioxide and other waste gases.
So our anatomy for breathing is split between two main zones: the biggest is the conducting
zone which features a bunch of pipes and chambers that filter and transport air from the outside
to the inside, as well the respiratory zone, which is where gas exchange actually happens.
I mention those two zones because that context makes our study of histology much easier.
The conducting zone will be lined with epithelium to make a smooth, clean path for air to travel
through, while the respiratory zone is going to be thin and porous so that molecules of
gas can escape.
But we’ll get to that.
Let’s start up top at the nasal vestibules, less than an inch behind the nostrils, and
like vestibules at church, these things are chambers of empty space.
They’re lined mostly with keratinized stratified squamous epithelium — that’s a lot of
words, so let’s break that down.
These are multiple layers of flat, or squished epithelial cells.
But they’re keratinized.
They’re no longer living cells, but sacs filled with the protein keratin, just like
the cells on the outermost layer of our skin.
We might also see some hairs which are there to filter big chunks of dust and keep them
out of our lungs.
On the superior part of the nasal cavity, we have the cells that actually do the smelling.
These are olfactory cells, olfactory just being the science word for sense of smell.
But it’s not just sensory cells in that region, there are also supportive cells.
The nasal vestibules and oral cavity transition into stratified squamous epithelium, so alive
cells this time, not keratinized cells, and then into pseudostratified columnar epithelium
with cilia.
We’ll see this exact same tissue type along most of the respiratory system, so we call
it respiratory epithelium.
Those cilia can move and beat back and forth, driving something called the mucociliary escalator,
a strategy for getting gunk out of our lungs.
Cells like goblet cells will pump out mucus which traps any kind of debris or dust, then
the cilia move it upwards into the pharynx so it can either get swallowed down the esophagus
or coughed up.
Either way, we need those cilia to move around mucus.
Behind the nose and mouth is the pharynx, but depending on the region, it’ll have
different histology.
The nasopharynx behind the nose is coated in respiratory epithelium while the oropharynx
in the throat is coated in stratified squamous epithelium.
And they’re different because they deal with different stresses, right, the nasopharynx
deals with air while the oropharynx is constantly being pelted with food and water, so it has
a beefier lining.
That stratified squamous tissue extends down the esophagus too, but that’s creeping into
digestive system histology which I already made a video about.
Link in the description if you want to check it out.
Before we go down the airway into the lower respiratory passages, we might bump into the
tonsils, bundles of lymphatic tissue, which technically count as connective tissue.
As that breath continues, it passes from pharynx to larynx.
Since those words sound so similar, I use their first letters to tell them apart.
The pharynx makes a P shape as it joins the nasal area, while the larynx is a narrow tube,
or a lower case L.
The larynx is mostly connective tissue and muscle, providing a stiff opening for the
rest of the airway as well as a seat for the vocal cords.
I picture the larynx as the nozzle on the end of a rubber hose, it’s a similar idea
— a stiff opening to the tube where some kind of flow regulation can happen.
It opens to the trachea, another stiff tube about 10-12 centimeters long and 2 centimeters
wide.
It’s made of cartilage and connective tissue on the outside, but lined in respiratory epithelium
on the inside with a smattering of those mucus-producing goblet cells.
But they’ve also got cells with little microvilli on them, called brush cells when present in
the trachea, and they can detect certain irritating chemicals and initiate an immune response.
That mucociliary escalator is still important as we move down the respiratory tree.
We still need to move gunk out of our lungs.
Now, when studying gross anatomy, we can see the C-shaped cartilage rings of the trachea.
So on histology, you can orient yourself easily.
The inside of the trachea is lined in epithelium while you’ll find lamina propria, smooth
muscle, and then the glassy, hyaline cartilage as you move away from the lumen.
The trachea then splits into the two bronchi, and as far as histology is concerned, they’re
really similar to the trachea.
More respiratory epithelium on the inside.
Scattered throughout the bronchi, we’ll see small clusters of cells that secrete different
chemical messengers.
These neuroendocrine cells are called Kulchitsky cells, and they mainly secrete catecholamines
which are those fight or flight hormones like adrenaline, but they also secrete serotonin,
calcitonin, and bombesin.
Eventually the bronchi split into bronchioles, which is where we see some big differences.
We’ll typically classify bronchioles as anything narrower than 1 millimeter while
bronchi are wider.
We’ll transition from respiratory epithelium to more ciliated simple columnar epithelium
— basically going from pseudostratified to simple, so a true single layer, while still
keeping the cilia.
Both of these structures are surrounded by smooth muscle, which lets them bronchoconstrict,
or get narrower, or bronchodilate, get wider.
Once we go further down the pulmonary tree, we end up at the respiratory bronchioles,
which is where we find alveoli that finally end in alveolar ducts, or alveolar sacs.
We’re now officially in the respiratory zone, which is where gas exchange actually
happens.
To make gas exchange even possible these alveoli need to be really thin and permeable to gases,
but still have some structural integrity.
So we’ll find really thin sheets of cells between neighboring alveoli, like 2 micrometers
thick, made of some wispy connective tissue and epithelial cells.
This separation is called the alveolar septum.
But the real money makers are the pulmonary capillaries between them — the microscopic
blood vessels that directly touch the outside lining of the alveoli.
This cell to cell contact allows oxygen, carbon dioxide and other gases to diffuse across
this thin wall and makes breathing possible.
At this point, we don’t see goblet cells anymore, which makes sense.
The passageways are getting tinier, so we don’t want a ton of mucus building up.
Instead, we see more club cells, a simple cuboidal epithelial cell which produce surfactant,
a substance that allows gases to diffuse through the air sacs more easily.
This is my biggest takeaway.
Alveolar anatomy is set up for optimal gas exchange.
Form influences function at the cellular level.
To maximize gas exchange efficiency, we need lots of surface area for gas to diffuse through
— just like how we have villi that increase the surface area for nutrient absorption in
the intestine.
Sphere shaped alveoli are a good start, but we need a lot, on average around 300 to 500
million of these that cover 80 square meters.
That’s a lot.
And alveolar cells are unique, they’re called pneumocytes: pneumo for lung, cyte for cell.
They come in two variations — the vast majority are type I, which are really small, like down
to 1 twentieth of a micrometer.
They’re thin squamous epithelium joined together with tight junctions, and they share
a basement membrane with the pulmonary capillaries around them, which makes them perfect for
keeping out fluid, but still allowing gas in and out.
Meanwhile, type II pneumocytes are bigger and form little bulges in the alveoli.
They’re larger cuboidal epithelium with microvilli on top, and like the Club Cells,
they secrete surfactant to decrease the surface tension of the alveoli and make gas exchange
happen more smoothly.
These alveoli buff up their immunity with alveolar macrophages, immune cells that constantly
patrol the alveoli in case anything bad snuck in.
The rest of the respiratory system was pretty good at catching things, but in case things
like pathogens or asbestos did get through, we have these immune cells.
Definitely a good idea to have a last line of defense, since your bloodstream is like,
/right there/.
Throughout this video, I referenced other pulmonary physiology videos I think you should
watch.
Definitely watch this one about the physics of breathing — I go over partial pressures
and explain the role of surfactant in depth.
The other is my spirometry video, a deep dive into some of the clinical metrics you’ll
have to interpret when learning this system.
Otherwise, thanks to my patrons on Patreon, and make sure to subscribe and click the bell
so you get notified when I post a new video.
Have fun, be good, thanks for watching.