I have a quiz for you.
Both of these circles are chopped up sections of your blood vessels.
One is an artery, the other is a vein.
If you think you know which one is which, leave your answer in the comments and I’ll
give you the answer at the end of the lesson.
All done?
Let’s get into the video.
Hello and welcome, if you’re new here, my name is Patrick and this channel is all about
anatomy and physiology.
Today we’re learning the anatomy and histology of blood vessels.
There are all kinds of different blood vessels around the body.
The biggest are about as wide as your thumb while the tiniest are microscopic.
But from a histology perspective, the general structure of any given artery or vein is the
same as any other artery or vein.
And by learning their general structure, we can learn more about their function, which
is transporting blood around the body.
Real quick terminology first.
Arteries are the blood vessels that take blood away from the heart.
You can remember A for artery and away.
In systemic circulation, they carry oxygen rich blood to the rest of the body while veins
carry deoxygenated blood back to the heart.
The big naming exception that you need to be aware of is within pulmonary circulation,
when blood vessels go from the heart to the lungs and back to the heart.
In that case, the pulmonary artery is still going away from the heart, and the pulmonary
vein is still returning to the heart, but since it’s getting shipped off to receive
oxygen, blood in pulmonary circulation is deoxygenated in the artery, and oxygenated
in the vein.
Now, since arteries and veins are hooked up to different structures and have slightly
different tasks, they’re built differently.
Arteries are under much more pressure than veins, and veins need to make sure that blood
doesn’t flow backwards.
And to that end, they have some anatomy that helps them do that.
At its most basic level, a blood vessel is a tube.
That empty space inside the tube, and all anatomical tubes for that matter, is called
the lumen.
Then each vessel — whether it’s an artery or a vein — is going to be wrapped up with
three different layers, each of which has its own job.
There’s the tunica intima, the innermost one, the tunica media, the middlest one, and
the tunica externa, the outermost one.
Tunica translates to coat in Latin, so I think of each layer of a blood vessel as a different
layer of clothing.
The tunica intima is a thin layer of cells meant to be smooth and accommodate the flow
of the blood through the vessel.
It’s kind of like wearing your base layer or undershirt.
It’s flexible and reduces friction between your skin and your heavier layers, but it
doesn’t offer a ton of protection by itself.
Around that is the tunica media, a layer of smooth muscle that can contract and expand
to accommodate blood flow.
This is like wearing your hoodie.
You can always zip or unzip, go hood up or hood down, or pull the drawstring depending
on how cozy you want to be.
A hoodie's main appeal is that it can adapt to changing needs — just like the tunica
media.
But to protect us against the turbulent environment of the human body, we’re going to need even
more.
The tough layer on top is the tunica externa, sometimes called the tunica adventitia.
It’s a layer of connective tissue, mostly type 1 collagen and some elastic connective
tissue if we’re looking at the arteries.
This layer connects the fleshy blood vessels to organs so that they’re not just flailing
around.
The externa is like your leather jacket.
It’s tough and durable, and if you lean against a wall for too long, you will stick
to it.
Now, I’m a big proponent of learning why things get their names over mnemonics, and
in this case it’s simple.
The lumen is always the empty void within a tube.
Lumen comes from the Latin word for light, like the Harry Potter spell lumos for the
flashlight wand thing.
So the lumen is the lightest, brightest spot on a slide.
From there, there are the tunicas, or coats around it which just get the layer names of
inner, middle, and outer.
Alternatively, you can remember LIME — lumen, tunica interna, media, externa.
Whatever works for you.
Okay, now that we know the layers, we can finally look at these things under the microscope.
Here’s a basic muscular artery, and right away, we can tell that we’re looking at
a cross section — slicing the artery across like it’s a banana.
Right away we see that empty space in the middle, so we know that’s the lumen.
That’ll be our starting point for the rest of the identification.
From there, we know we’ll find the tunica intima, media, and externa as we move out
from the lumen.
The tricky thing is finding exactly where one layer ends and another begins.
Luckily, each layer is made of totally different materials, so as long as you know basic tissues,
you’ll be okay.
By the way, I’ve got videos on all these tissue types linked in a playlist down in
the description or right here if you need a refresher.
If we zoom in, we’ll see a thin layer of simple squamous epithelial cells.
In anatomy, this layer gets a special mashup name — the endothelium since it’s the
innermost epithelium.
If this layer gets injured, it can actually lead to plaque buildup and eventually blockage
of the artery which is how you get a heart attack.
Around the endothelium is a thin basement membrane and a layer of connective tissue
that connects the tunica interna to the media.
This is the subendothelial connective tissue.
And that’s it.
The tunica interna is a razor thin layer of endothelium and some connective tissue.
On this diagram, it’s a pretty clear color separation.
That means this next layer is the tunica media, our layer of smooth muscle.
In general, this is what smooth muscle looks like.
It’s kind of spindle shaped, with a big nucleus right in the middle of each muscle
cell.
And when we look at the tunica media on a histology slide, ohh boy do we see those cells.
This thick layer goes all the way around the vessel.
Importantly, this is one of the big hints that lets you tell an artery apart from a
vein.
Arteries are much more muscular than veins.
So when you’re looking at a cross section under a microscope, you’ll see bigger rings
of muscle around arteries than veins.
That also gives arteries a bit more structure — so arteries will typically be rounder
while veins can get flattened out.
That’s it for the tunica media.
It’s all muscle.
Now we’re at the tunica externa, which is all connective tissue.
When I’m identifying connective tissue, I’m mostly looking for fibers and extracellular
matrix.
This layer is going to be light on nuclei and heavy on those jumbled chaotic fibers.
Then it kind of tapers off into the next structure, which will likely be connective tissue, or
you’ll just see the blank space of a slide.
So in summary: Step 1, find the lumen.
Step 2, identify the tissue type so you can find the boundaries of the three tunicas.
Now, that’s for the platonic ideal of blood vessels.
There are all kinds of different blood vessels throughout the body that deserve special attention.
Like not all endothelial layers are the same.
The most common type is a continuous layer of endothelium connected by tight junctions.
Just like all of anatomy, function follows form here — it’s a smooth tube meant to
transport blood.
But there are also fenestrated capillaries which is Latin for window.
This type of capillary has little pores in them to more easily transport certain materials
like water and big solutes out of the blood.
We saw these back in my kidney video when we looked at the glomeruli (video up here).
Finally, we have discontinuous capillaries that have bigger gaps between the epithelial
cells.
We see this type in the spleen, bone marrow, and liver.
And those are just differences in the innermost layer.
There are all kinds of differences in outer layers as well.
Take the elastic arteries for example.
As the name implies, they have much more elastic connective tissue in them.
You see this type of vessel in the aorta, pulmonary trunk, and common carotid arteries.
And that extra elasticity helps them deal with high pressures.
On the histology slide, that looks like a layer of wavy elastic tissue between the tunica
interna and media called the internal elastic laminae, and another layer between the media
and adventitia called the external elastic laminae.
Laminae is latin for a thin sheet by the way, just like a laminator machine.
Now, these arteries are big wide structures, which is usually their giveaway on a histology
test, but they also might have an additional structure.
Since these vessels are so thick, it’s hard to get the outer layers enough oxygen and
nutrients from simple diffusion alone.
So on some slides, you’ll be able to see the vasa vasorum, which literally translates
to the vessel of vessels.
It’ll be a little blood vessel poking out to the external layers of big blood vessels
like the aorta.
As the cardiovascular system branches off into tinier blood vessels, we see less elasticity
and more muscularity, but also with tinier diameter arteries.
And as the vessels continue branching out, we’ll get to the ultra thin capillaries.
This is where gas or nutrient exchange takes place, and their thinness allows molecules
to pass through their walls without leaking blood.
In histology, we’ll usually see these as cross sections.
Capillaries are only a single layer of endothelial cells, and they don’t have a muscular tunica
media.
That makes them flimsy vessels without any rigidity really, so slicing it lengthwise
like a hotdog bun would be really tricky.
Once we move over to the venous side of the vascular system, we see some other features.
Veins have larger lumens compared to arteries of the same diameter, which means they deal
with less pressure than arteries.
Now, that helps us identify them on the cross section, but there’s another big feature
we’ll see on the longitudinal section — valves.
Most veins will have valves that help prevent backflow of blood in circulation.
On a histology slide, these look like little branches that extend into the lumen.
They won’t always extend across the entire lumen depending on how the slide was prepared,
but if you see something thin poking into the lumen, there’s a good chance you found
a valve.
So let’s come back to the quiz from the beginning of the video.
If you guessed that Figure 2 was an artery and Figure 1 was the vein, you’re correct.
We can tell because Figure 2 has a much thicker tunica media, while Figure 1 has a much larger
lumen.
Now, blood vessels won’t usually be isolated like we’ve seen in this video.
They’ll be mixed in with the muscles and connective tissue of big organs.
Here’s an example of how that looks.
We can start identifying structures by first finding the big artery with its muscular tunica
media and round shape.
Around it, we see tinier blood vessels — what are probably arterioles.
Over here we have a more squished vessel, with a thinner tunica media, so it’s probably
a vein.
If you got it right,and you were just anticipating my every word on this one, let me know in
the comments!
If you want more examples of how I find structures on real slides, you can check out this video
where I walk you through a histology quiz start to finish.
And for more histology lessons, you can check out this playlist here.
Also if there’s a tissue type you want to see, feel free to message me on social media
and I’ll try to make it.
Otherwise, like the video, thank you to my patrons on patreon.
Have fun, be good thanks for watching.