Layers of a blood vessel Remember the 3 key layers of a blood vessel (Tunica intima, Tunica media, and Tunica externa) and how arteries, veins, and capillaries are all different from one another. Rishi is a pediatric infectious disease physician and works at Khan Academy.
Layers of a blood vessel
- Let's talk about blood vessels
- I'm going to talk to you about the different types of blood vessels that there are
- and we're going to see if we can come up with some general patterns.
- So let's start with the uh- let's say we take the blood vessel out of your body
- and examine it under a microscope.
- It would look something like this, where you have the middle: called the lumen.
- and right outside of the lumen you have cells.
- And these cells are going all the way around the lumen
- and they're the first type of cell that any blood would interact with, right?
- Because that's right immediately outside of the lumen.
- And on the other side of the cells, on the back side of the cells-if you think of the lumen as the front side-
- is a little line I'm gonna draw here to represent protein that sits there and
- acts as a scaffold, kind of like keeping all of the cells in place.
- So you have all of these endothelial cells.
- And then you have that little basement membrane- like a thin line of membrane- called the basement membrane
- And that keeps everything from falling out of place.
- And the thing that's in the basement membrane is mostly protein.
- So think about thinks like collagen which you can find
- in bones and your gums. That same protein is actually
- lining all of these blood vessels that are in your body.
- And together the basement membrane and endothelial cells, they make up a layer
- called the Tunica Intima.
- And that's kind of the word we use because "tunica" comes from the word for coat or cloak.
- And "intima" is kind of the intimate layer, the intimate coating of the vessels
- So Tunica Intima.
- Now right outside of this is another layer- and I'm gonna draw that right out here in red.
- And this will be a layer of muscle. Image these little lines
- represent smooth muscle cells. So this would be muscle and
- this would be the layer called tunica media, or middle.
- And the most important thing here is that there is smooth muscle in this layer.
- And finally, on the very outside, you have another layer.
- And I'm gonna draw it as a yellow line.
- And this yellow line represents more protein. This is another layer of protein,
- just like the basement membrane, but different composition of proteins.
- But again you see actually a lot of collagen, just as before, and some other proteins as well.
- And this is called the tunica externa. For external layer.
- Actually another word that they sometimes use for this is called "Adventitia." So you might see that word too.
- And the really interesting thing about the tunica externa is that you actually, on the large vessels,
- you find that they need blood to supply themselves, so some of these large vessels have little
- blood vessels on them. And you might think- well does this just go on endlessly?
- Ya know, vessels that have vessels on them and those little vessels have vessels on them and so on and so forth.
- Actually No. It's just the large vessels that have this. So you SOMETIMES, not always,
- see what's called vasa vasorum. And this is kind of a fancy name for it.
- But they're little blood vessels on blood vessels. Which I always thought was
- kind of a cool thing. So the tunica externa has the blood vessels
- and it also has nerve endings. And I'm not going to actually draw that in because it'd be
- kind of hard to show that. But nerve endings actually can be in that layer. In that tunica externa layer.
- So you've got three layers, each of them has some pretty cool things in them.
- And now what I thought we'd do is we'd go and think about each vessel type, and how this could look for that vessel.
- So let's start with the veins. And I'll do the veins over here.
- So let's say you have a vein. On the inside it has that tunica intima layer -and I'm not going to draw all of the
- cells out because now you know what that represents. That purple line represents the endothelial cells AND the basement membrane.
- And you also have a smooth muscle layer- so kind of a red line to represent tunica media.
- And veins have a tunica externa- a third layer.
- So veins kind of follow this general pattern. They have three layers and they're pretty straight forward in
- following exactly what I just talked about.
- Now arteries. Let me do a different color for arteries. Let's say red.
- I think of arteries in two different groups. So I think of large and middle sized arteries a little differently than I think about small arteries or arterioles.
- And you'll see that the small arteries and arterioles actually look quite similar to one another.
- So let's start with the large and middle sized arteries.
- So there you have, let's say, an inside. And then on the outside of that, on the tunica media layer-
- you start seeing a difference from the vein. You see that it's much larger, much larger.
- And in the vein you just had kind of a thin layer of smooth muscle, here you have a nice large layer of smooth muscle.
- And in addition to having these smooth muscle cells in the large and middle arteries you have an interesting
- new thing. You have something called elastin protein. And elastin is gonna be in here and this is a protein that
- helps make these very elastic. Right because they're going to have such high pressures that
- they have to be able to not break when these pressures are going through.
- So this elastin (this green elastin protein) helps make the arteries more elastic. More elastic for high pressures.
- And I'll write "P" for pressures.
- So that's what the middle layer looks like. And of course you still have your outer layer as before.
- So you've seen one key difference now with these large and middle sized arteries.
- Okay so now let's go to the small arteries and the arterioles.
- So here let's start out with the lumen, has that as before.
- And you also have a large tunica media.
- So unlike the vein, the tunica media in the arteries-in both types of arteries- is actually quite large.
- Except here, instead of having elastin in this area, you have just tons and tons of muscle.
- Lots of muscle. So they're very very strong in the sense that you can actually very easily squeeze down on the lumen
- if you wanted to by having all of this muscle contract.
- And that makes sense because we know that the arterioles - look at all of that muscle- the arterioles are actually
- going to be the ones that are going to create resistance. So here you have a lot of muscle.
- And this is to help create lots of resistance. If you need it, to change your blood pressures.
- So that's why the small arteries and arterioles are really helpful in squeezing down and changing blood pressures.
- And on the outside, they as before, have that layer of tunica externa.
- And this again is where the nerve endings go,and where the blood vessels, or the vasa vasorum go. So that's where that layer is.
- Okay. And you might be thinking there's gotta be one more vessel type and there is-
- and that's the capillary. So let me draw the capillary, so I'll do that over here.
- The capillary is actually going to look different from all of this other stuff that we've done because the capillaries
- are actually really really unique in the sense that-unlike what we had for the veins and arteries- this is down to the single cell level.
- So you still have a lumen but on the outside, instead of having a lot of endothelial cells
- you actually have, at times, one cell. One cell will actually form the entire wall of the tube.
- So you actually a cell travelling within a cell. Almost like the cell is giving the little red blood cell
- floating through it a big hug or something like that.
- So that capillary looks actually quite different as you can see from the other things that we've drawn: the veins and arteries.
- But I want you to just take a look at this again and see the key differences between the veins and arteries.
- The vein follows the general pattern. But the large and middle arteries have a lot of elastin in the tunica media.
- And the small arteries and arterioles have a lot of muscle in that tunica media.
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