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.Created by Rishi Desai.
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.