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Let's talk about capillaries. There are actually three different major types of capillaries

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I'm gonna just kind of sketch out all three. I've started with the continuous one. I've drew out to save us a little bit of time

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and the continuous capillaries is the one that you see the most commonly throughout the body.

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That's why I'll start with this one. Couple of things you'll notice, you'll see that there are 4 nuclei,

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so four cells here, making up the part of the capillary we're looking at. And there's red blood cell moving in right

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And you actually have the cross-section on the right side so you can actually see if we were to cut along that face that I've cut

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that's what you would actually see. Now there are two specific things that I want to point out.

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One is that there's a little gap here between these two cells. I'm sketching it in yellow just to point it out and

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that gap is called and intercellular,because it's between cells, intercellular cleft.

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So the intercellular cleft is that yellow streak that I just drew. And if was to point it out on that cross-section here

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it would be right here. You can see a little hole between the two where they don't really meet up.

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Now there are two more spots I want to point out. One right there and one right there in yellow. And they correspond to

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this spot and this spot. And there there is really nice joining between the two cells and we call them tight junctions.

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Kind of a good name for it as you can kind of see why they are called that. And those tight junctions are

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right there, labelled with my yellow arrows. Now one thing I haven't drawn, I'm gonna sketch out right here is in green

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and this kind of is a layer beneath all these cells. These cells make up the wall of the capillary but behind them

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so that the blood doesn't actually see this layer, except for at the intercellular cleft,

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is a layer called the basement membrane. So this green stuff that I'm drawing for you is the basement membrane.

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And this basement membrane basically like a foundation for a house.

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It's gonna keep our cells kind of grounded and keep them in place. And this layer is largely made up of proteins

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Let me now show you a second drawing that I did. This is our second type of capillary.

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This is a fenestrated capillary. You can see the major difference between the first one and this one

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is that the second one has little holes that will call fenestrations. Fenestrations

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So this is a fenestrated capillary. And these pores. I'm gonna just label them.

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You can also call them pores or holes. These pores are all over the capillary.

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So we still have, just as before, four cells, four nuclei and one little red blood cell poking its way through.

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And you still have the intercellular cleft. So, just to show you where it is on this one, it's right there

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where the two cells really don't meet up so nicely. There's a little gap

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there. And as before there is a basement membrane. So I'm just gonna sketch out the basement membrane

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all the way around. And on this cross section you can see now how I've tried to draw the best I can to show

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you the pores but you have to kind of now get a little creative to see where that intercellular cleft is

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versus where the pores are. So whenever you're looking at the cross-section, it is a little tricky

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cause you have to almost imagine it in three dimensions. Now the one thing that does help us is that

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on the inside of these endocellular cells, there is, I'm gonna draw in blue,

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a little layer, almost like a slime. And this slime layer is called Glycocalyx. Glycocalyx.

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And what glycocalyx is, is basically sugars that are attached to proteins.

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This kind of sugary protein mix is all over the inside layer of the endocellular cells

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and so what it does is that it actually gets across these pores.

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And even though there is a pore there, you might get a little glycocalyx expanding in the pore, and it will

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come across and look like that. The one place where you won't see it is in the intercellular cleft

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cause that's actually a real spot between cells. So you have the intercellular cleft

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like you do here, let me draw an arrow down there. Right there, you won't see any glycocalyx there

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So we call that little bit of glycocalyx that's bridging the pore, we call that the diaphragm. Diaphragm.

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So these cells. So these fenestrated capillaries actually have diaphragm over their pores.

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And I'm gonna put a little star next to that because sometimes you can have fenestrated capillaries that do not have this glycocalyx

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that's covering the inside and they therefore do not have diaphragm.

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So this is something that is generally true but not always true.

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So let me show you the third type of capillary. So let me show you this last drawing

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and this is actually the largest of the capillaries.

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And this one we call this a discontinuous, discontinuous capillary

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And another name for it, discontinuous capillary, is, sometimes they call it sinusoids

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I'm just gonna write that right here as well. Sinusoids.

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So these ones are often found in the liver, that's kind of the most popular place

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sometimes the spleen as well or bone marrow.

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These ones are actually-few things- are the largest one. So let me make a little list over here.

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They are very large. And they have a lot more of this intercellular cleft space. Look at all these gaps between the cells right

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And I'm just going to sketch it in yellow. Just to highlight it but there's a lot of gap here between the cells

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meaning that these capillaries end up being very leaky. So in addition to being large, they are very leaky

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And a final thing about these guys is that unlike the other two capillaries that we've just talked about

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they have a basement membrane that is often incomplete. So as long as there is a whole area missing just like that

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you might have some basement membrane right here and here but you can see whole chunks

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are missing. Basement membrane. And maybe there's a bit of basement membrane right here.

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So let me write that as a third point. Incomplete, I'm gonna write BM for basement membrane. Incomplete basement membrane

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So if this is the case, it'll be easier for things to kind of escape, even if you have a little glycocalyx here

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Let me draw a layer of glycocalyx on our discontinuous/sinusoid capillary but, even if you have this glycocalyx

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because of the fact that you have all that intercellular cleft space and you don't have many of the tight junctions,

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it's gonna be easier for things to get out. So moving down these three different types you're getting more and more leaky

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as you go down. So just keep that in mind is that the leakiness of the vessel is increasing

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In fact the most leaky is this guy down here, the discontinuous one

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So think with me for a second. Let's say you're a molecule in here

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In the capillary. And you want to get out here, into the tissue. What are the ways you can get there?

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One way would be if you actually just diffuse across. So one way could be diffusion.

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And that would work really well if you were a molecule of oxygen or carbon dioxide.

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It usually works well for these molecules. But let's say you're not one of those molecules

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Let's say you're a larger molecule or charged molecule, how would you get across?

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So the second way you could get across could be through a vescicle.

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Maybe you could get into a vescicle up here in the cell. And the vescicle could transport you from being on the inside,

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which is where this X is, to where it can actually get deposited on the other side, and of course

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it would slow up as it makes it way through the basement membrane but that's at least a way of getting past the cell

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So this is a second approach, maybe a vescicle could carry a molecule through

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A third way could be through this intercellular cleft. Again you still have to get across that basement membrane

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but at least you can get across the cell by simply going around the cell

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So maybe that intercellular cleft could be another ticket to freedom. So if you wanna get around, you can go that way

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that's a third way. So what's that fourth way.

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Now we have to go down to our second drawing, the fenestrated one.

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Here I would suggest maybe just going through-if you were that little X- maybe just going through that pore

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and you have to plow your way though the glycocalyx if there is some there. But maybe that's another way

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going through the fenestration.

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That could be another way across right. So these are four ways for things on the inside to get to the outside

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and as you look at this list that we made, these four options, you can see then that our idea

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around leakiness makes sense. Especially when you get down to the discontinuous vessels at the bottom,

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you've got large gaps between the cells, lots of intercellular clefts, you've got vescicles, that could apply anywhere

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diffusion can apply anywhere, and you've got fenestration. So really every opportunity for things to get out of the

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capillary is available in those discontinuous or sinusoid capillaries