Three types of capillaries Learn the differences between continuous, fenestrated, and discontinuous capillaries, and how they affect the movement of molecules. Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video.
Three types of capillaries
- Let's talk about capillaries. There are actually three different major types of capillaries
- 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
- and the continuous capillaries is the one that you see the most commonly throughout the body.
- That's why I'll start with this one. Couple of things you'll notice, you'll see that there are 4 nuclei,
- so four cells here, making up the part of the capillary we're looking at. And there's red blood cell moving in right
- 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
- that's what you would actually see. Now there are two specific things that I want to point out.
- 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
- that gap is called and intercellular,because it's between cells, intercellular cleft.
- So the intercellular cleft is that yellow streak that I just drew. And if was to point it out on that cross-section here
- it would be right here. You can see a little hole between the two where they don't really meet up.
- Now there are two more spots I want to point out. One right there and one right there in yellow. And they correspond to
- this spot and this spot. And there there is really nice joining between the two cells and we call them tight junctions.
- Kind of a good name for it as you can kind of see why they are called that. And those tight junctions are
- right there, labelled with my yellow arrows. Now one thing I haven't drawn, I'm gonna sketch out right here is in green
- and this kind of is a layer beneath all these cells. These cells make up the wall of the capillary but behind them
- so that the blood doesn't actually see this layer, except for at the intercellular cleft,
- is a layer called the basement membrane. So this green stuff that I'm drawing for you is the basement membrane.
- And this basement membrane basically like a foundation for a house.
- It's gonna keep our cells kind of grounded and keep them in place. And this layer is largely made up of proteins
- Let me now show you a second drawing that I did. This is our second type of capillary.
- This is a fenestrated capillary. You can see the major difference between the first one and this one
- is that the second one has little holes that will call fenestrations. Fenestrations
- So this is a fenestrated capillary. And these pores. I'm gonna just label them.
- You can also call them pores or holes. These pores are all over the capillary.
- So we still have, just as before, four cells, four nuclei and one little red blood cell poking its way through.
- And you still have the intercellular cleft. So, just to show you where it is on this one, it's right there
- where the two cells really don't meet up so nicely. There's a little gap
- there. And as before there is a basement membrane. So I'm just gonna sketch out the basement membrane
- 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
- you the pores but you have to kind of now get a little creative to see where that intercellular cleft is
- versus where the pores are. So whenever you're looking at the cross-section, it is a little tricky
- cause you have to almost imagine it in three dimensions. Now the one thing that does help us is that
- on the inside of these endocellular cells, there is, I'm gonna draw in blue,
- a little layer, almost like a slime. And this slime layer is called Glycocalyx. Glycocalyx.
- And what glycocalyx is, is basically sugars that are attached to proteins.
- This kind of sugary protein mix is all over the inside layer of the endocellular cells
- and so what it does is that it actually gets across these pores.
- And even though there is a pore there, you might get a little glycocalyx expanding in the pore, and it will
- come across and look like that. The one place where you won't see it is in the intercellular cleft
- cause that's actually a real spot between cells. So you have the intercellular cleft
- like you do here, let me draw an arrow down there. Right there, you won't see any glycocalyx there
- So we call that little bit of glycocalyx that's bridging the pore, we call that the diaphragm. Diaphragm.
- So these cells. So these fenestrated capillaries actually have diaphragm over their pores.
- And I'm gonna put a little star next to that because sometimes you can have fenestrated capillaries that do not have this glycocalyx
- that's covering the inside and they therefore do not have diaphragm.
- So this is something that is generally true but not always true.
- So let me show you the third type of capillary. So let me show you this last drawing
- and this is actually the largest of the capillaries.
- And this one we call this a discontinuous, discontinuous capillary
- And another name for it, discontinuous capillary, is, sometimes they call it sinusoids
- I'm just gonna write that right here as well. Sinusoids.
- So these ones are often found in the liver, that's kind of the most popular place
- sometimes the spleen as well or bone marrow.
- These ones are actually-few things- are the largest one. So let me make a little list over here.
- They are very large. And they have a lot more of this intercellular cleft space. Look at all these gaps between the cells right
- 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
- meaning that these capillaries end up being very leaky. So in addition to being large, they are very leaky
- And a final thing about these guys is that unlike the other two capillaries that we've just talked about
- they have a basement membrane that is often incomplete. So as long as there is a whole area missing just like that
- you might have some basement membrane right here and here but you can see whole chunks
- are missing. Basement membrane. And maybe there's a bit of basement membrane right here.
- So let me write that as a third point. Incomplete, I'm gonna write BM for basement membrane. Incomplete basement membrane
- So if this is the case, it'll be easier for things to kind of escape, even if you have a little glycocalyx here
- Let me draw a layer of glycocalyx on our discontinuous/sinusoid capillary but, even if you have this glycocalyx
- because of the fact that you have all that intercellular cleft space and you don't have many of the tight junctions,
- it's gonna be easier for things to get out. So moving down these three different types you're getting more and more leaky
- as you go down. So just keep that in mind is that the leakiness of the vessel is increasing
- In fact the most leaky is this guy down here, the discontinuous one
- So think with me for a second. Let's say you're a molecule in here
- In the capillary. And you want to get out here, into the tissue. What are the ways you can get there?
- One way would be if you actually just diffuse across. So one way could be diffusion.
- And that would work really well if you were a molecule of oxygen or carbon dioxide.
- It usually works well for these molecules. But let's say you're not one of those molecules
- Let's say you're a larger molecule or charged molecule, how would you get across?
- So the second way you could get across could be through a vescicle.
- Maybe you could get into a vescicle up here in the cell. And the vescicle could transport you from being on the inside,
- which is where this X is, to where it can actually get deposited on the other side, and of course
- 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
- So this is a second approach, maybe a vescicle could carry a molecule through
- A third way could be through this intercellular cleft. Again you still have to get across that basement membrane
- but at least you can get across the cell by simply going around the cell
- So maybe that intercellular cleft could be another ticket to freedom. So if you wanna get around, you can go that way
- that's a third way. So what's that fourth way.
- Now we have to go down to our second drawing, the fenestrated one.
- Here I would suggest maybe just going through-if you were that little X- maybe just going through that pore
- and you have to plow your way though the glycocalyx if there is some there. But maybe that's another way
- going through the fenestration.
- That could be another way across right. So these are four ways for things on the inside to get to the outside
- and as you look at this list that we made, these four options, you can see then that our idea
- around leakiness makes sense. Especially when you get down to the discontinuous vessels at the bottom,
- you've got large gaps between the cells, lots of intercellular clefts, you've got vescicles, that could apply anywhere
- diffusion can apply anywhere, and you've got fenestration. So really every opportunity for things to get out of the
- capillary is available in those discontinuous or sinusoid capillaries
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
Have something that's not a question about this content?
This discussion area is not meant for answering homework questions.
Share a tip
When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
Thank the author
This is great, I finally understand quadratic functions!
Have something that's not a tip or thanks about this content?
This discussion area is not meant for answering homework questions.
At 2:33, Sal said "single bonds" but meant "covalent bonds."
For general discussions about Khan Academy, visit our Reddit discussion page.
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
- disrespectful or offensive
- an advertisement
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
- a tip or thanks in Questions
- a question in Tips & Thanks
- an answer that should be its own question