Arteries, arterioles, venules, and veins Learn the differences between these blood vessels! 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.
Arteries, arterioles, venules, and veins
- I wanna figure out how blood gets from my heart, which I'm going to draw here,
- all the way to my toe, and I'm going to draw my foot over here and show you which toe I'm talking about.
- Let's say this toe right here.
- Now, to start the journey, it's gonna have to out of the left ventricle and into the largest artery of the body.
- This is going to be the aorta, and the aorta is very, very wide across, and that's why I say it's a large artery.
- And, from the aorta, I'm actually not drawing all the branches of the aorta,
- but from the aorta, it's going to go down into my belly, and it's going to branch towards my left leg and my right leg.
- So, let's say we follow just the left leg,
- so this artery over here on the top,
- it's gonna get a little bit smaller, and maybe I would call this a medium-sized artery by this point.
- Ah, this is actually now getting down towards my ankle.
- Let's say we've gone quite a distance down to my ankle,
- and then, you know there are, of course, little branches, and let's just follow the branch that goes towards my foot,
- which is this top one.
- Let's say this one goes towards my foot,
- and this is going to be now an even smaller artery.
- Let's call it small artery.
- Small artery.
- From there,
- we're actually going to get into what we call arterioles.
- So it's going to get even tinier, right? It's going to branch.
- Now, these are very, very tiny branches coming off of my small artery, and let's follow this one right here,
- and this one is my arteriole.
- So, these are all the different branches I have to go through,
- and finally, I'm going to get into tiny little branches,
- I'm going to have to draw them very, very skinny just to convince you that we're getting smaller and smaller.
- Let me draw three of them.
- Nah, let's draw four, just for fun. Ah, four of them,
- and this is actually gonna now get towards my little toe cells, so let me draw some toe cells in here to convince you that I actually have gotten there.
- Let's say one, two over here, and maybe one over here, and these are my toe cells.
- Toe cells.
- And after the toe cells have kind of taken out whatever they need.
- Maybe they need glucose, or maybe they need some oxygen.
- Whatever they have taken out,
- they are also gonna put in their wastes,
- so they have, of course, some carbon dioxide waste that we need to drag back.
- This is now going to dump into what we call a venule.
- A venule,
- and this venule is gonna basically then feed into many, many other venules.
- Maybe there's a venule down here coming in, and maybe a venule up here coming in.
- Maybe from the second toe,
- and it's gonna basically all kind of gather together,
- and again, to a giant, giant set of veins.
- You know, maybe veins are dumping in here now, maybe another vein dumping in here,
- and these veins are all going to dump into an enormous vein that we call the inferior vena cava.
- I'll write that right here.
- Inferior vena cava,
- and this is the large vein that brings back all the blood from the bottom half of the body.
- There's also another one over here called the superior vena cava,
- and this is bringing back blood from the arms and head.
- So these two veins, the superior vena cava and the inferior vena cava, are dragging the blood back to the heart,
- and, generally speaking, these are all considered, of course, veins.
- Let's back up now and start with the large and medium arteries.
- These guys together are sometimes referred to as elastic arteries.
- Elastic arteries,
- and the reason they're called elastic arteries,
- one of the, one of the good reasons why they're called that is that they have a protein in the walls of the blood vessel called elastin.
- They have a lot of this elastin protein,
- and, if you think about the word elastin or elastic,
- you know, obviously similar words,
- you might think of something like a rubber band or a balloon,
- and that's probably the easiest way to think about it.
- If you have a blood vessel, one of these large arteries for example,
- and let's say blood is under a lot of pressure, right, because the heart is squeezing out a lot of high pressure blood.
- This artery is literally going to balloon out, and if you actually looked at it from the outside,
- it would look like a little sausage.
- Something like this, where it's puffed out.
- So what's happened there between the first and second picture is that the pressure energy,
- so the heart is squeezing out a lot of pressurized blood, and of course, there's energy in that blood, right?
- That pressure energy has been converted over into elastic energy.
- It's actually converting energy. We don't really always think about it that way, but that's exactly what's happening,
- and when you convert from pressure energy to elastic energy,
- what you are really then doing is you're balancing out those high pressures.
- So you're balancing out high pressures.
- High pressures,
- and this is actually very important because the blood that's coming into our arteries is under, let's not forget, high pressure.
- So the arteriole system, we know, is a high pressure system.
- So this makes perfect sense that the first few arteries, those large arteries and even those medium-sized arteries
- are going to be able to deal with the pressure really well.
- Now let me draw a little line here just to keep it straight.
- The small artery and the arteriole, these two, these are actually sometimes called the muscular arteries.
- Muscular arteries,
- and the reason, again, if you just want to look at the wall of the artery, you'll get the answer.
- The wall of the artery is actually very muscular.
- In fact, specifically, it's smooth muscle.
- So not that kind of muscle you have in your heart or in your bicep, but this is smooth muscle
- that's in the wall of the artery, and there's lots of it.
- So again, if you have a little blood vessel like this, if you imagine tons and tons of smooth muscle on the outside,
- so let's draw it like this.
- Little bands of smooth muscle.
- If those bands decide that they want to contract down, if they want to squeeze down,
- you're going to get something that looks like a little straw because those muscles are now tight.
- They are tightly wound, right?
- So you're going to create like a little straw, and this process is called vasoconstriction.
- Vaso- just means blood vessel, and constriction is, kinda, tightening down.
- So vasoconstriction, tightening down of the blood vessel, and what that does is it increases resistance, right?
- Just like if you're trying to blow through a tiny, tiny little straw, there's a lot of resistance.
- Well, it's the same idea here, and actually a lot of that resistance and change in the constriction, or vasoconstriction, is happening at the arteriole level.
- So that's why they're very special, and I want you to remember them.
- From there, blood is going to go through the capillaries.
- I didn't actually label them the first time, but let me just write that here.
- Sometimes, they call them capillary beds.
- I'll write that out,
- and then it's going to go and get collected in the venules and eventually into the veins,
- and the important thing about the veins,
- I'm going to stop right here and just talk about it very briefly, is that they have these little valves,
- and these valves make sure that the blood continues to flow in one direction.
- So one important thing here is the valves,
- and remember the other important thing is that they are able to deal with large volumes.
- So unlike the arteriole side, where it's all about large pressure,
- down here with the vein side, we have to think about large volumes.
- Remember, about two-thirds of your blood at any point in time is sitting in some vein or venule somewhere.
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