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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. Created by Rishi Desai.

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  • leafers ultimate style avatar for user Sleuth
    How can the volume of veins be so much larger than the volume of arteries?
    I know that exactly the same amount of blood has to come back to the heart every second as is leaving the heart since the blood vessels can be seen as closed loop. Because otherwise if always more or less would come in than leave, there would be a buildup in that difference over time that would be catastrophic.

    Now if the volume of the veins is so much bigger, does this mean that the blood flows back slower (in terms of individual cells, not in terms of volume) once inside the veins?
    (31 votes)
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    • leaf yellow style avatar for user Taylor Logan
      Yes, the blood flows back to the heart from the systemic circulation much more slowly than it is ejected from it. As a result, over 60% of the blood is within the veins at any given time.

      The left ventricle contracting creates a relatively huge amount of pressure, which forces blood out and through all the arteries in the body. The increase in resistance in the arterioles causes the blood's flow to slow, and it is further slowed in the capillaries. By the time the blood gets through the capillary beds and venules and into the veins, the blood has quite low pressure and with no driving force behind it, moves slowly.
      (32 votes)
  • orange juice squid orange style avatar for user N Peterson
    the Aorta is the large artery exiting the left ventricle, does some large vein enter the right ventricle?
    (11 votes)
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  • piceratops ultimate style avatar for user Katie
    At it says that the very small arteries are called arterioles. When does an artery turn into an arteriole, and likewise for veins and venules? Does it depend on the size or the structure of the blood vessel or something else?
    (11 votes)
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    • female robot amelia style avatar for user amelia
      Size does define the classification of artery versus arterioles. When there is two or less medial layers of smooth muscle. This is typically less than 0.1 mm diameters.

      Structurally, arteries contain more elastic tissue and the arterioles contain more smooth muscle. Functionally, arterioles contribute more to restriction of blood flow and consequently control total peripheral resistance. There is also some gas exchange in arterioles.
      (17 votes)
  • starky tree style avatar for user Katie
    -- What is the purpose of vasoconstriction?
    (5 votes)
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    • female robot amelia style avatar for user amelia
      To increase the peripheral and systemic vascular resistance. This has many functions, but essentially it moves blood to where it needs to be. When you stand up your legs vasoconstrict. This displaces blood from your legs to the rest of your body. If this did not happen, you would have a large change in pressure everytime you stand. This is called orthostatic hypotension. To compensate your heart my race to pump more blood or you may even pass out, because your in danger of not getting enough blood (and consequently oxygen) to your brain. Passing out gives your body a chance to bring the blood to your brain, since gravity will no longer be competing.
      (12 votes)
  • leafers ultimate style avatar for user Prithvi Shetty
    Are there elastic veins ?
    (5 votes)
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  • purple pi purple style avatar for user Sarah
    - Are the valves in the veins tethered to the walls; or do they not need to be because of the low pressure?
    (6 votes)
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  • duskpin ultimate style avatar for user QueenTabby42
    At , How is the inferior venacava inferior when it is so big?
    (3 votes)
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    • orange juice squid orange style avatar for user briancsherman
      Inferior does not refer to size - it refers to location. The superior vena cava brings blood from the head, arms, and upper body to the heart, and the inferior vena cava brings blood from the legs and lower body back to the heart. In anatomical terms, superior is closer to the head and inferior is closer to the feet.
      (10 votes)
  • piceratops ultimate style avatar for user Ari Mendelson
    If I look at the veins in my arms, I wouldn't expect them to branch off as they get closer to the heart. But upon observation, they in fact DO branch off occasionally (presumably to join up as it gets yet closer to the heart). Why do they do that? Wouldn't it be more efficient to keep joining up smaller vessels and not splitting them up?
    (8 votes)
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  • blobby green style avatar for user keim6418
    What I find to be unlikely and it is implied with all the Red & Blue illustrations is that all the neighboring cells during the journey to the Toe did not take what they need nor deposit any waste. If that does not happen and the path become more blue and less red as it progresses then there must be a dedicated circut for each cell and that seems highly inefficent. Am I right?
    (4 votes)
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    • blobby green style avatar for user Angélica Parada
      Really great question! The thing is larger vessels (arteries AND veins) require their own capillaries to irrigate them, and those capillaries are located right in the outer layer of those vessels, through which those outer cells can deposit waste and obtain oxygen. As you may guess, the inner layers can do both by diffusion from the blood that runs inside of them.

      Regarding the "toe cells", there is not a dedicated circuit for each cell, which is why they are named capillary beds. As the nutrients and waste travel by diffusion, you do not need a single vessel for each cell (one capillary can supply quite a lot of cells).
      (3 votes)
  • male robot hal style avatar for user Andrew R.
    What are the valves in your veins called?
    (3 votes)
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Video transcript

I want to 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 going to have to go 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 going to get a little bit smaller. And maybe I'd call this a medium-sized artery by this point. This is actually now getting down towards my ankle. Let's say we've gone quite a distance down in my ankle. And then 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. From there, we're actually going to get into what we call arterioles, so it's going to get even tinier. It's going to branch. Now, these are very, very tiny branches coming off 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. No. Let's draw four just for fun. And this is actually going to now get towards my little toe cells. So let me draw some toes cells in here to convince you that I actually have gotten there. Let's say one, two over here, and maybe one over here. These are my toes 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've taken out, they're also going to put in their waste. 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. And this venule is going to 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 going to basically all kind of gather together, and again, to a giant, giant set of veins. 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. And the reason they're called elastic arteries, 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-- obviously very 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 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. 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're really then doing is you're balancing out those high pressures. So you're balancing out 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 arterial 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 are actually sometimes called the 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 the kind of muscle you have in your heart or in your biceps, 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, that they want to squeeze down, you're going to get something that looks like a little straw, because those muscles are now tight. They're tightly wound, so you're going to create like a little straw. And this process is called vasoconstriction. Vaso just means blood vessel. And constriction is kind of tightening down. So vasoconstriction, tightening down of the blood vessel. And what that does is it increases resistance. 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 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. Some, as 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 arterial side where it was all about large pressure, down here with the vein side, we have to think about large volumes. Remember about 2/3 of your blood at any point in time is sitting in some vein or venule somewhere.