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Pre-capillary sphincters

Precapillary sphincters are segments of smooth muscle that help direct bloodflow into capillaries. Because there is a limited amount of blood in the body, these sphinters are important in directing blood to the tissues that need it the most, and reducing the flow to inactive tissues. Created by Rishi Desai.

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  • orange juice squid orange style avatar for user Kutili
    If the pre-capillary sphincters of a metarteriole are closed, and the blood keeps flowing to the thoroughfare channel, will the blood in the thoroughfare channel be oxygenated? The gas exchange didn't happen, because it occurs on the capillary level, right?
    (12 votes)
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    • spunky sam blue style avatar for user Adeyemi47
      I wanted to ask the same question. Good thing you did. But I referenced to one of the previous videos, and there I learned that the veins carry some oxygen (at least, not as much as the arteries) and I guess that this lecture explains how the oxygen gets to the veins. since the oxygen is not used up by the cells, but went straight to the venule, then the blood there will be oxygenated. but to keep the distinction that the vein carrys deoxygenated blood will depend on comparing the percentage of oxygen in the artery to the vein. I hope this clarifies things for you too.
      (8 votes)
  • orange juice squid orange style avatar for user N Peterson
    Why would your body want to shut off blood flow? Wouldn't your cells in that area die?
    (7 votes)
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    • old spice man green style avatar for user Alf Lyle
      I asked a similar question a couple of weeks ago (and I'm still waiting for an answer). In my question I did make some guesses as to why the body would want to shut off blood flow to specific areas of the body. I suspect some reasons would be to shut off "unnecessary" tissue activity during a "fight or flight" response to danger or to shunt blood into the interior of the body when exposed to very cold temperatures.
      (4 votes)
  • male robot donald style avatar for user Kevin
    Does every cell have a capillary touching it?
    (5 votes)
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    • blobby green style avatar for user vandivej2001
      I think there is a 2-cell rule-of-thumb: no cell is more than 2 cells away from a capillary. I got that from an on-line pathophysiology course taught by Dr Geraldo Cizadlo at the College of St Scholastica at this link, http://bit.ly/1aqLsBa . The list of audio files there are poorly titled and there are 3 sets of them, but I think the Sept 14, 2012 file is the proper location. He's a good lecturer and emphasizes the significance of blood pressure in this area which affect nutrient flow in/out of cells. I'd like more information on that and how lymph separates out and gets into the lymph vessels.
      (4 votes)
  • blobby green style avatar for user Nom DePlume
    I've watched to the end of the blood vessels with the following question in mind hoping that it would be answered but I can't find an answer. This is concerning blood flow through the capillary beds

    From what I understand, blood flows through the arterioles into the capillary beds where gas exchange takes place. From what I can tell, the blood "leaks" through the inter-cellular cleft and fenestration pores to the environment of the cells which the blood is interacting with. My question is as follows; once the blood leaks out of the pores of the capillary beds, how does it "leak" back in once it's deposited its oxygen and picked up its CO2? I get that pressure from the arteries is pumping it through the capillary beds in the first place but once it leaks, it seems to me that the pressure that propelled it to leak out of the capillaries would be gone, no? I assumed some pressure or concentration gradient would exist to bring it back into the de-oxygenated side of the capillary bed but I can't figure out a mechanism under which that could take place

    If anyone could answer this question, I would be so grateful, the question is driving me crazy!
    (4 votes)
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    • leaf green style avatar for user Nahn
      There are only a few areas of the body (such as sinusoids of the spleen or liver) where there are holes in the endothelium large enough for red blood cells to actually leak through. In the case of almost all capillary beds, the blood cells will always remain within the capillary. What you may be thinking of though is the fact that on the arterial side of capillary beds, the pressure is great enough to cause fluid to leak out of the capillary bed (but the holes are too small for RBCs and many proteins to follow the fluid). Once the blood has reached the venous end of the capillary bed, the pressure has dropped enough that the concentration gradient you mentioned of the plasma proteins is enough to pull the fluid that leaked out of the capillary back in. One of the reasons that the capillary endothelium is so thin is so that red blood cells can exchange oxygen and carbon dioxide with the surrounding tissues without actually having to leave the blood vessel.
      (4 votes)
  • piceratops sapling style avatar for user saimohan1
    The difference between metarteriole and capillary is the presence of smooth muscle A capillary(true) does not have smooth muscle. A thoroughfare channel too does not have smooth muscle.
    Is thoroughfare channel a true capillary ?
    (4 votes)
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  • old spice man green style avatar for user Soha Samreen
    so whats the difference between thoroughfare channel and a capillary?
    (3 votes)
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  • mr pants teal style avatar for user Jovan Singh
    Does the venule have smooth muscle like the arteriole
    (2 votes)
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    • winston default style avatar for user Kenan
      I have looked up a diagram on the different layers of veins and venules as this was not covered in the videos. Although veins have smooth muscle (the layer is called Tunia Media), venues actually do not have this layer. Compared to large and middle arteries and arterioles, veins do not have a very think layer of smooth muscle. Venules, on the other hand, do not have any smooth muscle and only have the outer Tunica externa layer and the innermost endothelial Tunia intima layer. Hope this helps!
      (2 votes)
  • aqualine tree style avatar for user Mia Bricknell
    So is the difference between an arteriovenous anastomosis and a thoroughfare channel a structural one? They both seem to have identical functions, to provide a direct channel for the flow of arterial to venous blood. Is the difference that the arteriovenous anastomosis isn't surrounded by a capillary?
    (2 votes)
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  • leaf green style avatar for user Frank
    So, when the Pre-capillary sphincters of the metarteriole are shut, and the bloods keeps going through the thoroughfare channel, is there any change on oxygen ? or how does the change on oxygen occur since the blood is going freely from the arteriole to the venule ?
    (1 vote)
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    • aqualine ultimate style avatar for user invictahog
      The blood in the thoroughfare/venule with have, largely, the same degree of oxygenation as in the metarteriole. Some blood would be taken up by the cells in the vessels themselves but there would not be the same degree of diffusion as in the capillaries.
      (2 votes)
  • aqualine ultimate style avatar for user Jeshua De Jongh
    What is gained by closing the pre-capillary sphincters? I understand that this prevents the oxygenated blood from being expended, but if it is simply passing through the metarteriole and Thoroughfare channel into the veins, there is not opportunity for that oxygenated blood to be used by other parts of the body since it will just return to the heart, and go through the pulmonary cycle to be "re-oxygenated" (even though the oxygen was never used. What is the purpose in closing the pre-capillary sphincters?
    (1 vote)
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    • aqualine ultimate style avatar for user invictahog
      There are a lot of reasons for this. If all of the capillaries were fully dilated, you would die because there would not be enough resistance to maintain your blood pressure. So, some need to be closed for that reason alone. The gut is a good example - more blood is sent to the gut after eating and less when not eating. Another reason is for thermoregulation - if there is more blood going to the skin, it will radiate more heat into the environment. This may be good in warm weather but could kill you in colder weather.
      (1 vote)

Video transcript

So we often talk about capillary beds, but I thought that it would be fun to get really up close and really get a good understanding of how these things work. Couple of neat freaks(?) our body has developed to make sure that blood gets to the areas where it needs to go. So let's start up on the left side. This is an arterial, right. So an arterial is carrying blood that initially came from the heart. It's gonna come down --let's say, this way, through the arterial, and let's say some of the blood is gonna continue down this way, maybe to another capillary bed. But, here we have one right here. So, it's gonna divert and send some of the blood through this way. Now, the first vessels, this tiny little vessel over here, it actually is not a capillary, eventhough we would assume it's a capillary, 'cause it's coming after the arteriola, we call it a metarteriole, a [metarteriole]. And the reason I'm calling it that, is because if you look inside the wall of this metarteriole, it actually has some smooth muscle. And so this by definition then, not a true capillary, because it has some smooth muscle in the wall. The true capillaries are these guys out here. So, all these guys over here, these are the true capillaries. Let me actually-- just gonna call them that. So when we use the word [true capillary], we're really trying to distinguish form that metarteriole. So where is the blood gonna go? It's gonna go into all these true capillaries, right. It's course all the way through here. It's gonna go every which ways, gonna go down this way as well, into all these two capillaries. And it's basically trying to get to all the tissues, all the cells here. These are individual cells, kinda hanging out, all doing their job, and they need --of course-- nutrition. So this is at the sailevel(?) level, this is what it looks like, guys. Just a bunch of cells, put together, trying to get some nutrition, and when you kinda zoom out, you'd say, 'ah, what this is?' obviously, a bit of tissue. So we talk about tissue, is basically just talking about collection of cells like this. I'm not gonna draw them, but you get the idea. And some of the blood, of course, is gonna go and stay in this metateriole, it's gonna go through the other side. And it goes to the other side, the name changes. So, same basic vessel in a way, but its name changes. And it's called the [thoroughfare channel]. So the thoroughfare channel. And the distinction between the thoroughfare channel and the metarteriole, is that the thoroughfare channel does not have smooth muscle. So here you should not be finding any smooth muscle. That's really the key difference between the two sides. So this is really becoming more like a venule. So over here as the blood kinda exits, and goes back towards the heart, and of course it's gonna meet up with other blood coming back, this is the venule, or the vein side. So now you can kind of see how the blood flowing from the arteriole all the way to the venule. Now, here's the really cool thing. What the body does --this is the nifty concept-- is that there're these pre-capillary sphincters. [pre-capillary sphincters] Now I've drawn them as yellow -kinda- cells here. Smooth muscle cells. And these pre-capillary sphincters, what they do is they basically squeeze down. They basically squeeze down. That's these guys I've drawn --I think six sets of them-- but you can see that all basically looking the same. They will squeeze down. And if they do, let's say- we do not want blood to come to this capillary bed, we wanna save the blood for something else. Well, these smooth muscle pre-capillary sphincters, they squeeze down, and as the result, you really don't get any blood flow going through these areas. So these white arrows --they kind of do not exist. Yeah, the last(?) blood going this way. And all(?) the blood will end up just going right trough the middle. It'll just go right through that metarteriole, right through here. And it'll go into the thoroughfare channel. So basically what you're doing is, you're basically completely avoiding this capillary bed, by clamping down on these pre-capillary sphincters. So it's kind of neat use of smooth muscle, to control even at the capillary bed level, where the blood is flowing.