Pre-capillary sphincters | Circulatory system physiology | NCLEX-RN | Khan Academy
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Pre-capillary sphincters | Circulatory system physiology | NCLEX-RN | Khan Academy

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.

22 thoughts on “Pre-capillary sphincters | Circulatory system physiology | NCLEX-RN | Khan Academy

  1. so basically pre-capillary sphincters are made up of smooth muscles and they regulate where the blood is flowing.. but how they do this? they relax and contract?? open and close? thanks so much 🙂

  2. How could muscle ever open or close? Surely, it contracts constricting the lumen of the metarterioles so that the blood do not flow where it is not needed, and it relaxes whenever it wants to allow the blood flow into that region.

  3. Hi Rishi. If a pre-capillary sphincter muscle is contracted, and blood flow is ceased to a particular region, what happens to the cells of that region? If muscles do not relax, will the cells die eventually? thanks!

  4. Blood flow through a capillary bed is controlled by the capillary sphincters that direct blood to a particular tissue depending on their nutrient needs; precapillary sphincters are controlled by the pH of the tissue. When a tissue needs blood, the change in pH due to high levels of CO2 will cause the sphincter to open. When the tissue no longer needs freshly oxygenated blood and the balance is returned, the sphincter closes to allow other tissues to receive blood. (I'm pretty sure)

  5. …yes the pH is determined by the CO2. Carbon dioxide in the blood isn't simply carried as carbon dioxide, the majority is converted by carbonic anhydrase into bicarbonate and hydrogen ions. pH is simply a measure of the concentration of hydrogen ions in a material base on a log scale. The more hydrogen ions, the more acidic the material is. Therefore, if the tissue has a greater concentration of carbon dioxide, it will have more hydrogen ions and have a lower pH, causing the sphincters to relax and blood to enter perfuse the tissue.

  6. @Ciera Bellamy the precapillary sphincters are also highly sensitive to catecholamines (ie. epi and nEpi) and thats when they squeeze down, especially during episodes of shock for blood redistribution.

  7. @hijama hijam if the precap sphincters do not relax, and for some reason they do not open up, due to hypoxia from the lack of blood flow to the tissues in that region, vasodilators (such as histamine from mast cells) will loosen up the smooth muscle as well as the true caps, and there will be blood flow again.  But if the hypoxia persists for too long, when the sphincter opens up, a lot of RBC with poor ability to change conformation enters, coagulates, along with WBC and platelet adhesion, plasma viscosity increases, endothelial cells damaged, clot would then form, DIC then follows, and you are pretty much dead.  This is in the case of shock.

  8. The lecture is amazing! But I have a quick question: Does this smooth muscle exist in skeletal muscle capillaries? I just read on wiki, say, that "s of 2013 it is held that they are unique to the mesenteric circulation and some researchers have suggested the term precapillary resistance instead.[Sakai T, Hosoyamada Y (September 2013). "Are the precapillary sphincters and metarterioles universal components of the microcirculation? An historical review". The Journal of Physiological Sciences 63 (5): 319–31.]" I feel confused…

  9. Can someone explain the following question for me?? I had it in a exam and I can't seem to understand it: I posted the exam's explanation as a reply comment, its A LOT to read. lol

    Which of the following is an abnormal physiologic process that occurs at the capillary level during shock?

    A. Minimal capillary blood flow, which results in aerobic metabolism
    B: Sustained pre- and postcapillary constriction in response to hypoxia
    C: Precapillary sphincter relaxation in response to lactic acid buildup
    D: Postcapillary sphincter relaxation that causes capillary fluid depletion

    the answer was C.. but if I am correct, I thought C was is the only normal, not abnormal. And where are the post capillaries located?

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