Health and medicine
- Arteries vs. veins - what's the difference?
- Arteries, arterioles, venules, and veins
- Layers of a blood vessel
- Three types of capillaries
- Pre-capillary sphincters
- Compliance and elastance
- Bernoulli's equation of total energy
- Stored elastic energy in large and middle sized arteries
- Compliance - decreased blood pressure
- Compliance - increased blood flow
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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- 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)
- 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)
- Why would your body want to shut off blood flow? Wouldn't your cells in that area die?(7 votes)
- 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)
- Does every cell have a capillary touching it?(5 votes)
- 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)
- 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)
- 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)
- so whats the difference between thoroughfare channel and a capillary?(4 votes)
- capillaries actually go to different cells, they branch out more.
a thoroughfare channel connects the metarteriole to a venule. it is a direct channel path between the 2 that has capillaries around it.
there are also arteriovenous anastomosis, which directly connects the arteriole with the venule without capillaries around the channel.
- 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)
- Yes, I took this definition from Google, this is what they said: "Any of the fine branching blood vessels that form a network between the arterioles and venules." The thoroughfare channel is between the arteriole and the venule in the diagram Rishi draws. Also, at2:27of the Arteries, arterioles, venules, and veins video (https://www.khanacademy.org/science/health-and-medicine/circulatory-system/blood-vessels/v/arteries--arterioles--venules--and-veins), Rishi never changes the type of blood vessel he's talking/describing the part of the capillary that carries de-oxygenated blood.
Great question! I hope you get more votes, you deserve it. :D(2 votes)
- 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?(3 votes)
- I would be interested to learn more on exactly how the precapillary sphincters are controlled. Is it chemicals from the muscle metabolities (lactic acid, C02, NO, etc.), other extrinsic hormones or neurological stimulation? Thanks.(2 votes)
- So, I've had more time to look into this. Regulation of capillaries occurs automatically via the arteriole wall muscles in response to a large variety of factors. From my studies, precap sphincters appear to be specialized blood control mechanisms that primarily effect mesentaric blood vessels. The rest of the body's local blood supply is governed by "capillary autoregulation", which is based on arterioles that respond to changes in local capillary factors. Chief among these factors is oxygen levels in the capillaries. High levels cause the arteriole to clamp down and decrease blood to a set of capillaries. This will, of course have the effect of decreasing oxygen levels over time as cellular processes use up the oxygen without enough flowing in to replace what is used. This situation is sped up or slowed down by the cell's metabolic activity level. When the oxygen levels lower enough, this will signal to the arterioles to relax and increase local blood flow, returning the effected capillaries to a normal state. Other factors that cause vasodilation: hypercapnia, decreased pH, nitrous oxide, adenosine, and several other chemicals. Vasoconstriction is promoted, generally, by the opposite. Also, autonomic nervous system can play a powerful role. The question I haven't found an answer to is the specifics of how these muscles are responding to the chemicals.(2 votes)
- Two questions:
1. What is the point of the pre-cap sphincter? If the blood will just be sent to the thoroughfare and then the venule, won't the blood kind of be wasted, in a sense? Why wouldn't the capillaries just receive the blood and use what they can?
2. Do the thoroughfares, or venules, have valves like veins do? How does the blood know that it needs to move in the direction towards the thoroughfare and then onto the venule?(2 votes)
- 1. The body does not have enough blood to perfuse all of the tissues all the time. If too many precapillary sphincters open at the same time blood pressure would drop and the heart would not get venous return. This is called shock.
2. Venules do not have valves like veins, which have a larger diameter so blood pressure is lower in veins. Blood moves from high pressure to low pressure and pressure originates from the heart. Leaving the heart, bp might be 120/ 80 mmHg or around 100mmHg. In the capillaries, pressure is about 35 mmHg. Blood in the vena cavas before entering the heart is around 2mmHg. https://m.youtube.com/watch?v=6E_OLwVsf9w blood in arteriole.
https://m.youtube.com/watch?v=PQ-Oq6mnlTQ. Capillaries dilating.(2 votes)
- if blood does not go through the capillary bed(due sphincter squeezing) why are they even present there??(2 votes)
- The sphincter can open to let blood through and close to stop blood flow. Think of a faucet opening and closing to let us wash our hands or how a farmer irritates different fields using valves on his pipes to conserve and direct water to the driest crops. Why does the body do this? We do not have enough blood to irrigate all the tissues all the time. So what variables open the sphincter? High carbon dioxide in tissue, lactic acid (lower pH), warmth, histamine release, Nitric oxide release, etc. all cause the sphincter to open and blood to flow into that tissue bed. When we exercise, what happens to muscle tissue? It becomes warm, makes carbon dioxide and lactic acid to signal the sphincter to open so blood provides oxygen and removes carbon dioxide, lactic acid and cools the muscle tissue. What happens if too many sphincters are open at once? We faint due to a lack of blood flow to the head. It makes such elegant sense when you think about it. If you go to YouTube and search on 'capillary blood flow through tissues' you can find films of blood flow. This link will break at the question mark but if you copy it you will find an example of real blood flow through a tissue. https://m.youtube.com/watch?v=wu01vlf4ORM(2 votes)
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.