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Learn some important differences between arteries (high pressure/low volume) and veins (low pressure/high volume). Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Video transcript
Okay let's talk about the arteries and the veins and I'm actually going to talk about the both of them and simultaneously because there are some interesting differences I want to point out between the two. You know that they're both vessels but there are some really important differences between how they work and what they do. So this is your heart and coming out of your heart is the aorta and before this video I have been drawing the aorta quite differently, kind of showing its branches and here I am showing it kind of stretched out. and I'll show a few of the branches but really the point of the drawing is not to make it anatomically correct, you can tell that definitely it's not anatomically correct, because that wouldn't make any sense at all, that blood going off to one side. But really, I want to show you a concept in this video. A few concepts, actually. So, here we have --let's say, the vein. And I'm gonna stretch that out as well, and let's imagine that all of the veins are dumping into this one final vein. It gets back to the --to the heart. So I'll draw the branches kind of coming in to the vein here and here. And we know that the arteries and the vein are trying to get blood out to the capillaries. So these show you what a zoomed in version may look like of the capillaries. So we've got --let's say, some capillaries, here and maybe some here and here. And you know of course in real life you don't have three such a capillary beds, which is what I'm drawing here, but really of thousands and thousands, right? And let's not to forget, you know, these capillaries are basically found in different parts in the body, so you might have --for example, a bone, and some capillaries in the bone, you may have this second capillary, may be, going to your face, let's say even specifically, your nose. And this third capillary bed, may be going down to your foot. So you have blood in your foot. So you have different parts of the body all getting capillaries and the blood is gonna start at the heart, go to the capillaries through the arteries, and then return through all those beds. And we've kind of lumped it together. And what I want to show you now is, that these very very simply is the systemic circulation. Systemic circulation. Now, of course you know that there is also the pulmonary circulation. So what is that look like? I mean to draw that here --pulmonary circulation. So pulmonary, you know, means lungs. And I've shown you previously, in another video, that there is blood going to these lungs. In a very low pressure system. So we talked previously about low pressure and high pressure system. And now I just remember that this is kind of how it gets there, you have deoxygenated blood, or the --the blood in the blue vessel, kind of going up like that, and this should be the pulmonary artery. Pulmonary artery. All right, {Art.} for artery. And we have the pulmonary vein, bringing that blood back to the heart with oxygen in it. Pulmonary vein. So that's your pulmonary circulation, so your pulmonary circulation in your systemic circulation. And I'll get back to why I drew the pulmonary artery in blue, and pulmonary vein in red, in just one second. Let's put that on the side for a momment. And let's get into exactly how to think about arteries and veins in general. So in general, I think of arteries as taking blood {AWAY} from the heart. That's the easiest and most clear way of figuring out what an artery is exactly. So, arteries are taking blood AWAY from the heart. So let me just draw a little arrow here. And veins are bringing blood {BACK}. And so this becomes our first point. Point number one, and this's the point of difference between the two of them, is the direction of flowing, and even if you see at pulmonary system, you can see that --that remains true, that the arteries are taking blood away, and the vein is bringing the blood back. Okay. Point number two. Now you see that the arteries are drawn in red, for the most part. And so usually arteries have oxygenated blood, that they're bringing out to the tissues. And then the veins carry the deoxygenated blood, that they bring back from the tissue. So point number two, is really about the colour. And so we write that in here. So this should be like a little red blood cell with hemoglobin in it. And this is a little red blood cells -- still called the red blood cell eventhough its colour is more blue, but it still got the hemoglobin in it, but the hemoglobin down here --down here, actually doesn't have any oxygen left, and so the colour looks quite different. So the colour of arterial blood is usually red, because of the fact that oxygen is bound to the hemoglobin. And less oxygen bound in the venous side, so that's why it turns blue. Now, I want to --just a little star there, because that's not always true. And you can see where clearly it's not true, it's over here. So I'd like to --let me star the pulmonary circulation, where you can see that the pulmonary artery and vein --I'll star that, that's just really emphasize the point, or actually the reverse of what I've just said, right. Because the pulmonary artery is actually gonna carry deoxygenated blood, and the pulmonary vein is carrying oxygenated blood. So clearly that's the reverse of what I've just said. But that is one exception to the general rule, which is that arteries definitely look red, and veins generally look blue. Okay, so that --those are two points of difference between the arteries and veins. Let's keep going. So now you know, that arteries are high pressure, right. because the heart is pumping the blood out, and it's gonna be in a very very high pressure. And so I kind of think the arteries as being like a river, a fast flowing river. So you can think of this as fast flowing, high pressure. So that is the third difference. This is a high pressure system. Third difference, it's that it's high pressure system. In fact, maybe I'll even emphasize the point by writing the word very large: HIGH PRESSURE. And actually, the other thing we think about with these arteries is that there is not so much volume in the arteries. So if you actually look at --let's say, first time now, you should try figure out where all the blood was located, not too much of it is in the arteries. So it's very low volume system, right, its volume is very small. And the veins are the opposite. They're kind of a large volume-- system. And they have really kind of a low pressure. Alright, pressure is very small over here. And if you actually figure out what percent is where, in terms of the amount of blood, about 5% of the blood, at any point time, would be in the heart. And --let's say, about 5% would be in the capillaries. and then about 10% or so, would be in the lungs. So what we have to --now we're 5 and 5 is 10, 10 plus 10 is 20. And then we have another about 15% in the arteries. So about 15%. So that's actually not too much blood. And so the remaining --what is that, 65% of the blood, is actually in the vein, in any point of time. So that's remarkable, right. About two third of the blood in any point time is actually sitting in all the vein combined. And so the way I've drawn it, isn't really --not really a best way to draw it. I should really erase this. And really redraw this to make it much more accurate. It should look a lot more like this, right, like a big giant lake of blood. And so this is actually how to think about of the vein. It's actually very very low pressure but high volume system. So that may be the third point. I'll bring up on the blood side --or, on the vein side. it's that it's a high volume system. so, high volume. Now, we've talked about the direction of flow, we've talked about the fact that one side carries more oxygen than the other, and we've also talked about the fact that arteries are high pressure-low volume system. And the veins are high volume-low pressure system. So what are some other differences? So, one other difference is that in the arteries, you actually don't have any valves. Now --I don't know how to label that to make it super clear, but let me just find some sort open spot here, and write number four, there're no valves in the arterial system, whereas on the venous side, you actually do have valves. You have these valves come together --we make it very clear, but making the two sides touch. And the blood kind of goes through like that. So you have these valves keep the blood going in the right direction. So that would be the fourth point of difference between the two. So you have these valves. And you might get confused, because you think, "well, doesn't the heart also have valves?" And it does, the heart also has valves. But the veins have a different type of valves, and that's actually keep the blood flowing in the right direction. You can imagine that you probably don't even need that on the arterial side, because the pressures are so high, that the blood is forced to go one way. Or, it's in the vein, because it's kind of a giant lake. Water can kind of swirl around and around and around, and so it's helpful to have valves keeping going in the right direction. So that's the fourth point of difference. So now think about of the fact that you might have, a break in the arterial system. So let's say that you have an accident, and you cut an artery --or, a big artery, let's say. What would it look like? So if you have a break right here, you would actually have like a fountain of blood, coming out, right there. Because it's under such high pressure, it's gonna spew up, right. And so this is basically what it looks like when you actually cut your artery, again, because of the fact that it's under such high pressure. And on the venous side, on the vein side, if you have a break. It looks really different. Because you have --let's say your break is right here, you'd actually get a kind of --a pool of blood, just locally, before it clots off. Because again, you have a very low pressure. So it's just gonna kind of quietly pool and, you might get a bruise right there. But it's really won't look like a fountain of blood. And that's actually another final point of difference which, really gets back to the high pressure system on the arterial side, and the high volume-low pressure system on the venous side. ~o0o~