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了解动脉(高压/低容量)与静脉(低压/大容量)间的重要差异.里希(Dr. Rishi Desai)是一名儿科传染病医师,是可汗学院团队一员. Rishi Desai 创建




okay let's talk about the arteries and the veins and I'm actually gonna talk about both of them kind of simultaneously because there's some really interesting differences I want to point out between the two we 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 off your heart is the aorta and before this video I've been drawing the aorta quite differently kind of showing its branch isn't here I'm showing you 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 it definitely is not anatomically correct because that wouldn't make any sense at all to have blood going off to one side but really I want to show you a concept in this video a few concepts concepts actually so here we have let's say the veins 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 that gets back to the to the heart and so I'll draw the branches kind of coming in to the vein here and here and we know that the arteries and the veins are trying to get blood out to the capillaries so let me show you what a zoomed in version might 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 sets of capillary beds which is what I'm drawing here but really you have thousands and thousands right and let's not forget you know that these capillaries are basically found in different parts of the body so you might have for example a bone and some capillaries in the bone and you might have the second capillary you might be going to your face let's say even specifically your nose and this third capillary bed that might 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 of those veins and we've kind of lumped it together and what I want to show you now is that this very very simply is the systemic circulation systemic circulation now of course you know that there's also the pulmonary circulation so what does that look like let me draw that out here pulmonary circulation so pulmonary you know means lungs and I've shown you previously in another video that there are is blood going to these lungs in a very low pressure system so we talked previously about low pressure and high pressure systems and now just remember that this is kind of how it gets there you have the deoxygenated blood or the the blood and the blue vessels kind of going up like that and this would be the pulmonary artery pulmonary artery all right a RT 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 you have pulmonary circulation and you have the 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 but just put that on the side for a moment 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 probably the easiest and most clear way of figuring out what an arteries 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 is a point of difference between the two of them is the direction of flow and and even if you look at the 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 to the tissues and then the veins carry the deoxygenated blood that they're bringing back from the tissues so point number two is really about the color and so let me write that in here so this would be like a little red blood cell with hemoglobin in it and this is a little red blood cell still called a red blood cell even though its color is more blue but it's still got hemoglobin in it but the hemoglobin down here down here actually doesn't have any oxygen left and so the color looks quite different so the color of our chiral blood is usually red because of the fact that oxygen is bound to the hemoglobin and less oxygen is found on the venous side so that's why it turns blue now I want to just put a little star there because that's not always true and you can see where clearly it's not true is over here so I've actually let me star the pulmonary circulation where you can see that the pulmonary artery in vein I'll star that just to really emphasize the point or actually the reverse of what I just said right because the pulmonary artery is actually going to carry the deoxygenated blood and the pulmonary vein is carrying the oxygenated blood so clearly that's the reverse of what I 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 those are two points of differences 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 going to be under very very high pressure and so I kind of think of 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 so this is a high pressure system third difference is that it's a 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 arteries is that there's not so much volume in the arteries so if you actually looked at let's say I've froze time that she try to figure out where all the blood was located not too much of it is in the arteries so it's actually a very low volume system I'll write volume very small and the veins are the opposite they're kind of a large volume system and they have really kind of low pressures so I'll write pressure very small over here and if you actually figured out what percent is where in terms of the amount of blood about five percent of the blood at any point in time would be in the heart and let's say about five percent would be in the capillaries and then about ten percent or so would be in the lungs so what are we up to now we're at five and five 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 veins at any point in time so that's remarkable right about two-thirds of the blood at any point in time is actually sitting in all the veins combined and so the way I've drawn it isn't really not really the 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 the veins it's actually a very very low pressure but a high volume system so that might be the third point I'll bring up on the on the blood side or on the vein side is that it's a high volume system so high volume now we've talked about the direction of flow we talked about the fact that one side carries more oxygen than the other and we've also talked about the fact that the arteries are a high pressure low volume system and the veins are a 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 of open spot here and Ride number four there are no valves in that arterial system whereas on the venous side you actually do have valves you have these valves they come together actually make it and make it very clear by making the two sides touch and the blood kind of goes through like that so you have these valves that 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 it doesn't the heart also have valves and it does the heart also has valves but the veins have a different type of valve and that's actually to keep the blood flowing in the right direction you can imagine that you probably don't even need that in the arterial side because the pressures are so high that therefore the blood is forced to go one way or is in the veins because it's kind of like a giant lake water can kind of swirl around and around and around and so it's helpful to have valves keeping it going in the right direction so that's a fourth point of difference so now think about 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 that is just gonna spew out right and so this is basically what it looks like when you actually cut an artery again because of the fact that it's under such high pressure and now on the venous side on the vein side if you have a break it looks really different because if you have let's say a break right here you'd actually get kind of a pool of blood just locally before it clots off because again you have very low pressure so it's just gonna kind of quietly pool and you might get a bruise right there but it surely won't look like a fountain of blood and so that's actually another final point of difference which really gets back to the high pressure system on the arterial side in the high volume low pressure system on the venous side