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了解氧气从肺部到组织的两种输送途径,以及二氧化碳返回肺部的三种方式.里希(Dr.Rishi Desai)是儿科传染病医师,在可汗学院工作. Rishi Desai 创建




let's talk about exactly how oxygen and carbon oxide come into and out of the lungs so you know this is our alveolus in the lungs this is the last little chamber of air where the lungs are gonna interface with blood vessels right so this is our blood vessel down here and oxygen is gonna make its way from this alveolus it's gonna go into the blood vessel and it's gonna go from the blood vessel into a little red blood cell this is my red blood cell here he's headed out for the first delivery of oxygen that day and he's gonna pick up some oxygen and it's gonna get inside of the red blood cell through diffusion that's how it gets inside so the oxygen has made its way into the red blood cell and where do you think it goes first well this red blood cell is we sometimes think of it as a bag of hemoglobin right it's got millions and millions and millions of hemoglobin proteins so this is our hemoglobin protein it's got four parts to it and each part can bind an oxygen so hemoglobin I can shorten this to HB now oxygen is gonna bump into quite literally bump into one of these hemoglobins and it's gonna bind let's say right here and initially it's kind of tricky because oxygen doesn't feel very comfortable sitting on the hemoglobin or binding to hemoglobin but once a single oxygen is bound a second one will come and bind as well and then a third will find it much easier because what's happening is that as each oxygen binds it actually changes the conformation or shape of hemoglobin and so each subsequent oxygen has an easier time binding we call that cooperativity has the word almost like a cooperation in it and kind of an easy way to think of cooperativity the way I think of it is that if you're at a dinner party you're much more likely to sit where two or three of your friends are already sitting if you think of this is the table with four chairs rather than just sitting at a table by yourself being the first one to sit there so we kind of like sitting with our friends and oxygen is kind of a friendly molecule and so it also likes to sit wear or bind where other options have already bound what are the two than major ways based on this diagram how draw on it what are the two major ways that oxygen is going to be transported in the blood one is hemoglobin binding oxygen and we call that HBO - just HB for hemoglobin Oh - for oxygen and this molecule or this enzyme then is not really called hemoglobin anymore technically it's called oxyhemoglobin that's the name for it and another way that you can actually transport oxygen around is that some of this oxygen actually underlined it there is dissolved co2 is dissolved in plasma so some of the oxygen actually just gets dissolved right into the plasma and that's how it gets moved around now the majority the vast majority of it is actually going to be moved through binding to hemoglobin so just a little bit is dissolved in the plasma the majority is bound to hemoglobin so this red blood cell goes off to do its delivery let's say it's delivering some oxygen out here and there is a tissue cell it of course doesn't know where it's going to go that day but it's going to go wherever it's it's blood flow takes it so let's say it takes a path over to this size cell and you're let's say upper thigh so this thigh cell has been making co2 and remember you know sometimes we think of co2 is being made you know only when though when the muscles been working but you could be napping you could be doing whatever and this co2 is still being made because cellular respiration is always kind of happening so this red blood cell has moved into the capillary right by this thigh cell and so you've got a situation like this where now some of the co2 has is going to diffuse into the red blood cell like that and what happens once it gets down there so let me draw out now a large version of the red blood cell just so you get a closer view of what's going on and we're in the sigh and the two big conditions in the thigh that we have to keep in mind one is that you have a high amount of co2 or partial pressure of co2 and this is dissolved in the blood and the other is that you have a low amount of oxygen not too much oxygen in those tissues so let's focus on that second point if there's not too much oxygen in the tissues and we know that the hemoglobin is kind of constantly bumping into oxygen molecules and binding them and no they fall off and new ones bind so it's kind of a dynamic process now when there's not too much oxygen around these oxygen molecules are gonna fall off you know as they kind of always do in a dynamic situation except new ones are not going to bind because there's so little oxygen around in the area that less and less oxygen is free and is available to bump into the hemoglobin and bind to it so you're gonna literally start getting some oxygen that kind of falls off the hemoglobin simply because the partial pressure of oxygen is low so one reason for oxygen to come into the cells is gonna be a low po2 that's one reason so these are reasons and I'm gonna give you another one that's why I'm writing reasons for Oh to delivery so one of them is gonna be simply not having too much oxygen in that area a second reason has to do with co2 itself so let's actually follow what happens once co2 starts getting into the red blood cell now this first co2 molecule it's gonna meet up with a little water remember there's a lot of water in the red blood cell in fact there's water all over the the blood in fact it's made of mostly water and so it's not too hard to imagine that a water molecule might bump into the co2 and there's an enzyme called carbonic anhydrase and what it does is that kind of combines the water in the co2 into what we call h2 co3 or carbonic acid now if it's an acid try to keep keep in mind what acids do acids are going to kick off a proton so this becomes hco3 - and it kicks off a proton and notice that now you've got bicarb and proton on this side and this bicarb is actually going to just make its way outside so the bicarb kind of goes outside the cell and the proton what it does is it meets up with one of these oxyhemoglobin it kind of finds an oxyhemoglobin member there you know millions of them around and it literally binds to hemoglobin and it boots off the oxygen so it binds to hemoglobin and oxygen falls away so this is interesting because now this is a second reason a second reason for why oxygen gets delivered to the tissues and that is that protons protons compete compete with oxygen for one of the competing for for binding with hemoglobin so they're competing for hemoglobin now I said there is another thing that happens to the carbon dioxide so what's the other thing turns out that carbon oxide actually sometimes independently seeks out oxyhemoglobin remember again there are millions of them so it'll find one and it'll do the same thing it'll say well hey hemoglobin why don't you just combined with me and get rid of that oxygen so it also competes with oxygen so you've got some competition from protons some competition from carbon dioxide and when carbon dioxide actually binds interesting thing is that it makes a proton so guess what happens that proton can co and compete again by itself it can compete with oxyhemoglobin and try to kick off another one kick off another oxygen so this system is really interesting because now you've got a few reasons why you have oxygen delivery you've got protons competing you've got now co2 competing with oxygen so you've got a couple of sources of competition and you've got of course just simply the fact that there's just not too much oxygen around so these are reasons for oxygen delivery so at this point you've got oxygen that's delivered to the cells and these hemoglobin molecules they're they're still in our cell of course and start inside of a red blood cell and these hemoglobin molecules have now been bound by different things so they're no longer bound by oxygen so you can't really call them oxyhemoglobin anymore instead they have protons on them like this and they might have some co o- on them so they might have actually let me do that in the original kind of orange color so they basically have different kind of things binding to them and as a result the oxygen is now gone and our system so far looks good but let me actually now turn it around and let's ask the question how do we carry carbon dioxide from the thigh back to the lung let me start out by actually replacing the word thigh with lungs so now our blood has traveled back to the lung and the question is how much carbon dioxide did it bring with it and in what different forms do that carbon oxide come so we've got a couple of situations we've got a high amount of oxygen here and we've got a low amount of co2 so really quite different than what was happening within the thigh so when the blood is leaving the thigh headed back to the lung what's it got with it well it's got a few things one is that it's got hemoglobin that is bound to carbon dioxide's right and this is actually called carb amino carb amino hemoglobin and then it's also got some protons that are bound to hemoglobin so the protons themselves are attached to hemoglobin and just keep in mind that for every proton that's attached to hemoglobin you've also got a bicarb dissolved in the plasma right because it's a one-to-one ratio of these things so you've got a bunch of bicarb in the plasma as well and I'm writing in parentheses just so we don't forget that point and finally what else is in the blood we've got some co2 that just gets dissolved right into the plasma so this is sounding a little bit like what happened with the oxygen situation where you had some co2 in the plasma itself and this is what's headed back from the thigh to the lung so now in the lung what happens you've got all this stuff with you and the first thing that happens is that you've got a lot of oxygen now in the area right a lot of oxygen in the tissue of the lung and it diffuses into the cell goes into the cell and the oxygen is because there's so much of it it's gonna go and try to sit in these hemoglobins it's gonna try to find its spot and if it does what does in terms of equations is kind of the reverse of what happened before now you've got a lot of oxygen here you've got a lot of oxygen here and because these are reversible reactions you basically push this entire reaction to the left so now you've got a lot of oxygen and it basically competes for that hemoglobin again so remember before the protons actually ended up snatching hemoglobin away from oxygen and now oxygen returns the favor it says well I'm gonna snatch that hemoglobin right back and you've got this proton that's kind of left out by itself and on this side you've got this co2 that's kind of left out by itself so a couple of interesting things were happening let me actually make sure I keep track of them up here so what are some reasons now what are some reasons for co2 delivery how is it getting delivered back to the lungs and the first one probably the most obvious one is that we said that the lungs have a low co2 content so simply having very little co2 around means that whatever is there is going to diffuse into the alveolus so you're gonna get you know whatever's in the red blood cell is gonna diffuse in here simply because there's not a lot of co2 around so instead of diffusing into the red blood cell now it's gonna want to diffuse out a second reason though this is kind of the more interesting reason is that you actually have oxygen oxygen competing Auction competes with protons and co2 so it's competing with protons in co2 for hemoglobin and that's what we drew in our equation down there so what it does is it basically gets you back to the oxyhemoglobin that's the first thing that's what we've already drawn there that we've drawn oxygen bound to hemoglobin but it means that these little co2 is fall off they fall off these little protons fall off and they're back in the side of the cell back in the inside of the cell so if your co2 you can again you can just diffuse into the alveolus but if you're a proton let's say you're a proton and you just fell off of the hemoglobin because it got snatched away by oxygen well then this little bicarb is going to come back inside this bicarb comes back inside and it combines with the proton and these two form you guessed it h2 co3 so they kind of remember this is reversible as well so they kind of go back and they form h2 co3 and it turns out that you can actually go from h2 co3 over here also using carbonic anhydrase so you can basically kind of just do this whole reaction backwards and now you can see that you've got more co2 formed so by having bicarb dissolved in the blood or in the plasma it's kind of just staying there and kind of waiting it out and as soon as those protons are bumped off of the hemoglobin they go and combine with them and form the co2 so you've got co2 coming from here from the bicarb you've got co2 coming from the carb amino hemoglobin and you've also got the co2 that had remember we said that some co2 dissolved in the plasma so three different ways that co2 is actually coming back and once all that co2 is in the lungs it's going to diffuse right into the alveoli is because the amount of co2 in there is so darn low that the diffusion gradient gets it going towards the alveolus and of these different strategies the most important one the one that gets us most of our carbon dioxide transportation is this one this middle one where the protons are actually binding hemoglobin and all that bicarbonate is dissolved in plasma so of the three different ways that carbon dioxide comes back that's the one you should pay most particular attention to