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尽管妈妈控制着氧气的来源,胎儿也有自己的办法尽可能多的获得氧气!里希(Dr.Rishi Desai)是儿科传染病医师,在可汗学院工作。 Rishi Desai 创建




so here's a picture of mom and a little fetus and at this point when the fetus is still attached by the umbilical cord everything that goes into the fetus is really originating from mom she controls all the nutrients and all the oxygen that goes into that baby and with oxygen in mind there are a couple of interesting ways that the baby in this case this little fetus on the right has come up with to be able to get as much oxygen as possible for mom because remember the fetus is trying to grow and it wants to make sure all of its tissues that are growing and developing have enough oxygen so there are a couple of tricks and the first trick let me actually just draw it out for you is simply looking at a single vial of blood if we look at a single vial of blood from mom and compare it to a vial of blood from babies let me try to draw the files about the same height and width these are the two vials if I was to take now let's say a little bit of moms blood and spin it down let's say in this little tube and then do the exact same thing with the baby's blood take some baby's blood and spin it down that's spun blood once it's spun down would actually separate out into little little parts right you'd have three different little layers and this first layer would be something like this this is called the plasma the next layer right below it remember there's a little layer of white blood cells and platelets and below that right here is a layer of red blood cells and remember red blood cells are the ones that contain the hemoglobin they're the ones that are gonna move oxygen around and in mom the percent this red layer takes up is about 35% meaning this whole thing would be a hundred percent let's say this entire thing would be a hundred percent and of that just over one third or thirty-five percent exactly is that bottom red layer that's the red blood cell layer so we would call this the hematocrit right so this is mom's Matic red and this is a very typical number for a pregnant woman it varies depending on you know whether you're a man or a woman and what age you are but for a pregnant woman 35% is a pretty reasonable number now going over here to the baby side let's draw in what baby's blood probably looks like the baby has a lot less of the blood taken up by plasma so that layer is gonna be smaller and then the next layer the white blood cell later that's a very small layer anyway so that's not gonna change much and the final layer the third layer is a red blood cell layer this layer takes up about let's say about 55 percent so I hope I didn't kind of miss draw that but that's that's about right about 55 percent so here the hematocrit is much higher now what does that mean if the hematocrit is higher than the baby about 55 percent then that means that the baby actually has more red blood cells going around in a given amount of volume of blood and those red blood cells then can take up more oxygen because that's really the part of blood that we care about when it comes to moving oxygen around so that's one trick in terms of tricks for getting more oxygen simply having more of the red blood cells in a given volume of blood is kind of the amount of red blood cells is gonna go up in the fetus and that's kind of one trick when I say trick that's what I mean so what's another trick or strategy I guess that's another word not the baby or the fetus can come up with to get more oxygen from mom well if we think of the amount you can also think of the type and what I mean by that is thinking specifically about the type of hemoglobin so we know that the adult hemoglobin has 4 units to it so let me draw out the adult hemoglobin over here on the left let me just first write out adult hemoglobin so HB for hemoglobin and a for adults and I'll write adult over here just so we keep track of which is which and there isn't one type of adult hemoglobin there is a main one which is the one I'm gonna draw but there are a few different types that adults have the main one as I said is this one has a couple of alpha units this is just a protein peptide that is in some conformation we call it alpha and a couple of beta units and these are slightly different looking than the alpha ones and there's a 2 to 2 ratio so each hemoglobin has 4 units and here you can see that you have two of each type now on the fetus side you actually have something a little different so we also have over here hemoglobin HB this time F for fetus and just as before the fetus has a few different types of hemoglobin but the main one is HB f and actually this one also shares that alpha unit and has two of them just as before but instead of a beta unit this one has what we call a gamma unit this is the Greek letter for gamma now oxygen is gonna bind and both of the hemoglobins both the adult and the fetus can bind for oxygen so let me just draw in four oxygens here you get the idea now inside of red blood cells there's a little molecule and actually just gonna kind of sketch it out for you and this molecule has three carbons let me just number the carbons 1 2 3 and coming off of the 2 carbon this one right here is an oxygen and coming off of that oxygen is a phosphate remember phosphate has typically five bonds I'm just gonna show you what this little molecule looks like in fact the exact same thing is happening off the three carbon so this molecule that exists inside of red blood cells it looks like this has a couple of phosphates and coming off this number one is something like this so this is a little molecule and it's called and you can you maybe even take a stab at trying to guess what it's called it's called 2 3 and referring to this 2 and this 3 die because it's got two phospho so dye phospho and glycerate so that's dye phospho and then glycerate just refers to this part right here this is kind of the part that is being referred to when we see glycerate so dye phosphoglycerate and 2 3 dye phosphoglycerate let me just fix that is actually sometimes shortened down to two 3d PG because people don't like to say the whole thing so they'll say to 3d PG and that's what this molecule is so this molecule to 3d PG is inside of red blood cells and it actually helps the red blood cell get rid of oxygen and the way it does that it actually is a tiny little molecule I'll draw it now that you know what the whole structure looks like I'll draw it as a yellow dot this is the same thing let's just make the equal sign they equal the same thing this sort of molecule will go and bind in the middle here and it likes to bind to the beta subunits actually the beta subunits are shaped so that this thing can bind very easily and at night it's it's kind of nicely between all four subunits the betas and the alphas and when it does that it actually makes that conformation or the shape of the molecule change so that these little oxygens want to move off so basically what it does is it makes it easier for the oxygen to be released from the hemoglobin now when this molecule comes over on this side on the fetus side and tries to bond guess what happens well these gamma subunits basically say go away go away they don't want to bind to this to three DPG they don't have the right shape for it and so they basically want this little molecule to get lost and so this molecule doesn't bind as easily to hemoglobin F and as a result those molecules of hemoglobin don't get rid of oxygen nearly as easily as a hemoglobin ADA's now why would we even have a molecule like 2 3 DPG around what would it be doing there well interestingly the levels of 2 3 DPG actually go up in situations where you actually have more need for oxygen so let's say chronically you're without oxygen so what would a situation like that be where you're chronically without oxygen well let's say you live and I don't know what the the top of the Himalayan mountains and you know the altitude is so high you've got a high altitude that the air itself doesn't have a lot of in that situation your tissues are kind of always or chronically without oxygen now another situation could be let's say you have a lung disease let's say you have a lung problem or lung disease and it's a chronic lung disease where you're always having difficulty you know getting oxygen to the blood well again the tissues are really lacking in oxygen so there the red blood cells would make a lot of the 2/3 DPG or a final situation maybe you're anemic maybe you don't have a lot of red blood cells circulating around the body and if you're anemic the tissues are not getting as much oxygen as they as they wish they would and again you know in this situation you might have more 2/3 DPG so 2/3 DPG it's basic job is to try to make sure that oxygen is let off of the hemoglobin so that if you have tissue that really needs that oxygen it's more easy to actually deliver that oxygen to that tissue so going back to the tricks for the fetus you can see the fetus has a different type of hemoglobin from the adult so let me draw out a little curve and you'll see what this difference ends up doing so let me sketch out a curve let's just draw out a little graph here this will be the partial pressure of oxygen on this axis and this will be otwo or oxygen saturation looking at how many of those spots on hemoglobin are taken up so this will be going up that way now let's start out with mom's hemoglobin or adult hemoglobin you know it has a kind of an S shape because of the cooperativity that we've talked about in the past so this will be hemoglobin adult-type or hemoglobin a now if I had let's say really high levels of 2 3 DPG let me just draw out what that would look like so let's say we had a situation where at high levels of 2 3 DPG and it could be because of one of these reasons maybe you live on a high mountain or you have chronic lung disease or you're always anemic if you had one of these situations and you're 2 3 DPG levels were really high or higher than usual then what would happen to your curve it was actually it would look like this the curve for oxygen binding or oxygen saturation basically kind of shifts over to the right so we call this a shift because the whole thing looks like it's just kind of moved over a little bit and now at any point let's say I just choose a random point here and I choose the same point here so this is the same partial pressure of oxygen right which is somewhere down here now for the same partial pressure of oxygen my curve actually went down meaning I have less oxygen bound to hemoglobin in the presence of this molecule and that makes sense with what we just said because the molecule helps kick off of the oxygen now what if you had an opposite situation what if I actually drew out a curve like this and this could be let's say a situation where you have low levels of two three DPG well with low levels of two three DPG you can see that this would make sense because now all of a sudden that molecule is not around it's not doing anything to help get the oxygen off so of course oxygen is gonna stay bound to hemoglobin and at the same partial pressure of oxygen more of the hemoglobin will be bound by oxygen now think back to the idea of fetal hemoglobin remember fetal hemoglobin we said has this gamma unit and the gamma doesn't like two three DPG doesn't like to bind to it and so it says get lost go away and so in a sense the way I've drawn it for a low level of two three DPG I could just as well erase that and say well this is the situation in the fetus the fetal hemoglobin is basically this curve right so this is kind of the hemoglobin s curve if you just look at the curve it looks like it's left shifted but the but the real concept behind it is that it's because those hemoglobin molecules don't like to bind to three DPG and so of course it's going to go in the opposite direction of the blue curve so looking at these two curves now the white one and the red one the white one represents mom the red one represents baby and the white one if you want to look at a point where about half of the hemoglobin molecules are bound to oxygen that might be right about there meaning this is about halfway up to here this is actually 50% of the way there so 50% of the hemoglobin molecules are bound to oxygen when the pressure of oxygen of the partial pressure of oxygen is about 27 and for the fetus the same kind of point of reaching halfway saturation is reached when the partial pressure is about 20 so it's interesting for a lower partial pressure of oxygen the baby or the fetus is able to accomplish the same thing the adult can accomplish at only a higher amount of oxygen in the environment or in the blood and these values are called P 50s so if you see P 50 if you see that term you can remember now that the hemoglobin FP 50 is lower than the hemoglobin AP 50 and that is again the actual number is 20 versus 27 or thereabout so these are the two tricks then one is the eggs you know the amount of hemoglobin or red blood cells in the fetus and the other is the type and hemoglobin F binds oxygen much more tightly and has a lower P 50