Fetal Hemoglobin and Hematocrit Although mom controls the oxygen source, the fetus has a couple of clever tricks to get the most oxygen possible! Rishi is a pediatric infectious disease physician and works at Khan Academy.
Fetal Hemoglobin and Hematocrit
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- 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 from mom.
- because remember the fetus is trying to grow and it wants to make sure that all of it's 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 baby.
- So let me try to draw the vials about the same height and width.
- These are the two vials. If I was to take now, let's say a little bit of mom's 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 of baby's blood and spin it down.
- That spun blood, once it's spun down, would actually separate out into little parts, right?
- You'd have three different 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.
- Remember red blood cells are the ones that contain the hemoglobin.
- They're the ones that are going to move oxygen around.
- And in mom, the percent that this red layer takes up is about 35%.
- Meaning this whole thing would be 100%, and of that just over 1/3, or 35% exactly, is that bottom red layer.
- That's the red blood cell layer.
- So we would call this the hematocrit.
- So this is mom's hematocrit. And this is a very typical number for a pregnant woman.
- It varies depending on 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 going to be smaller. And then the next layer, the white blood cell layer,
- that's a very small layer anyway so that's not going to change much.
- And the final layer, the third layer, is the red blood cell layer.
- This layer takes up about 55%.
- So I hope I didn't misdraw that, but that's about 55%.
- So here the hematocrit is much higher, and what does that mean?
- If the hematocrit is higher in 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
- can then take up more oxygen because that's really the part of blood 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. Kind of the amount of red blood cells is going to 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, that 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 four units to it.
- So let me draw out the adult hemoglobin over here on the left.
- Let me first write out adult hemoglobin.
- So "Hb" for hemoglobin and "A" for adult.
- I'll write adult here 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 am going to draw.
- But there're few different types of little (...)
- 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 which hemoglobin has four 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 type of hemoglobin,
- but the main one is HbF, and it's actually this one also shares that alpha units
- and has two of them just as before,
- but instead of beta unit, this one has what we call it gamma unit,
- this is the Greek letter for gamma,
- now oxygen is gonna bind and both of hemoglobin,
- both the adult and the fetus, can bind four oxygens,
- let me just draw in four oxygens here, you got the idea.
- now inside of red blood cells there's a little molecule and
- actually I just gonna sketch it out for you,
- this molecule has three carbons, let me just number the carbon one two three,
- and coming off of the two carbon, this one right here,
- is an oxygen, and coming off of that oxygen is a phosphate.
- Remember that phosphate has typically five bonds,
- so I just gonna show you what this little molecule looks like,
- in fact, the exact same thing is happening of the three carbons.
- so this molecule that exist inside of the 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 the little molecule, and it's called --
- and you can --maybe when you can take a stab and try to guess what it's called--
- it's called 2,3 --I'm reffering to this 2 and this 3--
- Di --because it got two phospho--
- so, Di-phospho- glyceride.
- so that's Di-phospho, and then glyceride just refers to this part right here,
- this is kind of the part that is being refered to when we say gyceride,
- so Diphosphoglyceride.
- and 2,3-diphosphoglyceride, we just fix that,
- is actually sometimes shortened down to 2,3-DPG
- 'cause people don't like to say the whole thing,
- so they'll say 2,3-DPG, and that's what this molecule is.
- so this molecule 2,3-DPG, is inside red blood cells,
- and it actually helps the red blood cell get rid of oxygen,
- the way it does that is, actually is a tiny little molecule I'll draw it.
- now that you know what the whole sturcture looks like, I'll draw --it's a yellow dot,
- this is the same thing, I just make the equal sign, the equal of same thing,
- This little 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 it nit, it sits kind of nicely between all four subunits, the betas and the alphas,
- and what it does is that actually makes the conformation or
- 'the shape of the molecular' change, so these little oxygens want to move off.
- So basically what it does is that it makes it easier for the oxygen to be released from the hemoglobin.
- Now when this molecule comes around this side, on the fetus side it tries to bond,
- guess what happens, well this gamma subunits basically say, go away! go away!
- They don't want to bind to this 2,3-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 this result those molecules of hemoglobin don't get rid of oxygen
- nearly as easily as hemoglobin A does.
- Now why would we even have a molecule like 2,3-DPG around?
- What it would be doing there?
- well, interestingly, the level of 2,3-DPG actually go up
- in situations where you actually have more niver oxygen,
- so let's say chronically you're without oxygen.
- So the situation like that be, when you're chronically without oxygen.
- Well, let's say you live --I don't know what the-- the top of the Himalayan Mountains.
- And you know, the altitude is so high, if you got a high alitude,
- that the air itself doesn't have a lot of oxygen in it.
- and that situation your tissues are kind of always, or chronically, without oxygen.
- Now another situation could be, let's say you have lung disease.
- Let's say you have 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 of the 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 for anemic, the tissues are not getting as much oxygen
- as they wish they would.
- And again, in this situation you might have more 2,3-DPG.
- So 2,3-DPG, its basic job is to try-- to make sure that oxygen is left off of the hemoglobin,
- so that if you have tissue that really needs that oxygen,
- it's more easy to-- to actually deliver the oxygen to that tissue.
- So, going back to the trick for the fetus,
- you can see the fetus has different type of hemoglobin from the adult.
- So, let me draw a little curve and you'll see what this different incept doing.
- so let me sketch out a curve.
- let's just draw out a little graph here.
- this'll be the partial pressure of oxygen on this axis,
- and this'll be O2 or oxygen saturation,
- looking at how many of those spots on hemoglobin are taken up.
- so this'll be going up that way.
- let's start out with mom's hemoglobin, or adult hemoglobin,
- you know, has kind of S-shape, because of the cooperativity
- that we've talked about at the past.
- so this'll be hemoglobin adult type, or hemoglobin A.
- now, if I have, let's say, really high level of 2,3-DPG,
- let me just draw out what that would look like,
- so let's say we have a situation where that high level of 2,3-DPG,
- and it could be because of one these reasons,
- maybe live on a high mountain, or you've chronic lung disease,
- or you're always anemic, if you had one of these situations,
- and your 2,3-DPG level were really high, or higher than usual,
- then what will 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 right shift, because the whole thing looks like it has just kind of moved over a little bit.
- And now, at any point --let's say I just choose a random point over 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 molecule.
- and that makes sense with what we just said, because the molecule helps kick off 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 a low levels of 2,3-DPG.
- Well, with low level of 2,3-DPG, you can see that this would make sense
- because now of all sudden, that molecules are not around, it's not doing anything
- to help get the oxygen off, so of course oxygen is gonna stay down to hemoglobin,
- and at the same partial pressure of oxygen, more of the hemoglobin would 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 2,3-DPG, it doesn't like to bind to it,
- and so it says, get lost! go away!
- and so in a sense the way I drawn it for a low level of 2,3-DPG,
- I could --just this --well, erase that,
- and say, well, this is the situation in the fetus.
- The fetal hemoglobin is basically this curve, right?
- This is kind of the hemoglobin F curve.
- If you just look at the curve, it looks like it's left-shifted.
- But the-- but the real concept behind it,
- it's that it's because those hemoglobin molecules don't like to bind to 2,3-DPG,
- and so of course it's gonna 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 wanna look at the point where about half of the
- hemoglobin molecules are bound to oxygen, that might be right about there,
- meaning this is about half way 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 --the partial pressure of oxygen--
- is about 27. And for the fetus, the same kind of point of reaching half way 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 we adult can accomplish
- at only higher amount of oxygen in the environment or in the blood.
- And this values are called p50, so if you see p50
- --if you see that term, you can remember now that the hemoglobin F p50
- is lower than the hemoglobin A p50.
- and that is --again, the actual number is 20 versus 27, or there about.
- so these are the two tricks than one, is the-- 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 p50
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