Oxygen content Learn how oxygen content (CaO2) is related to Hemoglobin concentration (Hb), oxygen saturation (SaO2), and the partial pressure of oxygen (PaO2). Rishi is a pediatric infectious disease physician and works at Khan Academy.
- Let's talk about oxygen content. I'm going to actually spell it out two ways.
- One is the full word oxygen content, or the full term. And I'm also going to give you the short hand of what you might see.
- Sometimes it's written this way: CaO2. The C is the content, the little a is arteriole, and the the O2 is oxygen.
- So what it means exactly, and the way we think of it is, how much oxygen is there. How much is there.
- We measure it in milliliters per 100ml of blood. So per 100ml.
- Sometimes you might see deciliter instead of milliliter. Let me just quickly jot that down, that equals 1 deciliter per 100ml of blood.
- So this is the definition. Now let's use this definition right away. Let's see if you can think through this idea.
- So let's imagine I go down and I decide to get 1 pint of blood taken from my left arm.
- This is my left arm. Let's say I'm in a huge rush this day so I decide that I also want to get another needle stuck in my right arm, and they also draw blood out of my right arm.
- Kind of at the same moment, the same time. So the same kind of blood, same hemoglobin concentration, same amount of oxygen into my lungs when I was getting the blood drawn.
- Except, for some reason, maybe this second needle was larger, and they were able to get more blood out. 2 pints.
- Now some smart wise guy walks by and says, "Hey, which side, your left or your right, were you able to get higher oxygen content from?"
- Now just looking at the picture you might be tempted to say, "well, oxygen content, sounds like the right side is the winner."
- But actually this is a trick question, right, because it is per 100ml. So you've got to remember it is a certain volume that we are thinking about.
- And in this case since we know that the blood was drawn at the same moment from my two arms and I have no reason to believe that the left vs the right had a higher oxygen saturation, I would say actually that probably the two had the same oxygen content.
- That would be my guess based on this setup.
- So that is one important thing to remember, that it is per 100ml. So let's just keep that in mind.
- And now let me actually just jot down for you the exact equation, kind of the formula for if you want to mathematically calculate oxygen content, how would that look.
- Well, CaO2 is quicker to write so let me just jot that down. And the units on this are milliliters of oxygen per 100 milliliters of blood.
- So these are the units here.
- And this is going to equal, to figure this out I need to know the hemoglobin concentration, and there it's the grams of hemoglobin per 100ml of blood.
- And then I have to multiply this by a constant, and the constant is 1.34. And what that number is, is it's telling me the milliliters of oxygen that I can expect to find for each gram of hemoglobin.
- So that is actually quite a nice little number to have handy, because now you can see that the units are about to cancel, right. This will cancel with this.
- Now you end up with the correct units.
- But there is one more thing that I have to add in here, which is the oxygen saturation.
- Remember this is O2 saturation. If I know the O2 saturation, remember there is this nice little curve, this is O2 saturation, and if I'm looking at just the arteriole side I could right SaO2.
- And I could compare it to the partial pressure on the arteriole side of oxygen. Remember we have these little "s" shaped curves.
- And all I want to point out is that for any increase in my PaO2, the partial pressure of oxygen, I'm going to have an increase in the O2 saturation.
- So there is an actual relationship there, and we usually measure this in percentage. The percentage of oxygen that is bound to hemoglobin.
- And so this is the same thing here, a certain percentage.
- So this whole top part of the formula then, this whole bit in my brackets really is telling me about hemoglobin bound to oxygen.
- Now remember that that is not the only way that oxygen actually travels in the blood.
- Let me write out the second way that oxygen likes to get around.
- The second way is when it dissolves in the blood.
- So this is all going to be plus, and the second part of the equation is the partial pressure of oxygen, and this is measured in millimeters of mercury, so that is the unit,
- and this is times, now this is another constant, 0.003, and then keep track of the units here because we have to end up with these units.
- You know everything has to cancel out to end up with that.
- So I have milliliters of oxygen on top.
- Then I'm going to want to cancel my millimeters of mercury, so take that times 100 milliliters of blood.
- So these are the units on the bottom, and they end up the same as we just worked through.
- We've got this crosses out with that, and my units are going to end up perfect.
- And this bottom bit that I'm going to put in purple brackets, this bit, tells me about dissolved oxygen.
- So I have my oxygen bound to hemoglobin and I have my dissolved oxygen. These are the two parts of my formula.
- So let me actually just quickly, before I move on, circle in blue then the important parts that I want you to keep your eyeballs on.
- There is the total O2 content, hemoglobin, oxygen saturation, and partial pressure of oxygen.
- And remember this guy influences this guy, and we saw that on the O2 curve that I just drew. Let me bring it up again so I can remind you what I am talking about.
- In this graph you can see how to two are related, right? There is a very nice relationship between the two.
- So this is my formula for calculating the total oxygen content.
- So lets actually use this formula, let's think through this.
- And when I think through it I always go through my four variables, let me jot them down here so we keep track of them.
- Let's do PaO2, SaO2, and then hemoglobin, and then the total oxygen content. These are my four variables.
- Let's do a little problem together, let me make a little bit of space.
- Let's say I have two little containers.
- In the first container, this first one is full of blood. Here is a B for blood.
- And here is a second container full of plasma. Remember plasma is a part of the blood but is not all of the blood.
- Plasma specifically does not have any red blood cells or any hemoglobin, so let me right that down. No hemoglobin in the plasma side.
- So to make sure we don't lose track of that fact.
- Now plasma is yellow colored so let me make it yellow colored here, make sure we clearly see that that is plasma.
- And blood I'm going to keep as a red color.
- So now we have our two containers full of plasma and blood.
- So now let's say I decide to increase the partial pressure of oxygen in the air, so it's going to diffuse in here and here.
- So I increase the partial pressure of oxygen in the air and it's going to diffuse into those two liquids and will dissolve in those liquids.
- So my question is as we go through one by one each of these four variables, I want you to think through if they go up, if they go down, or if they stay the same.
- So let's start with the first one, PaO2.
- Well if the oxygen is going to diffuse into those liquids, then I would say the partial pressure of oxygen in the liquid would go up.
- Now it's a little bit confusing to use the words PaO2 in this case, or even down here, CaO2 or SaO2 because we are not really talking about arteriole blood here, we are just talking about blood.
- And we are not talking about arteriole plasma, we are just talking about plasma. There is no artery connected to these two tanks of fluid.
- But the concept is the same.
- So the partial pressure of oxygen is going to go up in the blood and in the plasma because it just dissolved into those liquids.
- Now what about saturation of oxygen.
- Well, O2 saturation goes up in the blood. Remember there is a relationship we said between PaO2 and oxygen saturation.
- So it's going to cause the SaO2 to go up here.
- Whereas in the plasma side there is no hemoglobin, so of course there is going to be no change here, or I would say Not Applicable.
- Because there is no hemoglobin, so how can you have an oxygen saturation curve for hemoglobin.
- Now what about the third variable, hemoglobin concentration.
- Remember that was grams per 100ml of blood.
- Well I'm not talking about adding or subtracting hemoglobin, so there should be no change here.
- I'll right no change.
- And on the other side, on the plasma side, again there is no hemoglobin, so it's not going to affect that at all. It's not really applicable.
- Plasma again does not have hemoglobin.
- So in terms of the total oxygen content, or the CaO2, would I expect it to go up in the blood? Definitely.
- It's definitely going to go up because the dissolved part of the equation goes up.
- But even then hemoglobin bound to oxygen part of the equation goes up because we said the SaO2 went up.
- Now that is an interesting point.
- On the other side, on the plasma side, it also increases, but only a little bit because here you only have the contribution from the PaO2.
- You have no contribution from any of the oxygen bound to hemoglobin, again, because there is no hemoglobin.
- So this problem illustrates some of the ideas specifically around trying to tie in and increase the partial pressure of oxygen, to how that could affect the saturation of oxygen.
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