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学习氧含量(CaO2)与血红蛋白浓度 (Hb)、血氧饱和度 (SaO2) 和氧分压 (PaO2) 的联系.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 oxygen content and I'm gonna actually spell it out two ways one is kind of the full word oxygen content or the full term and then I'm also going to give you kind of the shorthand what you might see sometimes it's written this way cao2 and the C is the content the little a is arterial arteriole and the o2 is oxygen so what it means exactly the way we we kind of think of it is how much oxygen is there how much is there and we measured in milliliters per 100 milliliters of blood so per 100ml and sometimes you could you might see deciliter instead of milliliter let me just quickly jot that down that equals one deciliter per 100 milliliters 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 one pint of blood taken from my left arm let's instead of bind to let me write pint and this is my left arm and let's say I'm in a huge rush this day so I I decided 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 and same amount of oxygen in my lungs when I was getting the blood drawn except for some reason maybe this needle the second one was larger and they were able to get more blood out two pints now some smart you know wise guy walks by and says hey which side you're left for you're 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 kind of a trick question right because it's per 100 milliliters so you gotta remember it's a certain volume that we're thinking about and in this case since we know that the blood was drawn at the same moment from my two arms and you know I have no reason to believe that the left versus the right had a higher oxygen saturation I would say actually probably the two had the same oxygen content that would be my guess based on this setup so that's one important thing to remember that it's 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 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 I said 100 milliliters of blood so these are the units here and this is gonna equal to kind of figure this out I need to know the hemoglobin concentration and there it's the grams of hemoglobin grams of hemoglobin per 100 milliliters of blood and then I have to multiply this by a constant and the constant is one point three four and what that number is is it's telling me the milliliters of oxygen that I can expect to bind for each gram of hemoglobin so that's actually quite a nice little number to have handy because now you can see that the units are about to cancel right they're gonna this will cancel with this right and I end up with the correct units but there's one more thing I have to add in here which is the oxygen saturation now this is o2 saturation Oh to saturation and if I know the o2 saturation remember there's this nice little curve this is Oh to saturation and if I'm looking at just the arterial side I could write s little a OH - and I could compare to the partial pressure in the arterial side of oxygen and remember we have these little s-shaped curves right these s-shaped curves and all I wanted to kind of point out is that for any increase in my pao2 and the partial pressure of oxygen I'm going to have an increase in the o2 saturation so there's an actual relationship there and we usually measure this in percentage right percentage of oxygen that is bound to hemoglobin and so this is the same thing here as 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's not the only way that oxygen actually travels in the blood let me write out the second way that oxygen likes to get around and 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's the unit and this is x now this is another constant 0.003 and then keep track of the unit's here because we have to end up with these units right so you know everything has to cancel out to end up with that so I have milliliters of oxygen on top and 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 kind of the same as as we just worked through we've got this crosses out with that and my units are gonna end up perfect right and this bottom bit that I'm gonna 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 kind of keep your eyeballs on there's the total co2 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 just bring it up again so I can remind you what I'm talking about in this graph you can see how the two are related right there's a very nice relationship between the two so this is my formula for calculating the total oxygen content so let's actually use this formulas think through this and when I think throw it I always kind of go through all of my four variables let me just jot them down here so we keep track of them let's do pao2 sio2 and then hemoglobin and the total oxygen content these are my four variables now let's do a little problem to you let me make a little bit of space and let's say I have two two little containers in the first container this first one is full of blood there's a B for blood and here's the second container full of plasma remember plasma is a part of the blood but it's not all of the blood plasma specifically does not have any red blood cells or any hemoglobin so let me just write that down no hemoglobin in the plasma side this would kind of make sure we don't lose track of that fact now plasma is kind of yellow colored so let me just kind of make it yellow colored here make sure we clearly see that that's 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 increase the partial pressure of oxygen in the air so it's going to diffuse in here and it's going to diffuse in here so I increase the partial pressure of oxygen in the air and it's going to diffuse into those two liquids it's going to dissolve into 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 if the oxygen is going to diffuse into those liquids then I would say the partial pressure of oxygen in the liquid would go now it's a little bit confusing to use the words pao2 in this case or even down here CA o 2 or SI o 2 because we're not really talking about arterial blood here we're just talking about blood and we're not talking about arterial plasma we're just talking about plasma because there's 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 it's going to go up in the plasma because it just dissolves into those liquids now what about saturation of oxygen well o to saturation goes up in the blood remember there's a relationship we said between PA o2 and oxygen saturation so it's going to cause the SAO to to go up here or is on the plasma side there is no hemoglobin so of course there's gonna be no change here I would say not applicable 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 100 milliliters of blood well I'm not talking about adding or subtracting hemoglobin so there should be no change here right I'll write no change and on the other side on the plasma side again there is no hemoglobin so it's not gonna 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 what I expected to go up in the blood definitely right it's definitely gonna go up because the dissolved part of the equation goes up but even the hemoglobin bound to oxygen part of the equation goes up because we said the SAO two went up that's an interesting point right 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 PIO two you have no contribution from any of the oxygen bound to hemoglobin because again there is no hemoglobin so this problem kind of illustrates some of the ideas specifically around trying to tie in an increase in the partial pressure of oxygen to how that could affect the saturation of oxygen