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now let's say that you have a vial of plasma and I'm actually going to label it as we go you've got some sodium some sodium floating in here and you've got some anion purple over here and this could be you know anything that really binds to sodium so if this is some negatively charged ion maybe chloride or bicarb those are the two most common and you've also got let's say some glucose in here and maybe some urea or what we call urea nitrogen urea as well so you've got a few things floating around the plasma and someone asks you well what is the total osmolarity of the plasma and you know that this is in units of Oz moles per liter blood actually I should write liter plasma to be more accurate since that's what we're talking about here so per one liter of plasma and these are the units that we have to think about to answer this question is what are the osmoles per liter plasma so let's go through this and I'm going to give you some lab values and we'll see how based on just a few lab values and really just four of the most representative solutes or most important solutes we can get a pretty close guesstimate of the osmolarity so you don't actually need to know every single Osmel that's in your plasma you can figure it out based on for the most important ones so let's go with the first one sodium sodium and let's say the lab tells you well your sodium value and I'm going to write the labs in kind of this gray color somehow that reminds me of the lab let's say they say the sodium value is 140 millicoulombs per liter so how do you take that and make it into oz moles per liter well our denominator is already okay but immediately you can say okay well 140 millimoles per liter is what that equals and you know that because sodium is a mono valent it's only got one charge mono valent ion so it's mono valent then that means that the equivalents equal the moles and now that you're in moles you can actually go across to osmoles you can say 140 oz moles or milliosmoles per liter and you know that because once sodium is in water it acts the same way that you would expect it to act it doesn't split up or or anything like that because it's one particle so it acts as a single particle 1 particle so if it's one particle it's going to it's going to have 140 millions per liter and we've effectively gotten 1/4 of this problem done because all we need to do is take the four different solutes that we've identified and add them up together so we figured out sodium and now let's move on to the anion and the trick to the anion is just thinking of it as sodium it's almost the same as sodium but just the reverse so we know that it's going to be a hundred and forty we're going to use 140 as the number here because our assumption our assumption is that sodium is a positive charge and for every one positive charge you need one negative charge so we're going to assume that all the negative charges are coming from these anions and these would be things like we said things like chloride or bicarb something like that so again we don't actually get these numbers or even need these numbers we simply take that 140 and we multiply by 2 and assume that the other half is going to be some anti on now we actually have to convert units till we have to get over to millions per liter and so we know that the anion is going to be mono valent and that gets us to milli moles and we use the same logic as above we just say okay off those millimoles and it's still one particle meaning it's not splitting up when it hits water and going you know in two different directions in a sense having twice the effect we're going to end up with 140 millions per liter just as before so this is our second part done all right so two parts are done figured out the sodium and we figured out the anion now let's go over to glucose so let's figure out how to get glucose is units from what the lab gives us which I'll tell you in just a second into something more usable so how do we actually get over to something usable let me actually help us switch over there we go make some space on our canvas so let's say we have our glucose here and the lab calls us and says hey we just got your lab result it was 90 milligrams per deciliter it's actually a very very common lab value or common range for a glucose lab value one thing we have to do right away is figure out how to get from milligrams to moles and you know that this is what glucose looks like this is the formula for it so to get the overall weight the atomic weight you could say well let's take six because that's how many carbons we have times the weight of carbon which is 12 plus 12 that's we have here times the weight of hydrogen which is one plus six times the weight of oxygen and that's going to equal this is 72 this is 12 and this is 96 and add them all up together and we get 180 so we have 180 atomic mass units per glucose molecule which means if you think back which means that one mole of glucose one mole of glucose equals 180 grams and since these are way way bigger than I mean this is grams and we're talking about milligrams over here so I'm going to just switch it down by a thousand so 1 millimole of glucose equals 8 or 180 milligrams all I did was divide by a thousand so now I can take this unit and actually use use our conversions I could say well let's multiply that by 100 and let's say 1 millimolar either 1 millimole per 180 milligrams it'll cancel the milligrams out and I also have to get from deciliters 2 litres right so I've got to go 10 deciliters equals 1 liter and that will cancel my deciliters out so I'm left with and this 10 will get rid of that zero so I'm left with 90 divided by 18 which is 5 millimoles per liter and just as above I know that the glucose will behave as one particle in water in solution so it's going to be 5 ozz moles or milliosmoles actually 5,000,000 moles per liter and that's the right units right so I figured out another part of my formula and I'll show you the actual formula at the end of this but I wanted to work through it piece by piece so we've done glucose now and we're ready for our last bit so let's do our last one which is going to be urea so specifically the lab is not going to call us about urea it's going to call us about blood urea nitrogen and actually it matters what this means so what that exactly means is that they're measuring the nitrogen component of urea and so they'll call you and say well you know we measured it and the value came to 14 milligrams per deciliter something like that so let's say that's the amount of urea we find in our little tube of plasma how do we convert that to moles per liter like we did before well again it'll be helpful if I draw out a molecule of urea so we have something like this a couple nitrogen's this is what EURion looks like it's a pretty small molecule couple nitrogen's carbon and oxygen and these nitrogen's have an atomic mass unit of 14 a piece that's 14 and this is 14 over here as well so what it actually measures the lab actually measures is just this part it's just measuring the two nitrogens it's not measuring the weight of the entire molecule so all it's going to give you is the weight of the nitrogen's that are in the molecule so what that means is that we say okay well that tells us that one one molecule one molecule of urea is going to be 28 atomic mass units of I'm going to put in quotes urea nitrogen because that's the part of urea that we're measuring and that means that one mole of urea is going to be 28 grams of urea nitrogen and because again this is much much more than what we actually have let me divide by a thousand so 1 milli mole equals 28 milligrams of urea nitrogen so that's how we figure out the conversion and I do the exact same thing as above I say okay well that's x let's say I want to get rid of the milligrams right so 1 milli mole divided by 28 milligrams and that'll get rid of my milligrams and I'll take let's say 10 deciliters over one liter and that'll help me get rid of my deciliters and so then I'm left with 14 over 28 which is 0.5 and then times 10 so that's 5/5 millimoles per liter and as I've done a couple times now we know that it's the urea nitrogen or the urea is going to act and behave like one molecule or one particle when it's in water it's not going to split up or anything like that so that means that it's going to basically be 5 milli awesomes per liter and so I figured out the last part of my equation so going back to the top we have sodium and this turned out to be a total of 140 140 millions per liter and then for our anion we had 140 millions per liter and then for our glucose we had 5 milli Azam's per liter and for our urea we had 5 milli ohms for a liter so adding it all up our total comes to a 140 times 2 plus 10 so we get if I do my math correctly I think that's 290 millions per liter that's the answer to our awesome Alera T our total osmolarity in the plasma is 290 millions per liter now that was kind of a long way of doing it let me give you a very very quick and dirty way of doing it let me actually make some space up here you could do the exact same problem you could say well this odds molarity osmolarity equals you could say sodium sodium times 2 times 2 plus glucose divided by 18 plus bu n divided by 2.8 and that takes all of those conversions and simplifies it down so if you ever get your sodium value your glucose value and your B UN and you want to quickly calculate your osmolarity now you know the fast way to do it