WEBVTT

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Now let's say that you
have a vial of plasma.

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And I'm actually going
to label it as we go.

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We've got some sodium
floating in here

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and you've got some anion
in purple over here.

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And this could be anything
that really binds to sodium.

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So if this is some negatively
charged ion, maybe chloride,

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or bicarb, those are
the two most common.

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And you've also got, let's
say, some glucose in here.

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And maybe some urea, or we
call it urea nitrogen as well.

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So you've got a few things
floating around the plasma

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and someone asks
you, well, what is

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the total osmolarity
of the plasma?

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And you know that
this is in units

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of osmoles per liter
blood, Actually,

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I should write liter
plasma to be more accurate.

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Since that's what we're
talking about here.

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So per one liter of plasma.

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And these are the units
that we have to think about

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to answer this
question, is, what

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are the osmoles per
liter of plasma?

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So let's go through this.

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And I'm going to give
you some lab values

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and we'll see how based on just
a few lab values and really

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just four of the most
representative solutes,

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or most important
solutes, we can

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get a pretty close
guesstimate of the osmolarity.

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So you don't actually need to
know every single osmole that's

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in your plasma.

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You can figure it
out based on four

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of the most important ones.

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So let's go with the
first one, sodium.

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And let's say the lab tells
you, well, your sodium value--

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and I'm going to write the labs
in kind of this grey color,

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somehow that reminds
me of the lab--

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let's say they say the sodium
value is 140 milliequivalents

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per liter.

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So how do you take that and
make it into osmoles per liter?

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Well, our denominator
is already OK.

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But immediately, you can
say, OK, well 140 millimoles

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per liter is what that equals.

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And you know that because
sodium is a monovalent.

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It's only got one charge.

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If it's monovalent,
then that means

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that the equivalents
equal the moles.

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And now that you're
in moles, you

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can actually go
across to osmoles.

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You could say 140 osmoles
or milliosmoles per liter.

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And you know that because
once sodium is in water,

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it acts the same way that
you would expect it to act.

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It doesn't split
up or anything like

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that because it's one particle.

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So it acts as a single particle.

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One particle.

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So if it's one
particle, it's going

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to have 140
milliosmoles per liter.

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And we've effectively gotten one
quarter of this problem done.

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Because all we need to do is
take the four different solutes

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that we've identified
and add them up together.

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So we've figured out sodium.

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And now let's move
on to the anion.

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And the trick to the anion is
just thinking of it as sodium.

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It's almost the same as
sodium, but just the reverse.

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So we know that it's
going to be 140.

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We're going to use 140
as the number here.

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Because our assumption is that
sodium is a positive charge

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and for every one
positive charge,

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you need one negative charge.

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So we're going to assume that
all the negative charges are

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coming from these anions.

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And these would be
things like we said,

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things like chloride or
bicarb, something like that.

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So again, we don't
actually get these numbers

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or even need these
numbers, we simply

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take that 140 and
we multiply by 2

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and assume that the other half
is going to be some anion.

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Now we actually have
to convert units still.

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We have to get over to
milliosmoles per liter.

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And so we know that the anion
is going to be monovalent

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and that gets us to millimoles.

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And we use the same
logic as above.

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We just say, OK, well
if that was millimoles

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and it's still one particle,
meaning it's not splitting up

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when it hits water and going
in two different directions,

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in a sense, having
twice the effect,

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we're going to end up with
140 milliosmoles per liter,

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just as before.

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So this is our second
part done, right?

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So two parts are done.

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We figured out the sodium
and we figured out the anion.

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Now let's go over to glucose.

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So let's figure out how to
get glucose as units from what

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the lab gives us, which I'll
tell you in just a second,

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into something more usable.

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So how do we actually get
over to something usable?

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Let me actually, switch over.

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There we go.

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Make some space on our canvas.

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So let's say we have
our glucose here.

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And the lab calls us
and says, hey, we just

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got your lab result, it was
90 milligrams per deciliter.

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It's actually a very,
very common lab value

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or common range for
a glucose lab value.

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One thing we have
to do right away

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is figure out how to get
from milligrams to moles.

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And you know that this is
what glucose looks like.

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This is the formula for it.

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So to get the overall
weight, the atomic weight,

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you could say,
well, let's take 6,

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because that's how
many carbons we have,

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times the weight of
carbon, which is 12,

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plus 12, because that's
what we have here,

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times the weight of
hydrogen, which is 1,

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plus 6, times the
weight of oxygen.

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And that's going to equal--
this is 72, this is 12,

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and this is 96, and add them all
up together, and we get-- 180.

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So we have 180 atomic mass
units per glucose molecule.

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Which means, if you
think back, which

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means that one mole of
glucose equals 180 grams.

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And since these
are way, way bigger

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than, I mean this is grams, and
we're talking about milligrams

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over here, so I'm going to
just switch it down by 1,000.

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So one millimole of glucose
equals 180 milligrams.

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All I did was divide by 1,000.

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So now I can take this unit and
actually use our conversions.

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I could say, well, let's
multiply that by 100

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and-- let's say, one
millimole rather,

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one millimole per 180
milligrams, that'll

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cancel the milligrams out.

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And I also have to get from
deciliters to liters, right?

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So I've got to go 10
deciliters equals 1 liter.

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And that'll cancel
my deciliters out.

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So I'm left with-- and this
10 will get rid of that 0--

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so I'm left with
90 divided by 18,

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which is 5 millimoles per liter.

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And, just as above, I
know that the glucose

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will behave as one particle
in water, in solution.

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So it's going to be 5 osmoles,
or milliosmoles, actually.

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5 milliosmoles per liter.

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And that's the
right units, right?

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So I figured out another
part of my formula.

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And I'll show you the actual
formula at the end of this,

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but I wanted to work
through it piece by piece.

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So we've done glucose now and
we're ready for our last bit,

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so let's do our last one,
which is going to be urea.

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Specifically, the lab is not
going to call us about urea,

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it's going to call us
about blood urea nitrogen.

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And actually, it
matters what this means.

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So what that exactly
means is that they're

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measuring the nitrogen
component of urea.

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And so they'll call you and
say, well, we measured it

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and the value came to 14
milligrams per deciliter.

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Something like
that, so let's say

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that's the amount
of urea we find

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in our little tube of plasma.

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How do we convert that to moles
per liter like we did before?

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Well, again, it'll be helpful if
I draw out a molecule of urea.

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So we have something like this.

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A couple nitrogens.

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And this is what
urea looks like.

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It's a pretty small molecule.

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A couple nitrogens,
carbon, and oxygen.

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And these nitrogens have an
atomic mass unit of 14 apiece.

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So that's 14.

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And this is 14
over here, as well.

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So what the lab actually
measures is just this part.

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It's just measuring
the two nitrogens.

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It's not measuring the weight
of the entire molecule.

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So all it's going to give you
is the weight of the nitrogens

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that are in the molecule.

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So what that means is that we
say, OK, well, that tells us

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that one molecule of urea is
going to be 28 atomic mass

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units of-- I'm going to put
it in quotes-- urea nitrogen.

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Because that's the part of
urea that we're measuring

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and that means that
one mole of urea

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is going to be 28
grams of urea nitrogen.

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And because, again,
this is much, much more

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than what we actually have,
let me divide by 1,000.

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So one millimole equals 28
milligrams of urea nitrogen.

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So that's how we figure
out the conversion.

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And I do the exact
same thing as above.

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I say, OK, well, let's
times-- let's say,

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I want to get rid of
the milligrams, right?

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So 1 millimole divided
by 28 milligrams,

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and that'll get rid
of my milligrams.

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And I'll take, let's say,
10 deciliters over 1 liter

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and that'll help me get
rid of my deciliters.

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And so then I'm left with
14 over 28, which is 0.5.

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And then times 10, so that's 5.

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5 millimoles per liter.

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And as I've done
a couple times now

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and we know that it's
the urea nitrogen

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or the urea is going to act and
behave like one molecule or one

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particle when it's
in water, it's

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not going to split up
or anything like that,

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so that means that
it's going to basically

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be 5 milliosmoles per liter.

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And so I figured out the
last part of my equation.

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So going back to the
top, we have sodium.

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And this turned
out to be a total

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of 140 milliosmoles per liter.

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And then for our anion, we had
140 milliosmoles per liter.

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And then for our glucose, we
had 5 milliosmoles per liter.

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And for our urea, we had
5 milliosmoles per liter.

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So adding it all up, our total
comes to 140 times 2 plus 10.

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So we get, if I do
my math correctly,

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I think that's 290
milliosmoles per liter.

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That's the answer
to our osmolarity.

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Our total osmolarity
in the plasma

00:12:01.970 --> 00:12:04.710
is 290 milliosmoles per liter.

00:12:04.710 --> 00:12:07.030
Now that was kind of the
long way of doing it.

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Let me give you a very,
very quick and dirty way

00:12:09.460 --> 00:12:09.960
of doing it.

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Let me actually make
some space up here.

00:12:12.750 --> 00:12:15.380
You could do the exact same
problem, you could say,

00:12:15.380 --> 00:12:24.900
well, this osmolarity
equals, you could say,

00:12:24.900 --> 00:12:40.800
sodium times 2, plus
glucose, divided by 18,

00:12:40.800 --> 00:12:45.450
plus BUN divided by 2.8.

00:12:45.450 --> 00:12:48.370
And that takes all
of those conversions

00:12:48.370 --> 00:12:49.650
and simplifies it down.

00:12:49.650 --> 00:12:52.850
So if you ever get your sodium
value, your glucose value,

00:12:52.850 --> 00:12:55.030
and your BUN, and you
want to quickly calculate

00:12:55.030 --> 00:12:59.230
your osmolarity, now you
know the fast way to do it.