Plasma osmolality, what is osmolality? Osmolality
is the number of solutes in plasma. In other
words, we are talking about the tonicity in
ECF and by that once again you know me now.
I am not just going to read to you. I am going
to have you conceptualize. In the ECF, what
is the most important compartment that we're
monitoring please? What is that? Good. That
is the plasma compartment and in terms of
actual fractions, what is it? A measly 1/4,
use that you will be fine. 1/4, quarter
that is which you are measuring in terms of
tonicity. Next what does isotonic mean? Normal
plasma osmolality. To make things simple,
we will keep it around 300 okay. Now, of course,
you want to be technical later on, which is
between 275 and 295 and then I will give something
dramatic. If they want you to know the plasma
osmolality has decreased too much, then I
will give something like 250 to 245. What
do you mean they? Well the patient or maybe
perhaps a question. And if it is too much,
let it above 300. Hypotonic what does that
mean? Where am I right now? In the plasma
okay. The plasma compartment and you find
that they have a hypotonic state. What does
that mean to you? What does that mean? It
means that things are diluted right. Now,
what does that mean to you in terms of plasma
osmolality? You should know you are less than
275 maybe at 260s. You're on the lower end of your plasma
osmolality. Keep that in mind because what
we are going to do? We are going to walk through
this and then well we are going to have some
important differentials. Okay. By the time
we are done with the section, all this entire
lecture series of water and sodium pathophysiology,
we will have gone through the details of SIADH,
diabetes insipidus and we will have gone through
psychogenic polydipsia. Okay. So these are the
three that always come into mind and these are
the ones that students tend to confuse because
they are not well. You are not going to do
this anymore. I am going to show you how to
think through this and know as to what you
are paying attention to so that you never
miss the question. Hypertonic state. What
does that mean to you? That means that you
have too much "solute." Maybe too much sodium
inside my what compartment? Good. Plasma compartment.
What does all this mean? You will see in a
little bit. Okay. An important formula that
you want to keep in mind now. I am going to
walk you through which you need to know for
physio and then of course what we need to
know in pathology. The most important component
that is then going to really dictate or determine
your osmolality is your sodium. But going
back to our initial statement in our philosophy
of electroneutrality, what does that mean?
Give a cation such as sodium and then with
it, it has to be accompanied by a chloride.
So therefore if I tell you the sodium is the
most important component to determining then
tonicity or plasma osmolality, then you must
take what into consideration? The chloride
So then one becomes two.
What does that mean? Two times sodium
will give you proper amount of plasma osmolality
physiologically okay. Physiologically, take
a look at the second component, glucose. Tell
me about glucose. We will take a look at the
formula, a serum glucose over or divided by 18.
So normally speaking if you have plasma glucose
approximately how much, please? Everything
that you do at some point you have to be able
to interpret your labs. So I am going to hammer
home glucose once again. Keep it simple at
100. 100/18 gives you a very small number
physiologically and so, therefore, the glucose
at this point could be negligible. Remember
you're clinician. Well if you want to research
in such, that is on your own time, isn't it?
Our first priority is to make
sure that that patient is surviving, is maintained
and properly managed. So what are you looking
for here is two times sodium physiologically.
The glucose at this point can be ignored.
But when does the glucose come into play? What
does diabetic ketoacidosis mean to you? Diabetic
ketoacidosis. Absolute uncontrolled diabetes
mellitus. Uncontrolled. Most likely occurs
in which diabetes type? Diabetes mellitus
type I, why? Because this was a child that
wasn't born or should I say that the insulin
levels pretty much got exhausted immediately
very quickly and so therefore if there isn’t
proper control and there is no insulin, what
is going to happen to that glucose level in
that patient? 400, 500, 600, 700, 800, extremely
high. Now this glucose becomes an important
factor in plasma osmolality. Of course, it
does. You go as far as that. You will be fine.
The physio part, two times sodium, you add in the
pathology such as diabetic ketoacidosis, you
will have to take glucose into consideration
and we will repeat this entire concept again
in the Darrow Yannet box. What about urea?
You should know urea and if you don't please
pay attention. Urea, extremely unpredictable.
Urea in the body physiologically is not at
all ever going to contribute to plasma osmolality.
Technically it is a part of the equation.
Yes, it is. Let it go. I beg of you. Time
is too precious. Now technical plasma osmolality
is between 275 to 295 and what is plasma osmolality?
It gives you an approximate correlation with
the amount of sodium concentration.
Plasma osmolality, well here we go but urea
I just want to focus and make sure that you
understand that urea is too unpredictable.
So mean to say at times it is just going to
shift back and forth between ECF and ICF.
It does not contribute to effective
osmolality. Nephrologists, as I said once
again as far as effective osmolality is urea
is not going to contribute to. What are the
two major things? Sodium, glucose when? If
your patient has hyperglycemia. That is the
equation ladies and gentleman that you are
focusing upon. Here we take a look at sodium
and glucose and its totality of ECF and what
it means to be impermeant, shall we? Okay.
What I'd like for you to do on this slide is
really be able to conceptualize what I
wish for you to see not in terms of the verbiage
here but what is actually taking place in
your body. And in my opinion one of the most
effective pictures that you can create for
yourself would be the Darrow Yannet box so
that you can see in a very organized fashion
as to other shift of fluid between ECF and
ICF is taking place. So with that said, that
will return a complex with the slide. Begin
with sodium and glucose. First and foremost
once again go back to your basis of total
body water. Work with me here. Simple questions
but yet also every effective and this is the
kind of stuff, this is the platform that requires
in which you will build
and build again into pathology and you are
just going to walk through the stuff and you
will feel good. You really will on every single
level and you won't be doubting yourself. Don't
you hate that feeling of just doubting?
Did I say that right? Did I come across right?
Did I chose the right answer? You will be
confident. So what is sodium and glucose?
For total body water, there are two major
barriers in the total body water system. The
first barrier is going to be between the ECF
and ICF. What is the name of that membrane
or that would be a simple question. That membrane
well, let me give you two choices. Is it
the cell membrane or is it the capillary membrane?
That is your cell membrane, isn't it? Between
what? ICF and ECF. It has to be the cell membrane.
That cell membrane now read impermeant to
whom? Sodium and glucose. Are we clear? That
is one membrane, but Dr. Raj, you said there were
two. I did. So where is the second one? The
second one actually is in the ECF, isn't it?
So with the ECF, what two compartments are
there again? The most important compartment
is which one at all times physiologically?
Plasma compartment, isn't it?
In terms of fraction, what are you going
to use? 1/4. Clinically understand
that it is approximately 1/3. Let us
go with what is most commonly asked, 1/4,
is your plasma. What is the name of that barrier
membrane that seperates the plasma from the
interstitium? That would then be your capillary
membrane. Good. Now that capillary membrane
is permeant to sodium, isn't it? Yes, it is.
That pitting edema that we talked about so
many different times, does that contain sodium?
Always. Yes, it does. Transudate, the pitting
edema, which is protein poor, sodium rich.
Protein poor, sodium rich equals transudate.
So that means the sodium did pass through
the capillary membrane that has to be cleared.
Now, let us move on.
Changes in the concentration will produce
an osmotic gradient obviously. Now these are
the steps that you will be taking chronologically,
forever more clinical practice, any board
exam in which you will get every single question
right as long as you follow this order of
pattern of the fluid shift. When you urinate
too much, for example diabetes insipidus, let
us say uncontrolled diabetes mellitus,
doesn't matter. Both of those have what? Polyuria.
Did you just hear what I just said? Polyuria.
You hear that. You automatically think actually
three differentials for the two important
ones at this juncture. Both begin with the
prefix diabetes. One is insipidus, one is
uncontrolled diabetes mellitus correct. Okay.
My point is this. If you are urinating in
great amounts, then what happens to your plasma
compartment? It is going to decrease. Is that
part of ECF? Yes, it is. So you tell me what
kind of change is now taking place with the
ECF compartment? A decreased in ECF volume,
number 1. And as we continue through here, we are
going to add more chronological changes so
that you eventually come to understand what
kind of shift we have between the two compartments
of ECF and ICF? That shift between ECF and
ICF is controlled by whom? The osmolality.
Now, this brings us to number 2 or step number 2. While step
number 1, when you are losing fluid, ECF volume. Now
the opposite could be held if you are putting
in too much infusion of saline, then you would
have increased in ECF volume. Anyhow, your
first step is going to be ECF volume either
a diminished effect or an increased effect,
what's the second step? It is the osmolality.
In diabetes insipidus as an example, if you
are losing too much fluid, ECF volume decreased
number 1, what happens to ECF osmolality? Increases.
Are we clear? Because you are losing fluid.
That is important. Step number 2, let us move on.
Now the water moves from obviously there is
osmosis here and you tell me. There is an
increase in ECF osmolality. What step is this?
Number 2, tell me about this fluid shift. Water is
being pulled out of the cell. This is due
to what? This is due to that osmosis. So in
your head clinically step number 3 would be ICF
volume and in diabetes insipidus that ICF
volume is then going to be decreased. Clinically
this then explains what we just talked about
water moving from a low to high solute concentration.
Now here we have water shifts. Do not alter
with urea concentration. As I told you earlier
and I emphasized this with our discussion
of urea. With urea, it is not part of your
effective osmolality in clinical situations.
Is that understood? So what are the two effective
osmolalities? It is the sodium times 2, why? Because
it is always attached to chloride and you
will take glucose into consideration if you
have uncontrolled diabetes.