Hyponatremia Explained Clearly (Remastered) – Electrolyte Imbalances
09
September

By Adem Lewis / in , , , , , , , , , , , , , , , , , , , , , , /


okay welcome to another MedCram lecture
now I’ve got a lot of requests out there to talk about a very difficult topic
we’re gonna talk about hyponatremia okay low sodium now this is a very
complicated topic because we have to talk about water balance we’ve got to
talk about the difference between osmolality and tonicity and antidiuretic
hormone and aldosterone and compartments of fluids and the different types of
hyponatremia are they isotonic are they hypotonic hypertonic and then of the
hypotonic hyponatremia is it hypovolemic hypervolemic and I saw bulimic and it
just goes on and on and on but I think after you get done with my series here
in fact I know after you get done with my series you’ll be able to work through
how to deal with the different types of hyponatremia so to do that we have to
build a little bit of a foundation so you can understand what we’re talking
about so bear with me and go through these lectures and you’ll see that it’ll
build on one another just like our other series on acid-base okay so the
definition of hyponatremia is a sodium concentration of less than 135
milligrams per deciliter okay that is basically the definition anytime you
have any word that says amia at the end it’s relating to the concentration of
sodium in this case sodium or any other molecule in the blood so hypokalemia
alkyl emia acidemia all of that is related to the blood so let’s go ahead
and build the foundation the first thing that you should know is what is the
definition of osmolality so osmolality OS m o l al ity so that’s serum
osmolality as an equation and it’s two times the sodium concentration plus the
glucose concentration divided by 18 and that glucose concentration
is in milligrams per deciliter plus the B UN and please look at our acute renal
failure lectures to get a update on the B UN and that’s again in milligrams per
deciliter divided by 2.8 and the normal for that is 285
mili moles per kilogram you can also say milligrams per deciliter as well so that
is the definition of serum osmolality 2 times the sodium plus glucose divided by
18 plus bu n divided by 2.8 and that’ll tell us if something has a basically a
low osmolality a normal osmolality or a high osmolality now something that’s
very similar to that is tonicity it’s a very similar concept and you’ll see it
looks very similar it’s 2 times the sodium concentration plus the glucose
divided by 18 once again so why is it missing the B UN component it’s missing
the b1 component because B UN can go freely between the plasma membranes and
so therefore if b1 can go between plasma membranes it really doesn’t have too
much of an effect on the difference between the inside of the cell and the
outside of the cell so from now on what you’ll see is we’ll use tonicity and
osmolality interchangeably but just be aware that if they ask you to calculate
the osmolality of something this is the equation that you should use ok we’ll
come back to that again and again the other thing you should know about is two
major types of hormones one of them is ADH this is antidiuretic hormone
antidiuretic hormone is secreted from the posterior pituitary it’s actually
made in the hypothalamus but it’s secreted from the posterior pituitary
and if you can remember where this works it works at the collecting tubules of
the nephron and the presence of ADH at the collecting tubules of the nephron
causes water reabsorption and what’s that going to do what’s
reabsorption gonna do it’s going to cause water not to go out to the urine
but to be reabsorbed into the blood and basically it’s gonna have the effect of
diluting out a lot of your substances but specifically it’s going to be used
to retain water the this is a protein hormone and so it works almost
instantaneously as soon as it secreted the other hormone is aldosterone you’ll
if you look at our adrenal gland lecture you’ll see that aldosterone is a steroid
hormone it is secreted from the zona glomerulosa and it works primarily at
thee so we’ll put down here adrenal cortex and it works primarily at the
distal convoluted tubules of the nephron and it’s action is to cause reabsorption
of sodium and because of that water and it causes excretion of potassium and
excretion of protons so just be aware of those things we’re going to talk about
that obviously because water and sodium are going to be intimately related to
the discussion that we’re gonna have today okay so any discussion regarding
intracellular extracellular fluid you’ve got to know about the different
compartments of the human body so let’s get into a discussion of that and this
drawing is going to be pretty important so I want to make sure that you
understand the difference nuances are regarding this so this is a diagram
essentially showing you the different compartments of fluid in the human body
okay so this is the intracellular fluid so this represents this compartment here
represents all of the all of the volume inside of cells this is the
intravascular volume this represents all the fluid inside arteries veins
capillaries etc and this is the interstitial volume
this is separated this intravascular volume is separated by the interstitial
volume by the capillary wall and we know the capillary wall is not a barrier to
the movement of electrolytes and fluid and I’ll represent that here by simply
drawing little openings in that so basically sodium and water can go
back and forth between these two and so basically this is effectively one
chamber for or one component or one container for electrolytes fluid things
of that in other words this wall does not separate their movement on the other
hand this wall here that separates if you will the extracellular fluid from
the intracellular fluid is separated and there is a pretty tight barrier this is
basically the the membrane of the cell now remember animals don’t have cell
walls like plants do but yet this is a pretty rigid wall when it comes to
electrolytes water is able to move through it very freely of course because
it’s a not ionic compound but in terms of sodium sodium cannot go through this
it is not permeable to sodium if sodium is going to go through it’s got to go
through channels and so this is very well regulated sodium is not to be able
to go through however water can go through all of this okay so water can go
through here and water can go through here and so because of that because
sodium is able to freely go through here and water is able to we’ll call this
simply this whole thing the extra cellular fluid compartment so this is
intracellular fluid extracellular fluid and generally if you want to look at
compartments this is about 4 liters this is about 10 liters and this is about 28
liters of fluid so the key here is that there are three compartments the
intravascular and the interstitial combined to form the extracellular fluid
this is where sodium and water mix water is able to go across this semi
of permeable membrane but sodium is not and that’s it that’s a key there so when
we’re talking about drawing blood with needles okay this is the fluid
compartment that we’re talking about when we’re talking about hanging IV
fluids okay it goes into the vascular and
extracellular fluid compartment so the questions going to be is what happens to
these fluid compartments when there are perturbations in sodium and fluid
management and we’re going to talk about that so the way we’re going to represent
this from now on in the lecture to simplify it is basically two
compartments and remember this is going to be the intracellular fluid and this
is going to be the extra cellular fluid now why do I have it drawn this way as a
vertical and as a horizontal well the way I’m gonna represent this if you can
imagine these are X and y axis is the x axis is going to represent volume so how
much so simply this dimension here represents how much volume is in there
and the y axis is going to demonstrate the sodium concentration so the higher
this is on the y axis the higher the concentration of sodium or the lower the
concentration of sodium and the farther out this goes on the x axis the more
volume there is or consequently the less volume this is now because of that
you’ll note that the area then is simply represents the number of sodium
molecules let me give you an example if I were to add water to this just
straight out water what would that do the water would add volume to this
compartment but at the same time it would cause the sodium concentration to
go down so instead of it instead of it being these dimensions we would have
something more along the line of this dimension but you could see that the
area in the box would be the same because the number of sodium molecules
wouldn’t change but the way this is depicted on the
graph would change because we have a lower sodium concentration and a bigger
volume so that’s how we’re going to depict this just to finish up the
analogy if I did by the way add h2o to this you would see what would happen
here the first thing that you would see will happen is that the sodium
concentration would go down and that the volume would get bigger that would be if
there was absolutely no transport of water across this membrane but we know
that that’s not the case because as soon as this sodium concentration would go
down remember this is a permeable membrane to water and if water is on
both sides of this and the sodium concentration goes down on this side
water is going to go to the side of the higher concentration that’s the law of
osmosis so you would have water going across from the extracellular fluid into
the intracellular fluid and you would see that would happen until what would
happen until this would be diluted down to a similar level well in fact what
would happen is is that this would go down and then this would go down to meet
this until this was equal but because this has a much larger volume in fact
it’s probably twice the volume of this that about twice the volume of water
that we’re putting in here would go into the intracellular fluid chamber and so
you can see here that when you give pure water or in medically speaking dextrose
d5w when you give water to somebody you can see why this does not expand the
intra the intravascular volume it’s because the majority of this volume as
soon as it goes into the extracellular fluid is going to go across the membrane
and basically fill up the intracellular volume rather than the extracellular
there will be some left in the in the extracellular volume but because this is
a larger volume more of this is going to go into the intracellular varm so it’s a
review free water goes in it causes a reduction
in the sodium concentration because of that the law of osmosis means that water
will keep going across this membrane until the sodium concentration in the
cell matches that in the extracellular fluid which means that most of the water
that you give the patient if it’s in the form of free water is going to go into
the intracellular fluid compartment now that’s as opposed to giving someone
normal saline or as we like to say 0.9% okay or 0.9% normal saline so normal
saline when is given it’s basically as if you’re just adding it on to the edge
and why is that because the fluid concentration that is being added to the
extracellular volume is already at exactly the same concentration or very
close to the same concentration of the fluid that’s already there because of
that that will simply be added on to the volume there is no change in the
extracellular fluid concentration of sodium because of that there is no shift
of fluid over into the intracellular fluid and that is why normal saline is
the best type of fluid to give if your primary person purpose is to expand the
extracellular fluid compartment and that’s how that is represented here we
see that the volume has gone up but the sodium concentration in that
extracellular fluid is the same okay so with these basics we’re gonna move on
and talk about the different types of hyponatremia the hypotonic hyponatremia
the hypertonic hyponatremia and the hypotonic hyponatremia so isotonic
hypertonic and hypotonic we’ll talk about those in the next lecture thanks
for joining us


20 thoughts on “Hyponatremia Explained Clearly (Remastered) – Electrolyte Imbalances

  1. Hi I have a question that I'm having trouble solving:
    Na- 120
    Urine Na- 10 (low)
    Serum osmolality- 280 (normal)
    the normal and low in the bracket was given by the question not me
    What is the reason for this patient's hyponatremia?
    The answer given is hypotonic hypovolemic hyponatremia.
    But the osmolarity is considered to be normal so is it possible for this to be hypotonic?

  2. Thanks for the video. I thought sodium concentration outside the cell was higher because of the sodium potassium pump, so why does that graph in the video have the sodium concentration as being the same inside and outside the cell?

  3. Nice video! Great overview of the physiology! I have a video on hyponatremia that might complement yours! It's a practical approach to hyponatremia in the clinic. It'd be great to hear some feedback! Keep up the good work sir!

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