[Music]
hello and welcome back to the nephrology
curriculum
today we're going to be talking about
water balance in the disney tremeos
so when we're talking about disorders of
water balance we're talking about
hyponatremia and hypernatremia
they're very common clinical problems
and you should know about them
and although the serum sodium level is
abnormal these clinical syndromes
reflect an abnormality in water balance
that may or may not be accompanied by
changes in sodium balance
disorders of water balance are
characterized by abnormalities in the
concentration of serum sodium
so hyponatremia means that we have too
much water
hypernatremia reflects too little water
and each of these particular disorders
can exist at
any level of total body sodium
so in order to understand this concept
it's really important to
discuss how total body water is
distributed
when we think about total body water
that's the percentage
of lean body weight so in men because
they have
more traditionally muscle mass or leaner
muscle mass than women
60 percent of their weight is total body
water
in women about 50 percent of their
weight is considered total body water
the way that that distributes is about
two-thirds
of total body water will be contained in
the intracellular fluid compartment
only one-third resides in the
extracellular fluid compartment
of that one-third three-fourths of that
will stay within the interstitial fluid
and only about one-fourth is actually
circulating
in the intravascular fluid compartment
now let's take an example
if we have an 80 kilogram man then
if we say 60 percent of his weight is
total body water
he has about 48 liters of total body
water
now remember two-thirds of that is going
to be in the intracellular compartment
that means 32 liters
are actually residing within the cells
we said that one-third is going to be in
the extracellular fluid compartment or
16 liters will reside within the
extracellular fluid compartment
of that we have 12 liters within the
interstitial fluid and only 4 liters
is actually circulating within the
vascular volume
now the next concept that's really
important when it comes to water balance
is to really understand the difference
between osmolality and tonicity
so plasma osmolality is really a measure
of solute concentration
it's determined by the ratio of plasma
solutes and plasma water
the majority of plasma solutes are
things like sodium salts the remainder
potassium calcium and glucose and urea
play less of a role
normal plasma osmolality is somewhere
between 275
to 290 milliamps per kilo
now glucose and urea contribute only a
small amount
to the plasma osmolality when they're
within the normal range
if they're abnormal they can actually
contribute quite a bit more
plasma tonicity is what we term the
effective plasma osmolality
that's the parameter that's sensed by
osmoreceptors
and it determines the transcellular
distribution of water
let's talk about that a little bit more
so plasma
osmolality is going to include osmotic
contribution of urea which is an
ineffective osmo since it can move
freely across cell membranes
so it has very little effect on water
movement
when it's moving between these two
cellular compartments
plasma tonicity on the other hand which
is the effect of plasma osmolality
really reflects the concentration of
solutes that do not easily cross the
cell membrane
so things like sodium salts for example
that will actually affect the
distribution of water between cells in
the extracellular fluid compartment
so this is an illustration that really
diagrams what we're talking about here
so in the top compartment or the top
part of the diagram
you can see the green which represents
our intracellular fluid compartment
that's our cell
and then the ecf which is adjacent to it
on the left
in our normal state our plasma
osmolality is somewhere between 275 and
290 milliamps per kilo
when we're in steady state water is
going to move freely between these
compartments
however if we have a situation where we
become hyperglycemic
then we have glucose that's added to the
extracellular fluid compartment
now remember that's an effective osmo it
cannot move
across that cell membrane freely
therefore the glucose
exerts a tonic effect on water and water
will then move from the intracellular
compartment
to the extracellular compartment so
again that's tonicity
that water is moving from that icf to
the ecf because of the presence of an
effective osmole
in that extracellular fluid compartment
and again
that's what's outlined in that left
lower quadrant
now what if we have a situation where we
have an increase in urea
so if a patient has an increase in urea
generation
even though the plasma osmolality
increases to 315 milliamps per kilo just
as it did with our patient who has an
increase in glucose
urea can cross the membrane freely
therefore it distributes equally between
the intracellular and extracellular
compartment
and thus does not exert a tonic effect
on water
all right let's also review some of the
osmolar substances within our body fluid
compartments
so in this graphical or in this table
what i want you to pay attention to is
our top
three labels which include our
intravascular fluid compartment
the interstitial fluid compartment and
then the
intracellular compartment now both the
intravascular and the interstitial fluid
compartment make up our extracellular
fluid volume
i want you to pay attention to a few
things just to keep in mind
let's look at sodium sodium if you note
is primarily located within the
extracellular fluid compartment
so highest in the intravascular volume
at 142 millimoles per liter
potassium on the other hand is highest
in the intracellular compartment
and we only have very small amounts of
potassium located in our extracellular
fluid compartment urea on the other hand
is equally distributed among all depart
all compartments
and then finally when we look at the
total osmolar concentration it's equal
across the intravascular interstitial
and the intracellular compartment
so when we think about water balance we
have to also think about something
called
oblique osmolar excretion this is how we
can handle water so obligate osmolar
excretion is the amount of osmoles
that needs to be removed by the kidney
in order to maintain osmolar
homeostasis it's dependent
on dietary intake so our basal
metabolism
in the fasting state we can generate
about seven milliamps per kilo per day
now our urine can actually be diluted to
about 50 milliamps per liter
and it can be as concentrated as 1000
milliamps per liter
so if we either want to get rid of free
water or we want to conserve free water
we can do that
through manipulating the concentration
of osmolarity in our urine
this way we can accomplish both osmolar
and water balance simultaneously
[Music]
you