Significance of clearance of water. So tell
me about water. Water, where? In the nephron
okay. There are two types of water and by
that we mean, it could be either free or it
could be obligated, right. What does obligated
mean to you? It is bound to electrolytes. In
the thick ascending limb, are you there, what are you doing?
We are moving the electrolytes. What are they?
Sodium, potassium, 2 chloride. Thus in your
nephron, what are you left with? You are left
with free water. Let us now take one step
further and we'll call this clearance of water.
Clearance can be zero. It could be positive
or it could be negative. Before we move on, can
you please tell me the concept of clearance?
Clearance means what you are actually evacuating
from the body. What is your urinating? What
is your clearing? Remember all the terminologies
that we did earlier, we have filtration, then
at the PCT you can modify it, you can reabsorb
or you can secrete and then finally you get clearance.
So it could be either zero or it could be
positive or it could be negative. What does
that mean? It is important that you understand
this clinically. The explanation of these
values are the following. If the clearance
of water is zero, what does that mean to you?
It means that the clearance of water is zero
when no solute-free water is excreted. What
does that mean? That means that what you are
here releasing is going to be what is called
the isosthenuria. In other words, this is
isoosmotic. What this means is it is obligated
water. So it is isotonic. There is no solute-free.
So that means it is bound with sodium making
it what? Isoosmotic, with plasma. You call
this isosthenuria. Interesting. Last time
we talked about isosthenuria and we will talk
about this again. This is a big deal. Isosthenuria
represents the fact that well let us say that
you had urinary tract infection. I bring that
up quite a bit because it is an excellent
example to really highlight and demonstrate
a lot of key issues. So if you have the ascending
infection come all the way to the kidney, polynephritis
and at some point that E. colis
in my pelvis, there it is, then it might cause
a little bit of destruction or lot of destruction.
It might have necrosis. Where are you in the
kidney? You're in the papilla. What is in the papilla?
What part of the nephron? How about the medullary
loop of Henle? Say the loop of Henle,
the medulla and it's responsible for doing what?
Creating hypertonic urine and if you have something
called renal papillary necrosis secondary
to an ascending E. coli type of urinary tract
infection, then, unfortunately, you won't
be able to properly form hypertonic urine
and you might have isothenuria. So the fact
that you have isothenuria in terms of clearance
of water, it'd be zero. But in terms of
pathology and urinary tract infection, it
could be a problem. Is that clear? Example
here, treatment with a loop diuretic. So if
you have a loop diuretic, what is it going
to do? You are going to block off the sodium-potassium-
2 chloride in the thick ascending limb and
all of that urination isn’t this the most
potent diuretic? Of course, it is. All
the urination is taking place subsequently
taking this drug, what kind of water? Clearance
of water is zero. Mean to say that it is not
solute-free. It is bound to sodium. Clearance.
Let us talk about further descriptions.
Clearance is zero. No free water is generated.
NaCl reabsorption is inhibited by? Well maybe
something like your loop. If free water is
not generated, it cannot be excreted, can
it? The ability to dilute the urine during
water drinking is impaired, why? Or you can’t
dilute the urine because of sodium-potassium-
2 chloride might be then impaired. The ability
to concentrate the urine during water deprivation
is impaired. It is all dealing with something
like your clearance and you are giving your
loop diuretic. Interferes with generation
of corticopapillary osmotic gradient, understand
what this is. Cortico, that's your Bowman space,
PCT. I want you to go into the papilla, where are you
now? In the medulla. Is that clear? So therefore
if you have a loop diuretic as your prototype
here and you have impairment of sodium-potassium-
2 chloride, what have you done? You have impaired
the corticopapillary osmotic gradient. Therefore,
what is your clearance of water here zip?
You are not producing any free water.