Continuing our discussion of the nephron where
at the level of the loop and the distal convoluted
tubule. Put these together and as we go through
this, it is important that you pay attention here.
Once again the physiology is important so
that you can then make sense of the pathology.
Otherwise things become really confusing and
you start memorizing. There is no end to it.
Overview: We are done with the PCT in its
entirety. We are moving down through the descending
limb, through the loop and as you move through
the loop, tell me about the urine tonicity?
Increased. Hypertonicity. As we move into
the area of C in ascending limb of thick ascending
limb and you have a symport there called your sodium-
potassium-2 chloride and then, later on,
we get into the portion where it says D as in delta.
There will be distal tubule and there we shall
take a look at our sodium chloride channel.
Let us take a look at our loop. Now the loop
is well as we moved down the descending limb,
it is impermeable to water and thus, you have
medullary hypertonicity. What kind of segment
would you then call this? The concentrating
segment. Good and makes the urine as you can
imagine where? In the medullary loop of Henley
as being hypertonic. Impermeable to water.
What is my next step? You are going to move
up the ascending limb and as you move up the
ascending limb, what then happens? While you
are focused, once again we have organized
this cell plenty. Let us do it here once more.
We have your urine on your left side, the
lumen. Then you have the epithelial cell or
the renal epithelial cell and then on the
right side, you have interstitium in your
blood that which is being reabsorbed or then
move through your symports .
So therefore what are you permeable to here?
Take a look at C as in Charlie. Here in the
thick ascending limb, you are permeable here
to the solute. Correct. If you are permeable
to solute, what are you impermeable to? Water.
Okay. Another important concept. Sodium travels
with whom? Chloride so that it maintains electroneutrality.
You know that from basics. Next, along with
sodium, wherever sodium goes with the solute
what is its pressure called? What is its method
of diffusion? It is called osmosis. Right. What does
osmosis mean? It means diffusion of water
from one compartment to another or you have
increased solute. Are you clear? Interesting.
So here in the thick ascending limb, you are
ripping the solute away from the water. Where
is the water in this picture? In the lumen.
You are separating and ripping the solute
away from the water. What do you call this
water now? Free water. It has been freed.
Freedom, liberty. Do as it wants. Prior to
this, what kind of water did you have? Clinically
we call this obligated water. Obligated water
is when you have sodium bound with water.
Here when you rip the solute apart and you
are reabsorbing it, then the water that's left
in your thick ascending limb is called free.
Thus what do we call the distal convoluted
tubule, the diluting segment? What is the
urine osmolarity? Hypotonic. You listen to
what I am saying. Hypotonic. What was it in
the descending limb? It was hypertonic because
it was permeable to water. What is this mechanism
in which you have exchange of water and solute
in this area between the descending limb and
the ascending limb called? It is called the
countercurrent mechanism. Remember this. Right.
The countercurrent mechanism. Now we don't
have enough time to go through all of that,
but at least you know that the counter current
mechanism is being established here so that
you can have proper tonicity information of
the urine. If you do not do all of this meaning
the counter current mechanisms within the
loop, you cannot properly concentrate the
urine, thus you cannot properly dilute the
urine. That is important for you to understand.
Once again if this counter current mechanism
is going to be pathologically affected, magic
word, pathologically affected, you cannot properly
concentrate this urine nor can you properly
dilute this urine. Why is that important?
You shall see. Now the important physiologic
aspects of this that we have talked about
already. Take a look at sodium-potassium-
2 chloride. You see it. That's your symport.
What is the name of the drug that knocks it
out quickly? Good. Loop of furosemide
Very good. Next if you bring in that sodium-
potassium into the cell, you increase the
concentration. Where do you begin all off this
process? Like everything else with the sodium-potassium
pump. There is potassium that is accumulating
inside my cell and this potassium is going
to be forced to do what? Back leak, which
then facilitates what? Take a look at the
picture please. You see the magnesium and
calcium that is being paracellularly reabsorbed.
So if this entire mechanism of the loop where
it destroys the sodium-potassium-2 chloride
mechanism, you lose everything.
Now the thick, continuing, actively reabsorbs
here sodium-potassium-2 chloride, indirectly.
What does it mean to you? Back leak will reabsorb
magnesium, calcium. Why? As the potassium
comes back out into the lumen, back leak, you
are increasing the positivity. Right? Transmembrane.
Increase in the positivity. So therefore you
are literally going to what would likes
do? Charges. Would you likes to do? Repel.
So there it goes. It is repelling the magnesium,
calcium between the cells. Think of it such.
You will be fine. Now it is important that
you pay attention to the potassium channel. You
want to know the name of this potassium channel.
Think of it as being the ROMK. At least know outer
medullary potassium channel the receptor for.
Receptor for the outer meduallary potassium,
ROMK. Know this, because at some point
in time when they refer to this, they now know that
this is the back leak channel responsible
for reabsorption of magnesium, calcium. You
need to know that detail. Yes you do. Trust
me. New questions all the time. Evolution of exams.
Evolition of what we know so that you have the proper
information and equipped properly so that
any question that comes at you, you are not
fooled and you know the level of detail that
is required of you. Move on. Impermeable to
water. We talked about, where am I? The thick
ascending limb. What we call this? The beginning
of the diluting segment. What does the diluting
mean? There it is. Hypotonic. So down here.