Now the PCT, the primary site of sodium reabsorption.
We talked about this. Now, an important point
and I actually have a separate piece of slide
that will further explain upon this point.
So stop here because you can't just read through
this. Okay. Bear with me. Listen, please.
Students get this confused and they keep getting
this wrong on boards, what have you? Listen
clinically what is it that is important for
this concept? The concept here is EABV. It
stands for effective arterial blood volume.
Is that clear? Effective arterial blood volume.
Stop. Go back one step, foundation. What is
that foundation referring to? Total body water.
Why am I bring that up? Total body water
equals ECF and ICF. Of that, you tell me about
ECF. 1/3 right. 2/3 is ICF. Let go ICF.
I don't want that right, not relevant
to our discussion. ECF yes, 1/3. What
is ECF? Plasma interstitium. Tell me what
is used in every single physiologic text book
that you have to know now? It is the fact
you have 3/4 is interstitium. What is
interstitium? Tissue. What is plasma? 1/4.
That is your effective arterial blood volume.
That is the most important parameter in which
your baroreceptors in the carotid sinus, your
juxtaglomerular apparatus in the afferent
arteriole is going to be paying attention to.
Correct? So let us say that you have a situation
where you lose your effective arterial blood
volume. How do that happen? Maybe there's
hemorrhage and the blood is on the floor. You
see it? Low blood. Or what else happen? The
blood escaped into the interstitium.
So now effective arterial blood volume is what?
Decreased, whatever reason.
If it is, now you walk me through this. If
it is effective arterial blood volume that
is decreased, then well the question is you
decrease your GFR and you also decrease your
RPF. What am I referring to when I say GFR
and RPF? What formula is that? Filtration
fraction. So your first instinct is if both
are decreased and there must be no change.
Nope, that is not what happens clinically
and that is why you are paying attention
to me. So when you have a decrease in effective
arterial blood volume granted, you have a
decrease in GFR because a decrease hydrostatic
pressure. And you have a decrease renal plasma
flow because you don't have effective arterial
So what might you want to do from the peritubular capillary?
I want you to go down to the peritubular capillary
now and your effective arterial blood volume
has narrowed. So you have very little blood,
which is coming through, into the peritubular
capillary. You see the corner there. Turn
the corner. Okay. So now you tell me about
what is known as your oncotic and hydrostatic
pressure? So what ends up happening here is
that your oncotic pressure is going to be
greater than your hydrostatic pressure when
you have decreased the effective arterial
blood volume? Why? Because you don't have
as much plasma or fluid, but then you have
a lot of protein. So, therefore, your oncotic
pressure is quite high in the peritubular
capillary. Take a look at this green line
here or the green highlighted area, and you
notice the P is referring to the fact that
this is your plasma and your oncotic pressure
is going to be much higher than the hydrostatic
pressure. Where? In the peritubular capillary.
So therefore, what you must keep in mind and
what you must accept is that your filtration
fraction is increased when you have a decrease
in effective arterial blood volume and that
only makes sense so that you allow for more
reabsorption to take place because you have
decreased effective arterial blood volume.
Now that is one example and if that physio
was weak there, we are going to keep adding
to it and give you specific pathologies that I
have been doing here a little bit. Now let
us do the opposite, shall we?
Let us take a look at that decreased EABV
in greater detail. It is an important concept.
Students get this confused because they either
overlook it or think they have it, but
they really don't. Please pay attention. So
you have a decrease in EABV, why? I'll give you
examples in a second. A couple of examples
that I already gave you was congestive heart
failure or perhaps you lost your blood, and there it is on the ground.
Now all comedy aside, let us take a look at what happens
seriously. You have a decrease in EABV. Okay
great. And more that you have decreased in
EABV, then tell me as to how was the kidney
going to interpret this? Decrease renal perfusion,
isn't it? So won't you think that you might
want you to increase the reabsorption of your
substances? That is just common sense. So
as you travel through here, you have an afferent
arteriole with decreased effective arterial
blood volume, in other words your plasma. Don't
worry about the green circle right now, I
will come to that in a second. I want you to skip
over to the efferent arteriole and how much
do you have of your EABV? Right now it is a
little bit decreased. I want you to go down
to the peritubular capillary, please. If you
don't have as much effective arterial blood
volume, would you please tell me as to what
the hydrostatic pressure is in the peritubular
capillary? It is decreased, isn't it? Why?
Because there is decreased effective arterial
blood volume. Now if that is decreased, now
the albumin or the protein never got filtered
so, therefore, the oncotic pressure in the
peritubular capillary, the Po that you are
seeing here and that's what you are paying attention
to, is then referring to the reabsorption.
If you take a look at that arrow from the
PCT for reabsorption, what does that mean?
It means that coming out of your urine, it
is extremely black and dark and bolded. And that to you should
indicate there is an increase in filtration
fraction when you have a decrease in EABV
even though technically of a decrease in both
GFR and RPF. That must be understood and I don't care how you
want to do this, but at this point, I have
given you an understanding as to why you have
an increase in filtration fraction so that
you can have an increase in reabsorption when
you have a decrease in effective arterial
blood volume. Now let me give you some examples
clinically so that you get this. Now this
part you are probably good in terms of the
pathologies, but are you able to then bridge
over to the physiology so that you get your
questions righ. And this is where it becomes
important clinically and this is where students
are able to follow a question or attending
to a certain extent, but then they trip over
themselves because they are just not thinking
enough. So examples of decreased EABV, here is congestive
heart failure, cirrhosis, and hypovolemia.
I already gave you two of these examples already
for decreased effective arterial blood volume.
If it is congestive heart failure, who is not
functioning properly? The heart. So, therefore,
what happens? Things are going to back up.
What things? The plasma. Specifically, you
will have your protein poor substances, which
are then leaking out of your plasma into the
interstitium, correct? Who is your patient?
Pulmonary edema, left sided, right-sided, positive
JVD and pitting edema. So there is my confirmation
that fluid is leaving my blood vessel vasculature.
If you have fluid leaving the vasculature
going into the interstitium, what then happens
to your effective arterial blood volume? Obviously
decreased. Are we clear? What happens to
filtration fraction? It is increased. Reabsorption
is increased in this particular setting. Cirrhosis,
tell me about cirrhosis and why is it even
here. Tell me as to what you look like in
cirrhosis. You see my tummy. I am joking.
I don't have cirrhosis. Why am I pointing to this?
If there's cirrhosis, the liver is dead. Next, what happens?
Let me go through the one in
which albumin synthesis is not being produced
adequately. So decreased synthesis of the
albumin, what happens to your oncotic pressure?
It decreases. Thus, what happens to your fluid?
Escapes into your interstitium. What then
happens to your effective arterial blood volume?
Example number 2 of decreased EABV and number 3 you literally
have got into state of hypovolemia, maybe
there is excessive sweating or maybe perhaps
there is a massive hemorrhage. What happens
to effective arterial blood volume? It decreased.
What happen to filtration fraction? Increased.
Do not forget that.
If that is our first example of an alteration
of EABV, let us take a look at the opposite
end of the spectrum. This is going to be increased
effective arterial blood volume, what does
this mean? I want you to conceptualize this
first and we will approach this exactly as
to how we did with the decreased EABV. With
increased EABV, effective arterial blood volume,
there is more fluid passing to efferent arteriole,
more across the GFR, more through efferent
arteriole, more where? Through peritubular
capillary. Would you please tell me about
the hydrostatic pressure in the peritubular
capillary? What can you predict? An increase.
Take a look. Increased effective arterial
blood volume. More fluid passing through.
Want you to skip over your glomerulus right now.
Go to the efferent arteriole. Continue through
the peritubular capillary. You told me or
you followed me with knowing that your hydrostatic
pressure H, you see the H, hydrostatic pressure,
peritubular capillary, it is increased. You
tell me about reabsorption right now. Reabsorption
increase or decrease with increased effective
arterial blood volume. Decreased reabsorption.
Decreased reabsorption, why? The hydrostatic
pressure in the peritubular capillary is pushing.
In other words, it is offering resistance
to reabsorption. So what then happens to your
filtration fraction with increased effective
blood volume? You are not reabsorbing. Your
filtration fraction is decreased. What does
that mean? Well, tell me about your blood
pressure with increased effective arterial
blood volume. Obviously increased. What might
you want to do with your sodium? Take a look
at that bottom part right there, as you pass
through the PCT. See the bottom line there,
it says lose sodium and water. Isn't that what
you might want to do physiologically, so that
you try to decrease your blood pressure? Yes,
you do. So you decrease your filtration fraction.
You decrease your reabsorption in the hopes
of getting rid of your sodium and water so
that you can perhaps decrease the blood pressure.
If you have ever been confused before, I am trying to bring your
clarity. If you have never even seen this before,
please make sure that you know this well and
those of you that are reviewing with me and
all you needed was a little bit of a trigger
of things that you know, you have in the back
of your head. Here it is. Make sure you know
this well. Now let us take a look at examples
of increased effective arterial blood volume.
Say that you have mineralocorticoid excess. Give
me some examples where you might have mineralocorticoid
excess? You have heard of Conn's syndrome?
Many times we have talked about that. You
have heard of licorice? Dr. Raj what the heck
does candy and a tumor in the adrenal cortex have
in common? It is the fact that licorice has
a component in it. Believe it or not, especially
black licorice has a component in it. If you
suck on a licorice that is black all of the
time, you are actually be introducing mineralocorticoid
into your body. Believe it or not. Believe
it. Whereas if you have Conn's syndrome, that
is a tumor adrenal cortex that is releasing
too much aldosterone. Is there mineralocorticoid
excess? Yes. I want you to go in collecting
duct, what are you doing? Reabsorbing sodium
and you are reabsorbing more and more water.
What then happens to your effective arterial
blood volume? It is increased. What happens
to your hydrostatic pressure? Listen to my
question. What happens to the hydrostatic
pressure in the peritubular capillary? It
is increasing hydrostatic pressure in the
peritubular capillary. What is it doing to
reabsorption? It is offering resistance. So,
therefore, what then happens to filtration
fraction? It is decreased because what you
are trying to do? You are trying to get rid of the
sodium and water. Trying to. Is that always
going to be successful? It depends, right.
And then what is the other one? Isotonic gain
in fluid. What happen here? Maybe this was
a patient that came in who was hypovolemic.
Okay. Now if you have a patient that is sweating
and has been sweating severely in the hot
hot sun, and you are going to bring them
in. What are you going to give them immediately?
IV fluids, good. What is that IV fluid that
you are going to give? Right off the bat,
0,9% saline. What does that mean?
0,9% percent represents your normal isotonic saline.
What if you give too much? What happens to
the effective arterial blood volume? It increases.
What happens to hydrostatic pressure in the
peritubular capillary? It increases. What
happens to filtration fraction? It is going
to be decreased. Are we clear? Spend a little
bit of time here, please. Make sure that you
get these concepts down before you move on.
It will show up. I can guarantee.
In some way, shape or form.
Here we have further, a little bit more in our discussion
of the PCT with the bicarb. Luckily we have walked
through this enough or we can quickly take
a look at the verbiage. Okay. So here is my
bicarb. Does it get filtered? Simple questions
that I am asking you. Answer them. Bicarb,
does it get filtered? Yes. Does it get secreted?
Not at all, but majority of your bicarb in
your PCT synthesized or reabsorbed? Reabsorbed.
80% of it, how? Indirectly. What do you mean indirectly? Remember
this. We talked about bicarb. As soon you see bicarb,
what formula is coming to mind? Carbonic
anhydrase formula. Verbiage, mechanism requires
bicarb joins with hydrogen and you
need the help of your carbonic anhydrase. Number 1,
you bring in your water and carbon dioxide
into epithelial cell and with the help of
your second carbonic anhydrase, you form carbonic
acid and let me ask you something. For every
bicarb, take a look at the bicarb in your
epithelial cell inside that green box. Where
is that bicarb going? It is going into the
blood because you are reabsorbing it. Are
we clear? My question is this. For every
bicarb that you reabsorb, what are you doing
to your hydrogen? You're secreting it. Are we clear?
The same picture and formula can be used
up in the stomach in the parietal cell that
we'll do and we have done in gastroenterology.
The same concept can be done in the RBC when
you are dealing with the transportation of
carbon dioxide. Okay. Are we good? Now with
the proximal convoluted tubule, here is the
exchange that you are looking for and carbonic
acid then disassociates into hydrogen bicarb.
The bicarb reabsorbed into the blood. For every
bicarb that you reabsorbed into the blood, what
are you throwing out into the urine? You are
throwing out the hydrogen. What do you require
for all this to occur properly? You need to
make sure that you have a sodium-potassium
ATPase pump on the basolateral membrane at
all times to always maintain proper gradient.
Let me ask you one little
thing, that hydrogen those are being put out into
the urine, was it a symport or antiport?
Antiport, mean to say that hydrogen was doing
what? It was being thrown out into the urine.
Take a look at the previous discussion, and
sodium was being reabsorbed, just like you
would expect. On where? The apical membrane.
PCT is the place that you definitely want
to pay attention because there are a lot of
things that are going and that affect the entire
body in numerous numerous methods as we have