Now you also have a few other
mechanisms to get hydrogen ions
out of this apical membrane
in the proximal tubule.
So, we talked about the sodium hydrogen
ion exchanger and that was very important
for eventually binding into bicarbonate.
You also have a secondary hydrogen ion transporter,
which is a hydrogen ion pump or ATPase.
So, this HATPase will also push hydrogen
ions across the apical membrane.
No, why is this going to be an important process?
Well, remember that a certain portion of the acid
that you took in by a non-volatile acids,
you are going to have to excrete the
hydrogen ion and make a new bicarb.
So, with the Titratable acids
is one of the ways that occurs.
So, let’s take phosphate as our example.
So, phosphate will be coming
down the renal tubule,
and you noticed phosphate
has a negative charge on it.
So, this is HPO4, 2-.
You will kick out hydrogen ion
from the hydrogen ion exchanger
or form the V type hydrogen ion pump.
It will bind then to phosphate. Once it’s
bound as phosphate, you can urinate it out.
So, this is how you can remove a hydrogen ion,
is you push it across the apical membrane,
have it bind to something and then,
have it be removed from the system.
You don’t have it bind to bicarb, why?
Because you want to reabsorb bicarb.
But phosphate, you can get rid
of some of your phosphate.
But you have to reabsorb your bicarb
so you don’t urinate out your bicarb,
you bind it rather to one of these Titratable acids.
The hydrogen ions that then
will be left will be in a bound form.
You may ask, how low can you get
your pH of your urine?
They say about 4,4 is about
the lowest pH of your urine.
but that could be anywhere from 4,4 up to
whatever is in the blood, maybe 7,4 or so forth.
So, that’s kind of your
range of pH of your urine.
You noticed that phosphate
has a pK of around 6,8.
This becomes important because it’s kind a maximally
bind hydrogen ions at it’s pK which is 6,8.
So, you can utilize this
when your pH is very close to 7.
Some of the other Titratable acids such as
urate and creatinine, these have a lower pK.
Therefore, the pH of the urine has to be lower
Before you’re going to be able to
utilize these two great extent.
You also have lactate and pyruvate but those pH’s
are actually, or pK’s are below the normal pH
That you can obtain in the urine.
So, these are less important. So, they have very
minor roles because of the lower pK values.
In or with Titratable acids, you can,
or make new bicarb in the proximal tubule,
the distal convoluted tubule and the collecting duct.
You make about 40 percent of your new bicarb using
titratable acids, phosphate, urate, and creatinine.
Which one are you going to use? Depends upon
the pH of the urine and it’s pK value.
So, if we look at something like a pH
range here for something like phosphate
has a wide pH range, so you can utilize
that throughout many different pH’s of the urine.
Something like creatinine, is a little bit tighter.
And so, you can’t always use creatinine
to bind those hydrogen ions
to be able to urinate them out.