Bicarb reabsorption occurs in the nephron.
About 80 percent of it is pulled off in the proximal tubule,
about 10 percent in the thick ascending limb,
about 6 percent in the distal convoluted tubule
and about 4 percent in the collecting duct.
And I like to highlight here that you
really absorb all of your bicarb.
you will not urinate any of it
out in normal conditions.
In the proximal tubule, but let’s now take
this into a little bit more detail.
How are you reabsorbing the bicarb?
It’s actually more complex than you think
because bicarb is a charged molecule.
If we don’t have a specific transporter, that’s going
to be able to move it across the apical membrane.
So, we have a little and genius way of doing this.
We utilize a transporter called the NHE3, sodium hydrogen
ionic exchanger. To kick out a hydrogen ion.
It kicks out this hydrogen ion
combines to form bicarbonic acid.
Carbonic anhydrase then converts
it into water and carbon dioxide.
So, now you have water, which
can be in the renal tubule
and carbon dioxide, which now can
freely move across the membrane.
It moves into the cytosol, you have a different
carbonic anhydrase, which is the enzyme
that helps convert this back to bicarbonate acid.
So, you have two carbonic anhydrases.
Carbonic anhydrase 4, which is on the apical membrane.
Carbonic anhydrase 2, which is in the cytosol.
This complex process all it did was allow you
to get a molecule across the apical membrane.
Now, once you have formed carbonic acid it
once again can disassociate into a hydrogen ion
which then can be used to be kicked back out
through the sodium hydrogen exchanger.
and the bicarb can be reabsorbed
across the basolateral membrane.
So, that’s how you reabsorb 80 percent
of your bicarb through this mechanism.
Across the basolateral membrane, which is
the opposite side of the apical membrane,
where the sodium hydrogen
ion exchanger occurs.
There is two different ways we move
bicarbonate out of the cell.
The first way to do that is by
a co-transporter that involves sodium.
The sodium co-transporter then allows for both
bicarb and sodium to travel across the basolateral membrane
following the sodium gradient to move out bicarb.
The second mechanism that you would have
to move bicarb out of the basolateral membrane
is through an exchanger.
This exchanger uses chloride, and chloride moves
into the cell and bicarb moves out of the cell.
This is an important process to think about because
you need to now get the bicarb, which was in the cytosol
out across the basolateral membrane.
So, either co-transported with bicarb
and sodium or exchanged for chloride.