00:01
Bicarb reabsorption occurs in the nephron.
00:05
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.
00:19
And I like to highlight here that you
really absorb all of your bicarb.
00:23
you will not urinate any of it
out in normal conditions.
00:27
In the proximal tubule, but let’s now take
this into a little bit more detail.
00:33
How are you reabsorbing the bicarb?
It’s actually more complex than you think
because bicarb is a charged molecule.
00:44
If we don’t have a specific transporter, that’s going
to be able to move it across the apical membrane.
00:52
So, we have a little and genius way of doing this.
00:55
We utilize a transporter called the NHE3, sodium hydrogen
ionic exchanger. To kick out a hydrogen ion.
01:08
It kicks out this hydrogen ion
combines to form bicarbonic acid.
01:14
Carbonic anhydrase then converts
it into water and carbon dioxide.
01:22
So, now you have water, which
can be in the renal tubule
and carbon dioxide, which now can
freely move across the membrane.
01:32
It moves into the cytosol, you have a different
carbonic anhydrase, which is the enzyme
that helps convert this back to bicarbonate acid.
01:42
So, you have two carbonic anhydrases.
01:45
Carbonic anhydrase 4, which is on the apical membrane.
Carbonic anhydrase 2, which is in the cytosol.
01:53
This complex process all it did was allow you
to get a molecule across the apical membrane.
02:01
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.
02:14
and the bicarb can be reabsorbed
across the basolateral membrane.
02:19
So, that’s how you reabsorb 80 percent
of your bicarb through this mechanism.
02:25
Across the basolateral membrane, which is
the opposite side of the apical membrane,
where the sodium hydrogen
ion exchanger occurs.
02:33
There is two different ways we move
bicarbonate out of the cell.
02:38
The first way to do that is by
a co-transporter that involves sodium.
02:47
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.
03:01
The second mechanism that you would have
to move bicarb out of the basolateral membrane
is through an exchanger.
03:09
This exchanger uses chloride, and chloride moves
into the cell and bicarb moves out of the cell.
03:17
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.
03:27
So, either co-transported with bicarb
and sodium or exchanged for chloride.