So getting our transporters in place, you
see that there’s a two-step process.
We need to talk
There’s acinar cells and
then there are ductal cells.
In the ductal cells, this is the portion
that is going to be reabsorption occurring,
and it means the secretion portion
occurs in the acinar cells.
Water is moved in an isosmotic
form into acinar cell lumens.
Chloride is reabsorbed
in the ductal cells.
Sodium is reabsorbed
in the ductal cells.
Bicarb is secreted.
Potassium is secreted.
But there’s no water movement so that your
secretions are going to be hypotonic.
So let’s go through the exact mechanism
of how these transporters work.
So first, you have to get sodium
out into the interstitial --
from the interstitial space into the lumen,
and that is done outside of the cell or
what we try to call transcellularly.
And that is done about the same time as
potassium is moved through the cell,
which is transcellular.
Then water follows the
sodium and the potassium,
and water will move
through via aquaporins.
You have nice transport of both chloride and
sodium across the basolateral membrane.
Then sodium and hydrogen ions are actually
taken back out into the interstitial space.
This leaves chloride in the cytosol,
which then moves across in a
cotransport mechanism with bicarb.
So that is how we get the
first level of secretions.
In terms of reabsorption, you can start to
reabsorb sodium across the apical membrane.
Sodium then is moved out of the basolateral
membrane via the sodium potassium ATPase.
Potassium is then moved
through the apical membrane.
And that hydrogen ion
is simply recycled.
Potassium will be secreted.
Where does the bicarb come from?
Well, bicarb is produced via metabolism
through the carbonic anhydrase reaction,
where you take water and CO2,
combine it to form carbonic
acid and then it can
dissociate into a bicarb
ion and a hydrogen ion.
The hydrogen ion is
removed from the cell via
and with sodium.
The sodium is then moved via the sodium
potassium ATPase out of the cell,
and the bicarb is
exchanged with chloride.
Therefore, you’ll get a
secretion of bicarb,
and chloride is reabsorbed.