So given our transporters in place, you see that in the -- there's a two step process,
there’s acinar cells and then there are ductal cells, In the ductal cells this is the portion
that it’s going to be reabsorption occurring and means the secretion portion occurs in the acinar cells.
Water is moved in an iso-osmotic 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.
In the salivary glands sodium travel paracellularly 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 basilar 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 basal lateral membrane by 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 disassociate into a bicarb ion and hydrogen ion.
The hydrogen ion is removed from the cell via this counter transporter 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.