Salivary Glands: Structure and Functions – Upper Gastrointestinal Secretion

00:00 So getting our transporters in place, you see that there’s a two-step process.

00:00 We need to talk about transporters.

00:06 There’s acinar cells and then there are ductal cells.

00:11 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.

00:22 Water is moved in an isosmotic form into acinar cell lumens.

00:29 Chloride is reabsorbed in the ductal cells.

00:34 Sodium is reabsorbed in the ductal cells.

00:36 Bicarb is secreted.

00:40 Potassium is secreted.

00:42 But there’s no water movement so that your secretions are going to be hypotonic.

00:48 So let’s go through the exact mechanism of how these transporters work.

00:53 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.

01:07 And that is done about the same time as potassium is moved through the cell, which is transcellular.

01:14 Then water follows the sodium and the potassium, and water will move through via aquaporins.

01:23 You have nice transport of both chloride and sodium across the basolateral membrane.

01:30 Then sodium and hydrogen ions are actually taken back out into the interstitial space.

01:39 This leaves chloride in the cytosol, which then moves across in a cotransport mechanism with bicarb.

01:48 So that is how we get the first level of secretions.

01:50 In terms of reabsorption, you can start to reabsorb sodium across the apical membrane.

01:57 Sodium then is moved out of the basolateral membrane via the sodium potassium ATPase.

02:04 Potassium is then moved through the apical membrane.

02:10 And that hydrogen ion is simply recycled.

02:13 Potassium will be secreted.

02:15 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.

02:32 The hydrogen ion is removed from the cell via this countertransporter and with sodium.

02:42 The sodium is then moved via the sodium potassium ATPase out of the cell, and the bicarb is exchanged with chloride.

02:52 Therefore, you’ll get a secretion of bicarb, and chloride is reabsorbed.