Exocrine Pancreas – Lower Gastrointestinal Secretion

00:01 So these enzymes from the pancreas involve amylase for carbohydrates.

00:06 Remember we also had a salivary amylase that is secreted in the salivary gland.

00:11 In terms of fats, we have lipase.

00:14 We have phospholipase A2 and we have cholesterol esterase.

00:20 Also recall that there was a lingual lipase that was also secreted in the salivary glands.

00:28 We also have enzymes for various proteins such as chymotrypsin, trypsinogen, pro-elastase and carboxypeptidase.

00:36 Now these protein ones are in addition to the pepsinogen and pepsin that is secreted in the stomach.

00:45 Finally, we have nucleotides and these will help break down RNA and DNA molecules.

00:51 So besides enzymes, there are other things that are secreted by the pancreas.

00:56 These are ions and water and the primary ion we need to contain ourselves with is bicarb.

01:03 And why do we need to do be so – important or have eye out for bicarb is because remember in the stomach, we secreted all those hydrogen ions.

01:12 We need some way to buffer those hydrogen ions once we get to the small intestines where those hydrogen ions of that low acidity will end up eating away portions of the small intestines.

01:27 So just like our other secretions, we can set up secretions based upon the flow rate of the secreted substance and the concentration of that ion.

01:39 So let’s start with sodium.

01:41 Sodium, there is no real change in terms of its flow rate and concentration.

01:45 And it is very similar to what it is in plasma.

01:50 Potassium also does not change very much across flow rates of various pancreatic secretions.

01:58 Chloride does change though.

02:00 As you increase the flow rate of pancreatic secretions, you have a decrease in the chloride concentration.

02:09 And that is lower than what occurs in the plasma, meaning that you are reabsorbing it.

02:18 Bicarb is the opposite of chloride in which as you increase the flow rate, you increase the amount of bicarb secreted.

02:25 How do I know it’s secreted is because it’s above the level of the plasma.

02:31 So that’s how we control and regulate some of the ions.

02:36 How do we deal with enzymes? Now enzymes can be situated in this kind of format where we have acinar cells.

02:45 We’re going to have a couple of different membranes.

02:48 We have the interstitial space, the basolateral membrane, the acinar cells themselves, the apical membrane and then the lumen.

02:58 We are going to be releasing enzymes from the endoplasmic reticulum, Golgi apparatus and those vesicles will then bind and fuse to the apical membrane and then submit their enzymes out into the lumen.

03:14 Ions encompass a little bit more complex process.

03:17 Here, carbon dioxide goes into the cells.

03:21 Carbonic anhydrase changes the carbon dioxide in the water into bicarb and hydrogen ions.

03:32 Then, hydrogen ions are exchanged for sodium.

03:38 Bicarb is exchanged for chloride.

03:39 This is how it secretes bicarbonate across the apical membrane.

03:47 Chloride completes the process.

03:49 And finally, sodium is excreted out of the cell.

03:55 Then, in regards to this bicarb exchange, both sodium and water are pulled into the lumen of these pancreatic secretions.

04:09 Now, what it controls and regulates is pancreatic secretions.

04:13 About 25% of the amount secretions are done via the cephalic phase.

04:20 And remember from the cephalic phase, this involves primarily the vagus nerve or cranial nerve number X, releasing acetylcholine.

04:28 About 10% of secretions are controlled during the gastric phase.

04:33 And these are through the vago-vagal reflexes.

04:35 Finally, the most important is the intestinal phase.

04:40 And these will involve two different substances.

04:43 One is called secretin and the other is cholecystokinin.

04:47 These will be the primary regulators during the intestinal phase and of pancreatic secretions in general.

04:57 So acid from the stomach releases secretin and secretin is released from the duodenal cells.

05:04 And this is in response to both fats and amino acids will cause the release of cholecystokinin.

05:11 So our two different substances are secretin and cholecystokinin.

05:17 These then will be absorbed in the blood stream, delivered through the pancreas and also the vagus stimulates the release of various enzymes into the acini lumen as well.

05:34 Both of these together cause large amounts of secretions of pancreatic fluid especially bicarb.

05:41 Now, how do each of these substances cause these secretions? Let’s start with acetylcholine.

05:50 So acetylcholine, cholecystokinin are two primary regulators to increase the amount of cytosolic calcium.

05:57 This cytosolic calcium causes phosphorylation of various structure and regulatory protein, which then causes docking and fusing of these granules or vessels that have enzymes in them.

06:11 Now besides acetylcholine and cholecystokinin, secretin is a primary regulator of these secretions.

06:21 These works via cyclic AMP mechanism to also phosphorylate structural proteins and help the process of these granules to dock and fuse into the apical membrane to spill their contents in the intestinal lumen.

06:40 The intestines also do some secretions, not as much as the pancreas itself, but they also will secrete things like mucus from goblet cells.

06:51 Endothelial cells will secrete some serous solutions as well as Paneth cells.

06:58 Their primary function though is to increase the amount of fluid and mucus in the intestines for protective reasons, not to undergo digestion, rather to protect the small intestine and large intestine walls.

07:14 To begin to talk about biliary secretions, let’s start off with the gallbladder.

07:17 The gallbladder is the primary storehouse for biliary secretions.

07:23 In this phase, what happens is from the hepatic ducts, bile is drained from the liver and will be stored then in the gallbladder.

07:32 The bile duct itself is where the bile will be released into the small intestine.

07:40 Why do you produce billiary secretions? In fact, they don’t do very much in terms of digestion.

07:47 But they do emulsify fats, very similar to the way detergents emulsify fats in your dishwater.

07:55 So they don’t breakdown the fats, they glob them together or emulsify them.