Upper Gastrointestinal Secretions: Salivary and Gastric

00:00 Now, we have these salivary glands, we know what the different salivary glands are, we know how the ions move.

00:08 But what is the control factors for salivary gland secretion? The cephalic phase is going to be very important phase in the digestive process especially for salivary glands.

00:20 In fact, this is the classic Pavlovian response in response to a stimulus.

00:27 So Pavlov trained dogs to salivate in response to a bell.

00:32 And he did that by having the dogs eat at a certain time when he rang a bell.

00:38 Then after a while, he removed the food and would just ring the bell, and sure enough, the dogs would still salivate.

00:44 Same kind of process that is involved here.

00:46 It’s simply the brain acting via the sympathetic and parasympathetic nervous system to engage salivary secretions.

00:54 So what are the neurotransmitters involved? The first one is acetylcholine.

00:58 Acetylcholine causes an increase in serous secretions from all of the salivary glands.

01:05 Vasoactive intestinal peptide is also released from the parasympathetic nervous system.

01:10 It acts primarily to increase blood flow, which increases salivary secretions.

01:17 Interestingly, both the sympathetic and parasympathetic nervous systems increase the amount of salivary secretions.

01:23 So norepinephrine acting through beta-adrenergic receptors can also increase the amount of salivary secretions, but not in this great of quantity or rate as the parasympathetic nervous system and acetylcholine.

01:40 So let’s review now gastric secretions and what their primary functions are.

01:46 First and foremost is gastric secretions are very important in protecting the GI system.

01:51 The amount of acid that is released will actually kill most bacteria in most other form.

01:57 Substances help denature proteins, and that will help protect the lower aspect of the GI system.

02:05 It does act as kind of a reservoir or a storage unit, which means it kind of holds onto food stuff for a while and then gradually releases it into the small intestine.

02:16 And that slow release is very important because it acts to kind of regulate how much food enters the small intestine.

02:25 You wouldn’t want to have this process to not be regulated because then as soon as you ate a very large meal, all of that substance would hit the small intestine at the same time.

02:35 And there wouldn’t be enough enzymes and enough surface area in the small intestine for there to undergo complete digestion.

02:43 So the stomach holds it for a little bit and then gradually releases it.

02:49 The nice thing about the stomach is there is a little bit of absorption function, but that has to be through lipophilic substances.

02:57 And finally, you’re going to prepare most of the food stuff for digestion of small intestine.

03:04 And you do that by releasing both acid, as well as an enzyme called pepsinogen, which is then converted to pepsin, and that helps to break down protein.

03:13 So some of that digestion can start to occur in the stomach before it gets to the small intestine, and therefore, improve digestive function.

03:22 Okay.

03:24 Now that we have the digestive functions overall, let’s talk about where these digestive juices are secreted from.

03:33 These are gastric pits in the stomach that are lined with surface epithelial tissue, mucous neck cells, and then finally, our parietal cells towards the bottom.

03:45 The mucous neck cells are very important because what they do is provide a little bit of protection from the endothelial layer so that the lining of the stomach is not eaten up by the acid that’s produced from the parietal cells.

04:05 What are the other constituents that are released? Well, hydrogen ions being the most important, this is something that we probably should discuss first.

04:14 Hydrogen ion secretion is actually very, very difficult to do because of the concentration gradients between what is in the plasma and what you need in the stomach.

04:25 So in the plasma, you might have a pH of around 7.4.

04:29 Usually, the norm is in between 7.35 and 7, but you need to move that all the way down to a pH of about 1 to 1.5 That takes a lot of energy.

04:41 So you should think of, if this takes energy, you’re probably needing to pump, and you’re right.

04:47 You need a hydrogen ion pump.

04:49 So, let’s talk a little bit more about how that process works.

04:55 Now, to get the hydrogen ions out because it’s an ion, we need to talk about three other ions One is chloride, which is going to be secreted, sodium and potassium.

05:08 So those are the ions involved besides hydrogen ions.

05:14 They are secreted in various flow rates that we’ll discuss in a moment.

05:19 The other interesting thing about parietal cells is they also secrete something called intrinsic factor.

05:26 And in intrinsic factor, we won’t deal with too much today, but it’s very important in vitamin B12 absorption, and we’ll discuss that in the lecture on digestion and absorption.

05:39 Pepsinogen is also secreted from chief cells, and that is converted from pepsinogen to pepsin, which is its active form.

05:49 The gastric secretions of ions are done in this secretion format, the same that you saw in the salivary glands where we have secretion on the X-axis and you have the ion concentration on the Y-axis.

06:03 If you look at a substance like sodium, if you have a higher value in the plasma than in the gastric secretions, you know that it’s being reabsorbed.

06:12 If the value is higher in the secretions versus the plasma, you know it is being secreted.

06:20 So this should be their first indication that sodium is reabsorbed, and it’s reabsorbed in the flow rate dependent manner.

06:28 Meaning that as you get faster flow rates, what you’re going to get is increases in its reabsorption.

06:38 Potassium is being secreted.

06:40 Why do I know it’s secreted? Because the values in the gastric secretions are greater than the plasma.

06:47 Chloride is also being secreted, and this is done in the flow rate dependent manner as well.

06:53 The faster the flow rate, the more chloride is secreted.

06:57 And finally, hydrogen ions are secreted in a great number.

07:01 In fact, the amount that is secreted in hydrogen ions is so many folds higher in the gastric secretions than it is located within the plasma.

07:10 Okay.

07:11 So now that we have these particular ions, let’s talk about the mechanisms or transporters available to get those ions to the right place.