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Video transcript
Voiceover: Did you know that the human stomach can store up to four liters of food at any time? That's about as much as two Coke bottles, insane! So, in this video we're going to see how our stomach helps us process food it just received, from the oral cavity and the esophagus. So, why don't we take a look just at the stomach here and I will zoom in on it, so that we can talk exclusively about what this guy is up to. The stomach is primarily responsible for three steps. First of all, it is going to receive a bolus of food from the esophagus above. This bolus is just food that has been turned into a sphere, that can now be processed. Then the stomach can do two things here to help process the food even more. The first thing the stomach does, is that it churns this bolus or it churns the food. The muscular walls of the stomach here allow it to compress down and break up this food even more. In addition, there's also a certain degree of hydrolysis. Hydrolysis, or enzyme assisted degradation, or breakdown of this bolus and we will talk about the enzyme that's responsible for this process in a moment. And then finally after we do all these things, we receive a product that is called chyme. Chyme is just a mixture of whatever the bolus has been broken up to, including the gastric enzymes and juices that we've used. And then, the food is going to actually just stay here for a little bit, because the stomach also stores food. It will store the food, until it's an appropriate time for the chyme to be squirted into the duodenum, or the first part of the small intestine to be processed. And that's why we can store up to two to four liters of food, at any given time. Now, what about the anatomy of the stomach here? How do we release these enzymes and breakdown this bolus? Let's take a look at a little bit of gastric anatomy right about there, and see how it's responsible for the breakdown of food here. We can imagine that the stomach is lined with all these infoldings. These infoldings of the gastric wall, that help to increase the surface area and in doing so there's actually a layer of cells that sit around here. This layer of cells actually secretes a lot of the components of the gastric juice we're going to see, and it is nicely ripe with a ton important components. And the main thing to remember in the stomach is that there are three types of cells that are involved here. The first type I am going to talk about are called parietal cells, parietal cells. And the main thing that is released from parietal cells is hydrochloric acid. Hydrochloric acid, it is a very acidic or very corrosive acid that's actually more acidic than battery acid. In addition to parietal cells we also have chief cells, chief cells. These guys secrete an enzyme called pepsinogen, pepsinogen. And this is the inactive form of the enzyme. Pepsinogen is not active, in fact, in order for hydrolysis to occur we need an enzyme that's call pepsin. How is pepsin made? Well, we're going to need pepsinogen to make it, but the pepsinogen and whenever you see "gen", at the end of the name, that means that it is almost there. It just needs to be processed. In order to turn pepsinogen in to pepsin, we need hydrochloric acid which will break down this protein to turn it in to this active form, that can then be used for hydrolysis. Now, what would happen if we just had a bunch of pepsin, or a bunch of hydrochloric acid present in the stomach all the time? What do you think would happen to the stomach? It would probably eat itself alive, isn't that right? I mean you've got your cell membrane that definitely has a considerable amount of protein that should be present there. And in addition you've got this really corrosive acidic substance, that can eat through your stomach. And that's actually how you have gastric ulcers, but not all of us have gastric ulcers. What do we also have in our gastric pits right here, that help us prevent gastric ulcers from occurring in the first place? Well, the third type of cell we have sitting in here are actually called mucus cells, mucus cells. And just like the name suggest, mucus cells release what's called mucin and this is a coating that will sit around the stomach to make sure that things like pepsin, or hydrochloric acid don't degrade the stomach. Without mucus cells we would eat ourselves alive. So, as a review, the three things a stomach will do for us is that it will churn with its very muscular walls to help break down the bolus, in a very physical activity. Also, there's an enzymatic or chemical process that occurs here, where we used pepsin to degrade food. Now, pepsin itself only degrades one type of nutrient and that's it protein. This will break peptide bonds or the bond that connects amino acids to one another, to degrade your protein macromolecules. So, that's the only type of nutrient that's broken down in your stomach. Once we hydrolyze a significant amount of the protein we're going to produce chyme, that is stored in the stomach until it's the right time for it to be released into the next part of your GI tract called the duodenum. The first part of the small intestine, and that's how your stomach works.