The stomach is a muscular organ where digestion really starts to get serious. And that's because the stomach actually creates this acidic environment that is ripe for protein digestion. Now, before entering the stomach from the esophagus, the bolus has to move through a sphincter, which is a circular muscle with a hilarious name that maintains the constriction of an orifice. Basically, it's like the gatekeeper for a particular orifice. Now there are 2 sphincters in the stomach worth noting, and you don't need to worry about the cardiac sphincter from the esophagus into the stomach, and then the pyloric sphincter. So this is cardiac, this one is pyloric, and the pyloric sphincter leads to the intestine. Now, the stomach is full of gastric juice. This is digestive fluids that are secreted by the stomach. And when the food mixes with the gastric juice it is no longer considered the bolus; it is now considered chyme. Which is just a lovely sounding name for something that I'm sure looks and smells fabulous.
Now, there are some interesting specialized cells in the stomach that perform the important functions needed for digestion. The first cell I want to talk about is the parietal cell. This is going to be the type of cell that not only secretes but makes hydrochloric acid. Acid. And this hydrochloric acid is going to help break down foods, in part, it's going to help denature proteins. If you recall, proteins are very sensitive to pH changes, and a lot of them are sensitive to acidic environments. So by creating an acidic environment, it's going to help denature proteins, making them easier to, you know, chemically degrade, which is going to be the job of this enzyme that we'll talk about momentarily called pepsin, and the hydrochloric acid produced by these parietal cells actually activates pepsin. It's stored in an inactive form called pepsinogen. Again, we'll get to that in just a second. And this hydrochloric acid is also going to help kill pathogens. Yeah, I mean, you know, you often will be eating bacteria that's on the food you're eating, or, you know, like other harmful things. This acidic environment can help kill some of those before they can really cause problems for your body. Now it should be noted there's a hormone called gastrin that's actually secreted in response to food entering the stomach, and this has the effect of causing the parietal cells to increase hydrochloric acid production. And it is production because, as I'm sure you can imagine, hydrochloric acid is not something that you can just store. Right? It's a very powerful acid. You don't want that just sitting around. So these parietal cells don't actually store it, they produce it. And you don't need to worry about the specifics of how they produce it. As you can see it, it involves a variety of pumps and channels and exchangers. You know, just very quickly we have this, pump here. It's an ATPase, meaning it, burns ATP to pump, to, pump particles and it's going to bring in potassium and pump out a proton. And you have this exchanger here that's going to, get rid of a bicarbonate and bring in a chloride ion, and then the chloride ion is going to be, also moved into the stomach lumen along with your proton, and guess what? Proton plus chloride ion, that's hydrochloric acid right there. So don't worry about the specifics there, just know that these parietal cells, you know, use a chemical process to make this substance on the fly. Because again, can't really be storing acid in your body. A little dangerous. Now there's also chief cells. Didn't mean to give parietal cells all the attention, chief cells are, you know, equally, if not more important because they secrete pepsinogen, which is again an inactive form of an enzyme called pepsin. This inactive form is actually a special type of enzyme called a zymogen, which, again, it's a type of enzyme that's going to be stored in an inactive form, and usually it's because it's safer to store when it's inactive. See, the thing is pepsin is a protease, so it's going to degrade proteins. You can't really be storing proteases around your cells because they're going to degrade all the protein products you make. I mean, cells make proteins for everything. These proteases are just one of the things they make, and a lot of the proteins they make are essential to their life. So yeah, proteases floating around, they're just going to be gumming up your day. They're going to be totally ruining everything you make, ruining all your hard work. So they have to be stored as zymogens. We call the zymogen pepsinogen. And at low pH, pepsinogen actually converts itself into pepsins, called an autocatalytic process.
Again, don't worry about memorizing all these details, just know that pepsin is kept in an inactive form, which we call pepsinogen, and at low pH, which is provided by this nice hydrochloric acid, it will turn into pepsin and start breaking down proteins into smaller polypeptides. And you can see a nice little model of that there, parietal cell and chief cell working together. It's a match made in heaven. Lastly, I want to mention the mucus cells, which secrete mucus to lubricate and protect the stomach from all that nasty acid. I mean, the tissues of the stomach need protection from the harsh stuff that's in the stomach, and the mucus cells provide that. So don't forget the stomach is a very muscular organ in addition to, you know, secreting this hydrochloric acid, and the protease pepsin, it's muscular. It's going to, you know, physically move and churn the chyme in there to help with digestion. Now just like birds had a crop that, or some birds have a crop that modified esophagus, some birds have a modified stomach called a gizzard. And basically, if you think about it, birds have beaks, so they're not going to be very good at chewing, and that can be a problem when you want to break down your food so that it's easier to chemically digest. Right? We have, like, teeth and mouths to do that. Birds, don't have that option, which is why they have a gizzard. And a gizzard is basically a special stomach that is going to contain stones, and sand, and grit, like, you know, harsh abrasive materials in there, and it's going to help grind up their food. Right? The stomach, again, it's a muscular organ. If you have all this grit and stones and sand in there, physically grind the food that as a bird, you know, you swallowed in a big chunk because you couldn't chew it. And it should be noted just because I think it's kind of funny that the way the stones and sand and stuff gets in there is the birds actually swallow them. So just, if you're ever in a bad mood, picture birds swal ... With that, let's flip the page.
