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 thesis, um, ACC from the esophagus, the bullets 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 two sphincters in the stomach worth noting, and you don't need to worry about memorizing the names. I'm just telling you. So you know you have the cardiac sphincter, which is going to be the sphincter that leads from the esophagus into the stomach. And then the pile oryx sphincter. So this is cardiac. This one is Hi Lorik, and the pile or ex victor 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 Bullis. It is now considered Kym, which is Ah, just lovely sounding name for something that I'm sure looks and smells fabulous. No there are some interesting specialized cells in the stomach that perform the important functions needed for digestion. First cell I want to talk about is parietal cell. This is going to be the type of cell that not only secretes but makes hydrochloric acid in this hydrochloric acid is going to help break down food. Uh, in part, it's going to help de nature proteins. If you recall proteins are very sensitive to pH changes. A lot of them are sensitive to acidic environment. So by creating an acidic environment, it's going to help denatured proteins, making them easier. Thio chemically grade, Which is gonna be the job of this enzyme that will talk about momentarily called Pepsi in. And the hydrochloric acid produced by these parietal cells actually activates Peps in. It's stored in an inactive form called Pep Synergen. Again, we'll get to that just a second. 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 is civic 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 gastric 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 could just store right. It's 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 at pus, meaning it burns ATP to pump thio pump particles, and it's going to bring in a potassium and pump out a proton. And you have this exchanger here that's going thio. 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, uh, you know, use a chemical process to make this substance on the fly, because again, can't really be store in 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 Pepsi. Neogen, which is again an inactive form of unending Sime called Pepsi in this inactive form, is actually a special type of enzyme Caldas, Imogen, which again it's a type of enzyme that's gonna be stored in an inactive form. And usually it's because it's safer to store when it's inactive. See, the thing is, pep sin is a Proteus, so it's going to degrade proteins you can't really be storing. Proteus is around yourselves because they're going to degrade all the protein products you make. I mean, cells make proteins for everything. Thes Prodi. Aces are just one of the things they make. And a lot of the proteins they make are essential to their life. So, yeah, Proteus is floating around. They're just gonna be gumming up your day. They're gonna be totally ruining everything you make ruining all your hard work so they have to be stored as imagines we call those Imogen Pep Synergen. And at low ph Pep Synergen actually converts itself into pep sins called on auto catalytic process. Again, don't worry about memorizing all these details. Just know that Pepsi has kept in inactive form, which we call Pepsi neogen, and at low pH, which is provided by this nice hydrochloric acid. It will turn into peps in and start breaking down proteins into smaller poly peptides, and you can see a nice little model of that, their parietal cell and chief cell working together. It's a 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, this stomach is a very muscular organ. In addition, Thio secrete ing this hydrochloric acid and the Proteus Peps in its muscular. It's going thio, you know, physically move and churn the Keim 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 gonna be very good at chewing. And that could be a problem when you want to break down your food so that it's easier to chemically digest, right? We have thanks teeth and mouths to do that. Birds don't have that option, which is why have a gizzard and a gizzard is basically a special stomach that is going to contain stones and sand and grit like, you know, uh, harsh, abrasive materials in there, and it's going to help grind up their food. Right? The stomach again, some muscular Oregon. If you have all this grit and stones and sand in there and the stomachs churning, all that stuff is gonna 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 ever in a bad mood picturing picture birds swallowing stones and hopefully that'll cheer you up. Now, one other really cool evolutionary, uh, modification made to the stomach is found in class of mammals called ruminants. Ruminants have a special four chambered stomach. One of the chambers is called the Rumen, hence the name. Basically, this four chambered stomach is specialized for digesting plant matter, and part of that actually involves fermenting it. So basically, when food enters through the esophagus, let me jump out of the way here. When food enters through the esophagus, it's going to come into the room in which is the largest chamber and contains bacteria. And protests that produce cellulitis might recall that plant matter could be particularly hard for animals to break down because of the plant cell walls that contain cellulose, well, cellular laces, an enzyme that could break down cellulose. And even though we don't produce it, bacteria and Protess can. So if we keep those dudes in our stomachs or if you're ruminant, if you keep them in your room in, then they'll give you the assist. Still, help you break down that plant matter Now. It should be noted that ruminants will actually often regurgitate what's called the cut. Basically, that chewed up kind of die partially digested plant matter. It's pretty yucky. If you think about it too much, just just don't think about it. Don't think about it. They're going to regurgitate that cut and reach you. It thio grind it down even more to ensure that they can extract as much. Nutrition is possible. Plant matter. It's very hard to get the nutrition out of it, So this is their way of just like really ensuring they're getting all the nutrition they can for the effort they put in into digesting it. With that, let's flip the page.