We now want to spend some time talking about digestion in the stomach because, for all the parts that we've talked about, the job of the stomach really is to start breaking down food so that we can eventually absorb those nutrients. So we're just going to start by saying that both mechanical and chemical digestion occur in the stomach. And we're going to talk about mechanical digestion first. Well, because it's a little simpler, but also you probably don't think of the stomach doing mechanical digestion as much as you do chemical digestion, right? We're all aware of stomach acid, but actually physically breaking up the food is something that the stomach does as well. So mechanical digestion, we just want to know that this is non-specific to any macronutrient. Right? So anything that's in the stomach is going to go through mechanical digestion, and it just works through well, remember, the stomach is this muscular bag. So waves of muscular contraction we call peristalsis, and it's just going to cause this mixing of the chyme and the gastric juices. Remember, the goal of mechanical digestion is to break up food and mix it in with the liquid so that there's a lot of surface area for those food molecules so that enzymes can access them. Well, in the stomach, as we said, it's happening through peristalsis. Peristalis is this wave of muscle contraction, and we can see this in our image here. We see sort of 1, 2, 3. You see this ring of muscle contraction sort of going down the stomach, so it's squeezing in, and that wave sort of moves downwards. And as it does, it's just sloshing around the chyme that's in there. It pushes down. Some of this chyme actually is going to go through the pyloric sphincter. Now, if it doesn't go through the pyloric sphincter though, as this sort of wave of muscle contraction makes it all the way to the bottom here, well, then that chyme sort of squishes back in the other way. So what you can think of, imagine if you had a piece of bread and you put it in a plastic bag, and you didn't add any acid or anything to it. You just added water. And now just squeeze down on this bag over and over again. You can imagine that pretty quickly that bread is just going to break up and turn into mush. Instead of being a piece of bread, it's going to be all these bread little particles with tons of surface area so chemicals can access them and start chemical digestion. So that's what we're doing now. Chemical digestion. Alright. So in the stomach, we're going to have multiple chemicals that contribute to digestion of macromolecules. The first one is the one I'm sure you're aware of is hydrochloric acid, HCl, that stomach acid. Alright. So this hydrochloric acid causes a very low pH. We're talking about somewhere in the range of 1.5 to 3.5, very acidic, and that acidity is going to denature proteins. So remember, proteins are these chains of amino acids that are folded up in three-dimensional structures. To denature them, we're breaking up that three-dimensional structure, but we're not actually breaking the bonds between the amino acids yet. Now, the hydrochloric acid, this acidity is also going to break down a lot of cell walls. This allows us to sort of access what's inside the cells and digest those molecules. And importantly, it's also going to kill a lot of stuff in your mouth; you don't want every bacterium going in colonizing your gut or making you sick. So the stomach acid kills a lot of that bacteria as a sort of first line of defense. Alright. So that's the acid, but we also have some enzymes in there. The one that we really want to identify is pepsin, and pepsin is going to be this enzyme that breaks proteins into amino acids. Right? So the hydrochloric acid denatures the proteins, the pepsin, that's actually going to break those peptide bonds and break it into amino acids because that's what can get absorbed later on in the small intestine. Now pepsin is only going to work. It's activated, and it's only going to work in a low pH environment, the stomach. All right. Now we're also going to have some lipases in here. Lipases. Remember, these are enzymes that begin lipid digestion, or what we can call lipolysis, breaking those lipids into fatty acids that can be absorbed. Alright. Now quickly, if you want to remember your pepsin and your lipase, just simple alliteration. Pepsin is for protein. Lipase is for lipids. All right. So that's how digestion works in the stomach. If we want to break these things down anymore, we gotta move on to the small intestine. But before then, we got practice and examples. I'll see you there.