 Hey, you guys. So today we're going to talk about the digestive system. In the last lecture, we looked at membranes, and that let us look at the skin, our first body system. The digestive system is our second body system. We also looked at mucus membranes and serous membranes. In this lecture, basically the entire digestive system is lined with mucus membrane. So there will be some review happening when we think about the fact that really the digestive system is the inner tube in our body plan. And there's a whole bunch of serous membrane fun times that we are going to explore with the peritoneum, because that is a serous membrane that lines the peritoneal cavity and is a pretty interesting structure, and has some interesting structural consequences in the digestive system. So let's start out with sort of an overview of the functions of the digestive system. And because my brain just works this way, I have a diagram of the anatomy of the digestive system here, but I'm going to do a rigged style, for a more diagrammatic view of the digestive system. And it probably looks familiar, because it's the tube within a tube body plan. And we're going to call the top of the tube the mouth dog, and the bottom of the tube the, oh yeah, anus. Maybe we'll make it look like a little sphincter over here, got to go for the realistic details if we can have them. So here's the mouth, here's the anus. All we did is take this tube, and what in life has expanded portions and wholly like wound up tangled portions. We just sort of drew it out so that we don't have to worry about all the thickenings and the anatomy, and we can just look at, like what does this thing do? So I've got a list for you of functions of the digestive system. And the first function is ingestion. So the digestive system plays a role in ingesting fuel. And hold on just a second and think about, dude, why do you need fuel in the first place? The big picture reason we need the digestive system is because we need to get energy. So really the purpose of the digestive system is to provide us with energy. These are the strategies that the digestive system utilizes in order to accomplish that big picture goal of getting us energy. Energy is just fuel. There's a reason that you eat. You need the fuel provided by the food that you are ingesting. So let's draw a picture of some stuff that we might ingest. And you can imagine that goes in the mouth. Interestingly, it doesn't stay in your mouth. That may seem super obvious, but a second function of the digestive system is movement. So there are structures in the digestive system that are set up just to move stuff through the tube. So to move it from the mouth to the anus. Anything that you leave in your tube is going to come out the anus. I can't think of any examples. If you can get stuff out of your tube, but if you leave it in there, if you don't take it out, it will move through and come out the other end. Don't think about that too much because I'm sure there are lots of examples of things that don't come out. All right. Moving this food on through, there's a couple of things that are going to happen. First of all, remember what our tube is actually made out of. Do you remember? If I were to blow this up, what is this? What am I drawing you? I'm just drawing you the epithelial lining of the tube, the mucous membrane, the epithelial part of the mucous membrane. Good Lord, we don't need that in our picture. And the epithelial tissue does a third function, and that is secretion. These are sort of in a logical order, but the fact is, gosh, dang it, why does this happen? Whatever. What was I saying? Oh, I was saying that they happen in an order, but the order is mixy. Good God. Of course it's mixy. Secretion is the process of taking stuff from inside the body and moving it into the tube. Things that get secreted and things like digestive enzymes. So you've got food in there, you've got substance in there, you're moving it along. You secrete stuff that's going to help the other processes take place. Digestive enzymes are one thing you can secrete. Dude, you can secrete some slippery, fun time mucous. That's another thing you can secrete. You can secrete all sorts of interesting bicarbonate. That is a basic substance, an alkaline substance. It's baking soda, and it will neutralize stomach acid. Okay, so there's all sorts of things that you can secrete. Secretions often assist with the process of digestion. Digestion is nothing more than taking a substance and breaking it down into smaller pieces. There are multiple forms of digestion that can happen. So we just took that green box and we broke it into pink circles. Digestion can be mechanical, which means actually what happened up in the mouth, or the mouth is one place that mechanical digestion takes place. With these guys, it can also be chemical. In chemical digestion, we're actually breaking the particles apart using chemicals. We don't smash them and chomp them and squish them. We actually drip chemical on them and the chemical itself does the breaking, does the digesting. So digestive enzymes can be secreted and then they break apart the substance. You can also have, once you get those pieces broken down, another task of the epithelial tissue is to absorb those substances, whatever they are, and absorption really, I would argue, is the whole point. All of it, the secretion, the movement, the ingestion in the first place, breaking it down into little pieces, all of it is done so that you can absorb the goodies and send them to your body cells who can use them. And the goodies are used for energy. They're also used as building blocks, and it really is also about energy. Whatever is left, so if you did some break it down, break it down, break it down, and there's some motility happening, whatever is left in there, it goes through a process of elimination. And that's the final task of the digestive system, elimination. And those are all the things that the digestive system will do. We're going to look closer at each part of the digestive system and see how those structures enable these functions.