 We talked in the last lecture, we hinted in the last lecture about the different strategies that are the different structural characteristics of the digestive tubing that will enable increased surface area. So now we're going to get a little more specific. So there are four things. We listed them out last time. Go ahead and push pause and list those puppies out there. I'm going to use this diagram to help you remember your layers. So I'm going to try and color code it similarly. So I'm just going to get a little bit of fresh color or fresh space here so that I can draw these pictures. Now, the first strategy for increasing surface area was length. And that was the whole, like, dude, you got 30 feet or something ridiculous of digestive tubing in your gut. Yes, that's long. That increases the surface area. Why do we care about the surface area? Because the whole point of the whole thing is to absorb the nutrients that you are breaking down in the lumen of the tube. If you don't absorb them, then it's sort of pointless. They're eating if you're not going to absorb that stuff. So the more time, surface area, whatever you have to carry out the absorption, the better. Make the road longer. The longer the nutrients are on the road, the more likely they are to be absorbed. Okay, I'm not going to draw that one. That one's easy. The second one was this circular fold idea, circular fold. And I'm just going to get crazy. So bear with me while I draw. Actually, let's just list them all out and then I'll draw them all. Circular folds, we have villi and we have microvilli. And I believe, I believe I can draw them all on one drawing. Okay, watch. You'll notice, oh geez. I mean, it looks super awesome. I mean, look at that thing. What? Okay, this is my epithelial layer, okay? Now, I'm drawing, I'm going to draw it all, even though I drew like five billion of these things. This is like a cross-section, cross-section. Kind of like what I just drew, except now I'm drawing the texture. Consider this a textured lecture. That looks very textured. Epithelial tissue. I'm not going to draw in my basement membrane. However I am, okay, going to change the color of what layer was that, do you remember? That was the laminopropria. I'm going to change the color of the laminopropria because I want to make sure that you are cool that the laminopropria is green. Yes, I'm doing this on purpose. Laminopropria, yeah, yeah, get out of there. I don't know why it does that. It makes those cute little circles. They're perfect. I would never make a perfect circle like this. I'm drawing you laminopropria. What is, yikes, what is something? If this is laminopropria here, what will my little green line actually have associated with it? Uh-oh, did you see that? I think my computer's hot. It's like five billion degrees in my office. I don't know why it's five billion degrees. Dude, yes, I was up all night and I'm still going. Laminopropria, if we want to do our whole mucosa layer, we also have to include, this is not looking good, you guys. We also have to include our muscularis mucosi, which is what I'm trying to draw right here and leave room because what is going to be basal to muscularis mucosi? We're going to have submucosa in there. Now look, here's muscularis mucosi. Ah, why is it doing that? And now, now I'm putting in this color for submucosa. Now, I'm about to blow your mind. Look, what is going up here? What? It's submucosa. Submucosa is going all over the place. Yes, and what is basal to submucosa? Well, this is where we've got another couple of layers of muscularis externa. This is my smooth muscle layers, right? Look at this. This is actually really beautiful. I hope I do not have to re-record this thing since I'm glitching in and out over there. Don't make me re-record it. And then we have our final little adventitia or serosa. Let's call it a serosa. Oh, I'm almost ready. I have to draw a couple of epithelial cells. Each one of these is an epithelial cell, okay, with its own nucleus, right? And on the surface of each cell is a set of extensions. They look like crowns. They're not crowns. These are not frog princes or princesses. No, these are intestinal cells with microvilli. Microvilli are these little extensions of cell membrane and cytoplasm found on individual cells. Every single one of these cells is covered in microvilli on the luminal edge. That increases surface area. In addition to that, in addition to the microvilli, each one of these lamina-propria-filled extensions, each one of them is a villus. So here's one villus. Here's two villus. I mean, here's one villus, here's two villi. And the villi are made up of cells that have microvilli. What's this third thing? This third thing that does not have, it does have microvilli, it does have villi. This is my circular fold. Circular folds have submucosa inside of them. If you don't see submucosa, you're looking at a villus. You're not looking at a circular fold. Circular folds are actual folding in, like pinching down of the entire tube all the way around. So the tube narrows. But as the tube narrows, the surface area increases. Which is the point. Okay. You should be able to compare and contrast all those different ways of increasing surface area. And now that you have the generalized tube and how we can increase surface area, now we're going to look at each of the parts of the tube specifically. Sounds like fun. I'll be right back.