 Holy digestive fun times, dudes. So, small intestine, I think that I read today that the small intestine is 20 feet long. It's longer than that, or it's longer in a dead body than in a living body. In a living body, the whole digestive tube has some tone because of the smooth muscle layer that surrounds it. So once the digestive system dies, it actually kind of stretches out a little bit longer, but still, it's ridiculous. It's like, dude, 20 feet, 30 feet, whatever, any number of feet, like 20 inches, I could imagine fitting in my gut, but 20 feet, are you kidding? That's a lot. So there are three parts of the small intestine. The first part is the duodenum. So the pyloric region of the stomach empties food substances, mush, into the duodenum. The duodenum is not very long. It's about 10 inches long. And it's the main site for digestion, chemical digestion of substances. And so there's a lot of juices. Ow, I just hit my electron on my binder. There's lots of juices being secreted into the duodenum. After we secrete all those juices, we're secreting lipases, we're secreting proteases, we're secreting different kinds of sugar digesters. And, dude, you just sent in pH one stomach acid into your tender little small intestine, you better adjust the pH. So we also secrete bicarbonate from the pancreas. A whole bunch of stuff gets secreted into the duodenum. It gets mixed, it sits, it does its thing, and as the stuff moves through the tube and into the next part of the tube, it's called the jejunum, and the jejunum is longer. The jejunum is seven and a half feet long, whatever. I don't really care about these numbers. I just think it's super interesting to think about them. The jejunum is where absorption happens. So duodenum, we're having secretion and digestion. The jejunum, we're absorbing what we digested. And in the last section, the ilium is the same, and the ilium apparently is 10.8 feet long. That's a lot of feet. Now, ilium, you know, you have an ilium in your pelvic girdle, it's a bone, and it's spelled with an I. The ilium in your gut is spelled with an E, and I remember that because ilium is where you eat. So make sure you spell ilium correct because it will be wrong if you put an I in that space. Ilium has a majority of absorption also happening here. Now, if we are, most of the small intestine is about absorption. So there are a couple of things that are true about the structure of the small intestine that help with this, and these things have to do with increasing the surface area of the tube itself. There's four ways that you can increase the surface area, and we're going to look at them again in more detail, and I'm going to list them out for you now. You can have a long tube increase the length of the tube, a big long tube, I mean, dude, 20 feet long, yes, that's a long tube. The longer the tube, the more surface area you have to absorb the nutrients that you need. If you can imagine, if you just went directly from your duodenum 10 inches down to your large intestine, you wouldn't have very much time to absorb what you are digesting, and really you'd probably have some pretty gnarly diarrhea. So thank you for the length because that gives us time to absorb the nutrients out of there. We also have these things called circular folds, and these are a phenomenon where you can imagine the tube, hello, the tube of the digestive tract, and then there are, as if I'm looking down the tube at you, there are these places where the tube actually constricts and then comes back out again, and constricts and comes back out again, and there are these big circular folds in the tube itself, which you can imagine actually functions to increase the surface area, and somewhere I have a picture of that for you, whether it's today, I actually can't remember. You also have these little structures called villi, which are finger-like projections. So you have circular folds, and on your circular folds, you have all of these finger-like projections that burst out into the lumen of the small intestine, again, increasing the surface area, and then on individual cells, you have things called microvilli, and those are projections on an individual cell. So you have like a cell with projections, which increases the surface area for absorption, and then those cells make up these projections, and then those projections are on these circular fold projections on a freaking long, long tube that's crammed into your belly like that. Small intestine isn't messing around. We're going to talk about the accessory organs that actually secrete stuff into the duodenum in like a time or whatever, the next lecture. Right there, we're going to talk about accessory organs, so we'll talk about the pancreas and what they secrete and the liver, what it secretes. There's only one pancreas, so what the pancreas secretes. Oh, dear. How about if we talk about the large intestine and figure out what it does?