 Let's talk about the small intestine. So the first thing I think about the small intestine is just a shocking surface area, right? It's about 200 square meters. Imagine the surface area inside your small intestine alone would be a hundred times the surface area of your skin. So how does it have this massive surface area? I'll show you some images in just a little bit, but the small intestine has what are called circular folds. So it has folds inside of it, and then these folds are covered in fingers called villi, and then the cells of these fingers are covered with microvilli. So you have folds with fingers on them, and so that's how you get this massive surface area. So we'll explain why and everything. But first, let's dive into just the three key parts of the stomach. You can see them color-coded here. First you have the duodenum. So quickly, I'll just quickly go over them. The duodenum I like to call the mixing bowl, and that's because the duodenum is going to be where the chym from the stomach is mixed with the bile from the liver slash gallbladder, the bicarbonate from the pancreas, and the digestive enzymes from the pancreas. So it's the mixing bowl. It's going to take all those things and mix them, turn them together. It's only the first 10 inches or so of the small intestine. You see it as this C-shape. That's because it's wrapping itself around the pancreas. So the pancreas is going to be connected to the duodenum, and I'll show you that when we actually cover the pancreas. There's a structure called the apatopancreatic ampula, which is where the bile duct and the main pancreatic duct are going to fuse together. So that's where the digestive enzymes and bicarbonate mix with bile, and they're going to be squirted into the small intestine. There's a sphincter that controls that, so it's not always dumping in there. It's only going to be opened up when we need to digest food based on hormones, but it's usually going to be closed. So we'll talk more about that later, but that's the duodenum, and that's why its job is as the mixing bowl. The duodenum is the most important part of the small intestine. It's the simplest way to look at it. It is about three feet long, but this is where we always say the small intestine is where 80 to 90% of digestion and absorption take place. You could say that the duodenum is where 80 to 90% of that takes place. So the most important organ in your body or portion of an organ in your body for digestion and absorption of food is the duodenum. And then the ilium is kind of famous for being the longest portion of the small intestine. So it's about six feet long. It connects your small intestine to your large intestine at an area what's called the cecum, and there's a valve there called the iliosecobalp. So that's how food, as it's traveled through your small intestine, it's going to be moved into the cecum of the large intestine. Now the ilium's primary job would be just to capture a little bit more digestive products. It's going to be absorbing the B12, absorbing bile, hand flow, things like that. But most digestion absorption takes place there at the duodenum. All right, so here we see, I already mentioned the folds that have these villi. So these circular folds are covered with villi. There's 20 to 40 of these villi in every square millimeter of the small intestine. So obviously there's a lot of them. Each of them, as you can see here, has a capillary with a small artery and arterial and a venule, but they also have the lacteal. And the lacteal is going to be part of your lymphatic system. And that's because your long chain fats and the fat soluble vitamins are not absorbed right into your blood. They're too big. They're absorbed into your lymphatic system, and that's how they're carried through. So we'll cover the actual digestion of food later. So that's these villi, and then each of these villi are going to be covered with, as you see here on the right side, the microvilli. And these are about a micrometer wide, so they're very, very small. Each villi, there's like 200 million of these microvilli per square millimeter. So they're everywhere, and they look like bristles is how they're described. That's why they're called the brush border. So the lining of your small intestine is covered with these microvilli in what's called the brush border. And they're important for digestion because in the brush border are enzymes that finish off digestion. We focus on enzymes that break small things or large things down in small things, but the brush border has enzymes that finish off digestion. Imagine like the enzyme maltase, which would take maltose and split it in two or, you know, sucrose. These are the kind of enzymes that are going to be found here in the brush border. So just remember that term, if I talk about the brush border, when we do actually cover chemical digestion. All right, I think that's, well, I did have one more thing I wanted to cover. Your intestines, you know, if you watch enough yogurt commercials, you know that 70 or 80% of your immune system is lining your intestines. And that's because your intestine is full of potentially toxins in your food, you know, trillions and trillions of bacteria. Your immune system is trying to keep what's in your intestines out unless it's invited in, as far as digested food. So you see these areas of what is called malt, mucosa-associated lymphoid tissue. There's going to be a lot of them in your intestines. But the key area here are what are called pyres patches. These are going to be at the end of your ilium. So if you think about it, your ilium certainly has lots of bacteria. Your small intestine has lots of bacteria, but not near as many as the trillions that live in your colon. So the large intestine is supposed to be full of bacteria. The small intestine's not. So if bacteria climb through that iliocecal valve into the small intestine, well, at the end of the small intestine, you have these special collections of lymphoid and immune tissues whose job is to capture them before they can be absorbed. Your large intestine is not going to be a place where bacteria should be absorbed. But if they get into the small intestine, it is possible. So that's just a protective mechanism there, but not a huge deal, at least in this class. All right, so that is the structure and function of the small intestine. A lot of cool stuff there. I hope this helps. Have a wonderful day. Be blessed.