I'm Femi Kassam, as you heard, I'm from Loyola, just outside of Chicago. Welcome for those of you who traveled. And we're going to kind of talk about the middle part and the last part of the GI tract here, small and large intestine. And here's the disclosure slide. So Jennifer already talked about the esophagus and the stomach, so we're going to be really talking about this big kind of chunk of small intestine here and the large intestine on the outside. So the small intestine is the longest part of your GI tract, 20 to 25 feet long. I'm actually Canadian, so I use a metric. So I usually think of it as about 600 to 800 centimeters or 6 to 8 meters or 6 to 8 yards, all kind of wrapped together in the center of your abdomen. So the stomach empties the contents, the partially kind of digested food into the small intestine, and while the stomach, you heard, is really all about kind of mechanically and starting to chemically digest food, the small intestine continues to break down and digest the food, and where you really start to see absorption of nutrients and waters and propelling the digestive fluid or chyme through the GI tract onto the large intestine. So the small intestine is broken down into three general parts. So the duodenum is the first part. That's where the stomach empties into. The ilium is the—or sorry, the jejunum is the mid part that's the longest, where a lot of the absorption takes place, and the ilium is the shortest segment at the end. So the duodenum has several very well demarcated—or not well demarcated, but endoscopically and anatomically obvious parts. So you have the duodenal bulb, which is the very first part of the duodenum, and then you have the descending, or we sometimes call it the second portion of the duodenum, and you'll see here there's an opening called the major papilla, which goes into the bile ducts, which you'll hear more about later. Then the third portion of the duodenum, also called the horizontal portion, is down here, and then you have the ascending portion of the duodenum here, and then that empties into the jejunum. And the first section and the—the first section and the shortest part of the small intestine is the duodenum, about 9 to 12 inches in four sections, as we talked about. So this is where the—this is the second portion of the duodenum, where the major papilla that you saw that indicated before, it's also called the ampulla vater. And that is where the pancreatic ducts, which is the pancreas—which the main pancreatic duct coming through the pancreas empties into. So that allows various pancreatic enzymes to basically be added to the digestive fluid to help the digestion. That's also where the gallbladder empties into. So you have the gallbladder coming into the cystic duct, which merges with the common hepatic duct to form the common bile duct. And both of those come together to empty into the duodenum to aid with digestion. So this is basically what I just talked about, but the bile duct and the pancreas are emptying into the duodenum to help break down the food. So you've got the enzymes that are secreted by the small intestine itself, enzymes from the pancreas, and enzymes from the liver and gallbladder that are all working together to digest the food. Now absorption of nutrients, basically there are some nutrients that are only absorbed in the duodenum, but for the other ones it's just the start of the absorption process. So the jejunum is the longest part. It's the mid part of the small intestine, 8 to 10 feet. And there's no clear demarcation between the jejunum and the ileum, but they do have different functions. So the jejunum, most of its job is going to be absorption of nutrients. So in particular, carbs, fats, proteins, all of your macronutrients, minerals, and some vitamins. Again, there are some nutrients that are only absorbed in the duodenum and some that are only absorbed in the ileum. So for the vast majority, it's going to be through the jejunum. And this is a histology or pathology slide that shows all of the glands. So basically each of these projections is what we call a villi. I think there's some pictures, or it might be on the next slide, that shows what the villi look like when we actually look at them endoscopically. But essentially each villi, if you magnify it, you can see here it's very complicated with multiple different cells and glands that either secrete enzymes or absorb. Oh, yes. So here's a beautiful picture of the villi. I think this is like one of the prettiest things that we can see endoscopically. So each of those little purple projections that you saw on the pathology slide is basically one of these little things, and you can kind of see them floating in either fluid or air. Now villi are only present in the small intestine. And essentially it allows the surface area of the small intestine to be increased even further. Because if it was just flat and you had all of those cells, then yes, you'd be able to absorb some. But because there's cells going up and down each villi, that increases the surface area. And if you stretched out the small intestine flat, it would be the area of a tennis court. Now the ileum is the final 8 to 15 feet of the small intestine, and that connects into the large intestine via the ileocecal valve. And so its primary function is to continue the absorption of nutrients. In particular, there are some nutrients that are only absorbed there, including bile salts, fat-soluble vitamins, and vitamin B12, which is very important in folks who, because of disease or because of surgical resection to have their ileum removed, sorry about that, you will start to see deficiencies in those vitamins, and we'll talk about that later. So the large intestine is also called the colon. That's where I spend most of my time, and most general gastroenterologists as well, doing colonoscopy. So it's the last part of the GI tract, about 4 to 5 feet in length, or I usually say it's about a meter or about a yard in a taller person. And so its function is mostly to continue the absorption of water. It's the primary place where water and electrolytes are absorbed, and also the elimination of solid waste. So if you see here, basically this is the start of the large intestine here, and the rectum is the end. So if you can imagine those pictures that we had of the small intestine kind of wrapped in here, you would have the ileum that attaches via the ileocecal valve into the cecum. This little part hanging off here is the appendix, which doesn't really have much of a function in terms of digestion. Then the different segments include the ascending colon, transverse colon, descending colon, sigmoid colon, the rectum, and the anus. Now motility, that is the movement of fluid through your large intestine, or through the entirety of your GI tract is very important, as we've heard, for not just pushing the digestive chyme and fluid down, but also segmentation and mixing. And it's basically due to coordinated contraction and relaxation of smooth muscle. So after you eat a meal, even before your intestines have sensed the fact that there's physically food there, there's a neural kind of pathway that's initiated to start that peristaltic motion, and it continues kind of consecutively throughout the GI tract. Now the rectum and the anus, the main purpose of the rectum is to hold solid stool until you're ready to defecate. So the rectum is kind of just the reservoir right at the end of your colon, and the anus is the opening, which has two sphincters around it that allow you to control defecation. So very important. So the rectum retains the stool until it's appropriate. There's an appropriate time to release. Babies, children don't know that, and so they don't have voluntary control. But once we're adults, usually we try to. So the anus is wrapped by two sphincters. So you have the internal anal sphincter, that's under involuntary control. So once the rectum is full and your body senses, wow, maybe there's something else coming in, this is a good time, it will relax involuntarily. And then the external anal sphincter is under voluntary control. So once you sense that, yes, there's something in the anus, it's time to have a bowel movement, it's you controlling the external anal sphincter that allows the stool to pass. Now defecation, these two are showing, these two diagrams are basically showing, looking at you from the side essentially, where this is your spine here, and this is the rectum coming down in the anus here. So essentially, when you have solid stool that's sitting in your rectum, and you're at rest, it's not going to go anywhere because you have the external anal sphincter, the puborectalis, which is that kind of sling in the external anal sphincter as well, holding it in. Now when you are ready to have a bowel movement and you're straining that sling, the puborectalis muscle will relax. And also you'll have relaxation of the internal anal sphincter and the external anal sphincter as well, and a straightening of the anal rectal angle. Now this is a normal defecation. Now in some people who have something called pelvic dyssynergia or pelvic floor dysfunction, very common in women who have had multiple children, other obstetric complications, sexual trauma, they can have basically an inappropriate contraction of these muscles when they're trying to have a bowel movement and can lead to pretty significant constipation. So does anyone have any questions? I know I went through that pretty quickly. Any questions for Dr. Kossin? Okay. Yeah. And we will be talking about the disease part of this later on today as well. So thank you. Thank you, Femi.

small intestine

large intestine

gastrointestinal tract

duodenum

colon