Okay, so now we're just moving down the GI tract. So we heard about the esophagus and stomach. So now we're going to hear about the small intestine and the large intestine. Okay, so you saw this slide before, again, outlining the digestive organs and the digestive tract. So we'll start with the small intestine. So in terms of length, it's about 20 to 25 feet long. So that's a lot of bowel in your belly there. The stomach contents empty into the small intestine. So that's the first part it reaches after food goes through the stomach. And the small intestine's really important, again, further breaking down that food, so digesting the food. And this is where, again, the majority of the nutrients, as well as water, are absorbed in the GI tract. And again, your intestine is just one long tube. So starting in the esophagus, stomach, it's just moving through the tube, and that's where the food goes along. When we talk about the small intestine, you'll hear us talk about three separate parts. The first part is the duodenum. The second part, the jejunum. And the third part is the ileum. So the duodenum within itself actually has four parts. It's in this sort of C shape. The first section is the shortest. It's about nine to 12 inches in length. We call that the duodenal bulb. After that, it goes to the descending portion, because it just goes down, goes horizontally, and then up a little bit, so the ascending. So there's four portions. The duodenal bulb is what we say in the peritoneum, so in the abdominal cavity, versus when we say retroperitoneal, it means behind the peritoneum. You can think retroperitoneal if you have your body. It's the posterior part, so it's in sort of the back. So this, the duodenum, again, is the shortest segment of the small intestine. And that's where a lot of nutrients, vitamin, minerals, iron, and other nutrients are stored. Are absorbed. Within the duodenum, a lot is happening, actually. So you see these other organs here. Whoa. Let me go back, sorry. So in that diagram, we're gonna see other organs. So in the green, you'll see listed the gallbladder. The gallbladder is actually just a storage for what we say bile. So the liver, which you don't see in the picture, but is above the cystic duct. And the common hepatic duct, so the liver should be above there. But basically it produces bile. And the bile gets drained through these ducts. And the gallbladder's just a storage for bile. Bile travels down, and eventually gets emptied into the duodenum. You also have the pancreas there on the side. So the pancreas also produces enzymes to break down food, amylase, lipase. And the enzymes actually go through the ducts. And they do meet up. So the pancreatic duct does meet up with the bile duct and empties into the ampullae of the bladder. So that's in the duodenum. So again, it's just delivering more enzymes to help break down that food. Because imagine, you know, when you're eating, you have the steak, right? That steak has to be broken down into amino acids. So there's a lot of enzymes, the acid, all breaking down that food into minute particles for absorption. Oops. Okay, so again, the function is to break down the food from the enzymes in the small intestine and pancreas, and getting the bile from the liver and the gallbladder. After the food leaves the duodenum, it enters the jejunum, which is, again, that mid portion of the small intestine. It's about eight to 10 feet in length. And there's actually no clear demarcation between where the jejunum is and then goes to the ileum, which is, again, is the last part of the small intestine. A jejunum, again, most of the function of the jejunum is really, again, absorbing those nutrients, carbohydrates, fats, proteins, minerals, and vitamins. The picture here is sort of one of these villi that you see. So in the villi, a lot is happening. So you have what we say the columnar epithelium. So that is, again, the barrier between the outside world, meaning the food, and then crossing, the nutrients need to cross that layer to get into, you can see the blood vessels. So that's where, again, nutrients are absorbed. So you can see arterioles, so arteries and veins, and then the capillaries sort of connect the two. So again, that's where a lot of nutrient absorption is happening. Fat is actually absorbed through the lacteals, and you have nerves in there too. So a lot is happening in the small intestine just to absorb these nutrients. This is one of my favorite pictures. And when we scope, I think one of the greatest things in terms of being a GI doctor is being able to do endoscopy. So we see a patient, we get symptoms, but it's really gratifying just to see sort of what's going on in the inside. Not many people can do that. But this is a picture of the small intestine. This is the villi. The body is pretty amazing, right? So because of these villi, what we call these finger-like projections, you're increasing the surface area for absorption versus having just a flat sort of surface. So the villi are present in the small intestine to help absorb all those nutrients that we're eating. If you were to stretch out the villi, they said the surface area would be about that of a tennis court. So this is, I think, one of the neat things about being able to scope and looking at that. It looks like sea anemone, so when you're scoping, it's pretty cool. The ileum is the final eight to 15 feet of the small intestine. So after the small intestine ends, it connects to the colon, and you do have this ileocecal valve that connects the colon to the small intestine. The ileum also does absorb nutrients too, in particular bile salts. And the bile salts add in terms of aid in the absorption of vitamins, the fat-soluble vitamins, which are K, A, D, and E, in addition to B12. So after food travels through the small intestine, we hit the colon. So this is the last part of the GI tract. It's about four to five feet in length. The main function of the colon is actually to absorb water and electrolytes and elimination of solid waste. You actually can live without a colon. Some of you may know patients or people who don't have a colon. So that is possible. Sometimes we need to remove the colon because of cancer, disease. I would say those patients are at increased risk for dehydration, but they're able to live normal lives. Usually within a year after a patient gets a colon out, the small body actually can increase its absorption capacity and sort of make up for it. But the colon, again, primarily to absorb water and just to eliminate waste. We do break up the colon to different parts, as you can see labeled here. So starting at the right, you'll see the cecum, ascending, transverse, descending. So we're not that fancy. In descending, it just goes down. The sigmoid is sort of like this S shape. It can be a little bit tortuous in some patients. And then the. rectum and then the very last part, the anus. So again, your whole bowel is moving because again, you're eating food, breaking it down, absorbing it, absorbing water and then eliminating sort of the waste or residue. So that is the motility or the movement of food throughout the GI tract. We call that sort of peristalsis. You can have reverse peristalsis. So again, everyone probably in some time in their life has experienced that with vomiting, right, so things are going the wrong way and we call that reverse peristalsis. Mass peristalsis occurs when you're actually having a bowel movement. Those are a little bit longer and sustained contractions to actually eliminate waste. The motility is important for a couple of reasons. Again, moving things through the GI tract but also you have mixing things to break things down further too and absorbing. So it's a coordinated contraction, relaxation of the smooth muscle, so again, involuntary. The very last part again of the colon is the rectum and the rectum is actually really important. If it's diseased or has inflammation, you know, people can end up going to the bathroom a lot or they're not relaxing, they can inhibit them from passing stool. In terms of the rectum, we have sphincters, an external and internal anal sphincter. Again, you have voluntary and involuntary control of these sphincters, meaning again when you want to relieve yourself or defecate, you need relaxation to allow the stool to pass. The dentate line just states that, again, the lining of the intestine does change. So at the very bottom, it's a squamous transitioning to columnar, what we say lining or epithelium and so you can actually see that like you can see it in the esophagus. Again the rectum is a reservoir for stool, so the rectum can hold stool until, again, we signal that we need to release it and then again the sphincters are involved in controlling that. Passing stool education is a coordinated event, let's say, meaning a lot is going on and actually for you to pass stool. So we have this muscle called the puborectalis, it's sort of like this sling that you see over there. So in order to have a bowel movement, you have relaxation of the internal as well as external anal sphincters and this muscle, the puborectalis muscle, and that opens up the angle. As you can see now, the rectum sort of straightens out to allow the passage of stool and you also have pelvic floor muscles which relax a little bit. So it is a very coordinated event and there are patients who have, what we say, discoordination of the pelvic floor, the muscles or the processes that can cause constipation. So that in a nutshell is a quick overview of the small intestine and colon questions. Thank you for watching! Yeah, right, so good question. So you probably have heard patients needing their gallbladder removed. Usually if there's stones or again, it's not draining and can get inflamed or infected and again, people can live without a gallbladder and be fine. So the bile instead is sort of just, there's no storage vesicle, it's sort of just, yeah, exactly, yeah. So after a patient gets a gallbladder out, a lot of patients do have, can have diarrhea just because you have a leakage of bile salts in there and that can cause what we say is secretory diarrhea, but yes. Can you comment on the role of the microbiome in the small intestine? I think about it a lot in the large intestine and the role it plays in breaking things down in nutrient absorption. Is there an appreciable contribution in the small intestine at all from the microbiome? Yeah, the microbiome is very interesting because throughout the GI tract, there is a certain microbiome depending on a lot of different factors, right, depending on patient's diet, the pH, but yes. The microbiome is very important because you can imagine that barrier from the outside world to the inside world in terms of what's going on there, you have this bile film. So it is very important in small, throughout your GI tract. You know, it's interesting actually, so when, I do inflammatory bowel disease and so when we have patients and you divert sort of the stool because they needed surgery for obstruction or something and you don't have the normal nutrients or the stool going through, that mucosa actually does atrophy and when we, sometimes we have to scope that part for various reasons, but it is very, very friable, so I mean you touch it and it bleeds. So the microbiome is really important actually in gut health too. So at the spine layer, you know, there's a lot of research going on in that to figure out sort of, you know, define, you know, we were in the phase of sort of defining what is, you know, quote-unquote normal and health and disease and how we can manipulate that in terms of treatment. So a lot of active research in that area. I'd say nothing necessarily prime time for everyone, but I think that's definitely a goal that people are looking into. Yes. or is that just for a diagnosis? Good questions. I think we'll talk about more of that with the scopes. So in GI, we do scopes. And so in terms of the extent that we do, the typical for the upper, again, what we call EGD, or upper endoscopy, we're looking at the esophagus, stomach, and the first part of the small intestine. For a standard colonoscopy, we get through the entire colon. And then we can get through to the last part of the small intestine, usually just the ilium. There are other scopes that you might have heard of, like enteroscopies, which are designed to look at the small bowel. They can go from above and below. And sometimes they need to go through both sides to take a look at the entire GI tract, as you imagine that is a little bit of a feat, meaning it does take a lot of time. There's lots of intestine to get through. I'm not sure if all places do it. We do at Northwestern. Most of the academic centers do. Maybe some of the small private hospitals don't. But yes, we do have a way to evaluate the whole GI tract with scopes. In addition, you may have heard of what we say capsule endoscopy, where people swallow a capsule. And we use that primary to look in the small bowel. But the small bowel can be, what I say, a little bit of a mystery. Because with our senior scopes that we do every day, we don't see the majority. But with these balloon endoscopies, is what we call them, they're able to look at the entire GI tract.

gastrointestinal tract

small intestine

large intestine

nutrient absorption

gut microbiome