nicely went over the esophagus and the stomach with you guys. I'm going to focus more on the small intestine and the large intestine. So the small intestine is about 20 to 25 feet long, so pretty long structure here, all squished into our abdomen. So the goal is to have the stomach contents empty into the small intestine, and the small intestine has some important key factors. It's going to help us get our digestion of the food, absorb nutrients and water, along with moving the food along the GI tract to the colon. So the small intestine is further subdivided into three sections. Duodenum is your first section, jejunum is the middle, and then ileum is your distal third. Looking at the first section, which is our duodenum, this is right after the stomach. So the pylorus is our right here. Let's see if I can... Okay, good. The pylorus is right here. This is our valve from the stomach going into the small intestine. So the first portion is called the duodenum, and the duodenum is called the bulb. And this is within the peritoneum. It's anatomically important. And then we make this curve down here, which is called the second portion of the duodenum, and that's another name is descending. And its important anatomical structure kind of coincides here, which is called the major papilla, which I think Dr. Guha mentioned in the kind of slides that we're showing and the box flow diagram. We'll have the liver, the gallbladder, and the pancreas kind of come together with their digestive juices, and they all empty here in this common channel into the small intestine, and this is where the mixing process and the interaction with those juices happens. And further down, it makes a sweep down here. This is the third portion of the duodenum, which is the horizontal, and then going further here, the fourth portion, the ascending. And all of these are behind the peritoneum, so not so much anterior. And then further down, we'll get into the further second section called the duodenum. So the duodenum is smaller. It's 9 to 12 inches in length, and like we talked about, it's four sections. So this is the diagram we were referring to. So up here is your liver above the screen, and it has the production of bile, and I think we'll go into that further later on in the day. And then you have draining system to the right and left hepatic, and then they come together in the common hepatic and then to the common bile duct, and the gallbladder is kind of a holding vesicle there. You have the pancreas, another digestive organ. It sits behind the stomach, below and behind the stomach, and you have this draining tube called the pancreatic duct, and all the digestive juices coming from these two ducts, you know, empty commonly into the second portion of the duodenum right here. So the goal also within the duodenum is to have the bowel, duct, and pancreas empty into the duodenum, and the function of that is to break down the food. These enzymes, you know, help with the absorption of nutrients as they break it down and also with the absorption of particularly certain nutrients like fat and protein and other things that'll happen here. So the jejunum, which is the middle portion of the small intestine, is 8 to 10 feet long. There's really no clear demarcation in the second and third portion. Like anatomically, we can tell, like surgeons can tell, where the duodenum and the jejunum start, but it's difficult sometimes to know, and that's, you know, pertinent to devices and things where we're doing enteroscopies. We kind of have an estimate of where we are, but not a clear demarcation. So the jejunum and the small intestine in general, their absorption, again, the function is to absorb nutrients, carbohydrates, fats, proteins, minerals, and other vitamins, and this is the only area in the small intestine where we have this villi-like structure. These are like finger-like projections that help us with the absorption. And so on the right, you'll see this pathology, histology slide, and you can see if you take a biopsy or cross-sectional image or representative section of that, you'll see these finger-like projections, and they're made up of these outside. You'll have these columnar epithelial cells, and intermixed, you'll have these mucus cells producing mucus, and then you'll have the lamina propria right here, the plain tan section, and admixed into the middle of this is a whole network of arterioles carrying oxygenated blood, venules carrying deoxygenated blood back. You have lymphatics, and then you also have nerves entangled into all of this. So what does that look like in an endoscopy or like a video capsule endoscopy? So these are, I call it a shaggy carpet. So you won't find that in a lot of stomach and colon. It'll be more of a flat mucosa. So only in the small intestine will you know, and sometimes that's how we know we're in the small intestine, if we have an altered anatomy, like somebody who's had bariatric surgery or something different than the routine, and so this helps. And it's only present in the small intestine, helps to increase the surface area, and our body's pretty smart that way and can give us more absorptive surface. So if you really kind of stretch it out, it's equivalent to a tennis court, which is quite impressive in our belly. So the last third section is the ileum, which is about 8 to 15 feet of small intestine, connects to the colon at the side of the ileocecal valve, which is our anatomical connection. And the function, again, is to continue to absorb nutrients, like we've been saying. And they have some specialized goals. So here we can have absorption of bile salts and fat-soluble vitamins, thus with the help of the bile salts, so vitamin A, D, E, K, and then the vitamin B12. So it's important if somebody's had surgery or have disease that's affecting these types of areas, surface areas, that we can have malabsorption of these types of nutrients. And so we as physicians need to know that and to look for these things so we can replete them or be monitoring for these things. All right, so that was the small intestine. Moving forward to the large intestine, also called the colon. This is kind of the end of the journey for our food and our nutrients. So the last part, it's the last part of the gastrointestinal tract. It's not as long. It's 4 to 5 feet in length. And the goal is actually not so much nutrition absorption, but more water and electrolyte absorption, and to also eliminate waste and solid waste. So the colon is divided into multiple sections. So coming from the right to the small intestine attaches to the colon somewhere here where the ileocecal valve is. On the outside of the beginning of the colon called the cecum, you'll see the appendix, you know, if it was there. We don't actually visualize the appendix through colonoscopy. We just see something called the appendiceal orifice, which will be kind of like an umbilical cord, like a belly button opening. But we don't get into this. So the colon starts with the cecum. Then we go upwards. So it's called the ascending colon. And then we go across, again, transverse colon. Then go down, called the descending colon. And then this S-shaped section is called the sigmoid. And then the rectum. And the very last is the anal canal. So I think Dr. Guha kind of talked about peristalsis and motility a bit in the esophagus and so forth. So it's very important in the intestinal tract to have peristalsis. This is how we move contents forward. In the colon specifically, you'll have peristalsis, which is slowly kind of moving things forward. But you also have some reverse peristalsis to mix and allow surface area to, again, absorb nutrients and the water and the electrolytes. And then we have large amplitude waveforms in the colon, which give these big mass peristaltic passages. So, like, this is where, you know, some people wake up in the morning. They have their morning coffee and breakfast. And then you feel the urge to go to the bathroom. So these are the large bowel movements. They happen a few times a day. And then throughout the day, you're having these smaller peristaltic movements moving forward. So, again, they're happening throughout the GI tract. And it's important in mixing and propelling of contents forward. And it's helped by the – coordinated by the contraction and relaxation of smooth muscle. Rectum and anal canal are kind of in its own function. It's a very important function. So we'll dedicate a little bit of – a few slides on that. So the rectum is obviously the distal part of your colon, right here. And then you have the anal canal at the bottom. And it's broken down with two different sphincters that have a very important role in all of us. So we have the external anal sphincter and then the internal anal sphincter. And this is called a dentate line where the transition happens. So the rectum, of course, holds and retains the stool until it's an appropriate time to release. The anal canal is the one that helps us with the exit of our contents. So the internal anal sphincter is an involuntary control. So you do not have control over this. So this is – you know, stool bolus hits your rectum and this naturally will open up. But then if it's not a socially acceptable time to use the restroom, like say we're in a conference like now and we don't want to go to the bathroom right away, then our external anal sphincter will tighten and it's a voluntary control until we're ready to defecate. So the defecation process is a multifaceted process. It has muscle groups, innervations, and, of course, social cues. So we have the puborectalis muscle, which is like a sling right here, that helps in the angle of the anorectal canal. And so if we're at rest, we don't want to go to the restroom, we'll have this relaxed area, and then we're going to have to tighten the external anal sphincter because we don't want to go to the bathroom. And so this is an angle that also prevents us from leaking. And then if we're straining and we want to actually go to the bathroom, we're going to have the relaxation, and then we finally have the descent in the pelvis floor, and then we have expulsion of our contents. So that was it. Mine wasn't as long and less physiological component. Any questions about that from your pretest or any other questions that I didn't cover? Yes, go ahead. Are there parts of the colon that are maybe riskier to take a resection from, like they're more vascular, or is it different for every patient generally? So I wouldn't say vascular, and my colleagues can correct me if I'm wrong, but I would say maybe the muscle thickness is somewhat important. So usually we are taught for, like, polypectomies and large, you know, we're going deeper and thermal energy and things like that, that they respect the right side of the colon, which is thinner, and then the left side is thicker, like the sigmoid descending. And so you have a little bit more, you know, room, whereas in the right colon, you have increased risk for perforation if you're doing, you know, if you're very aggressive. I don't think vascularly we have any difference. They're fed by two different SMA and the IMA, which is your mesenteric arteries. But I do not think that there is one more hypervascular than the other. There are now areas of less vascular support. So we'll have these watershed kind of areas. Let's see if I can go back to the colon, maybe this one right here. I don't know if there's a—oh, no. There's not like—okay, I'll just use my cursor. I don't think there's a laser is what I was looking for. Oh, sorry. Yeah. Okay, thank you. Perfect. I see it. Thank you. So right here, these bends are where we have less blood flow supply just because of the nature of the arcades of our blood supply. These are called the watershed marks so we can get more ischemic injury if we have low blood flow situations or blood clots, things like that. We'll address that in another section when we get to more diseases of the bowel so we can lead to ischemia. And, you know, we kind of see that commonly in a pattern. It doesn't always fit, but more than none, it's usually in those zones. So I would say that would be the one thing that stands out as less so hypervascular. That's a good question. Yes? Oh. Go ahead. Yeah. You started describing the villus structures in the jejunum, but I'm just wondering how it would vary if you were looking at the duodenum compared to the jejunum or, say, the ileum. Similar. Exactly similar. And so now the function are different. So in the jejunum we sometimes absorb iron and those kind of nutrients more, and then the ileum we're absorbing B12, fat-soluble vitamins, because of the bile salts that are also getting reabsorbed. So the function is different. But structurally, if you're looking at it in an endoscopy, it'll look just like this. Maybe this isn't the right forum to discuss this, but if the structure and cells are similar, how is the function different? Right. So that has to do with the physiology of the cellular structure and makeup. Yeah. So I think that would be going a little bit in more detail. But there are certain cells like, you know, in the stomach we have G cells and parietal cells and things like that that function to make. So same way the border interaction with those vitamins are happening in certain sections. But overall, this is what it looks like in general. So if I gave a section, you know, like a histology section, I think like this. Oh, sorry. And I gave this to a fellow, they would know that we're in the small intestine. Now, you know, here we have some changes, and we can stain them in histology and things like that to know what type of cells there are. So we would know maybe it's more jejunum versus ileum versus jejunum. But overall, I think for our knowledge today, we should just say that this is the brush border or like the villi structures for the small intestine. Thanks. I think you had last. Yeah. The jejunum, you know, and you say 8 to 15 feet. I mean, how do you articulate to 10 in length? Is that because of the size of the person? I mean, like I understand children, but when you get, I mean, how do you know it's 8 to 10 feet? I mean, is that because of the size of the body? I'm just kind of wondering. I think it's an average, and you can have more petite people and large. But, you know, even in like an esophagus, we tend to see similar, you know, we can see the Z line in the shorter patients a little bit lower than an average person maybe at 40 centimeters from the mouth to the esophagus. I think it's a similar thing. It's an average of they must have taken like either surgical measurements or, you know, post-mortem measurements and averaged those. And this is probably what we're seeing. So it's not to say like, oh, petite people will have 8. You can still have up to 10. Okay.

small intestine

large intestine

digestion

absorption

GI tract