So, I'm going to talk about the small and large intestine. Okay. So, we'll start with a poll. In the small and large intestine, the small intestine, A is divided into two main parts. B is the main location for nutrient digestion and absorption, or C is approximately six feet long. Awesome. Yep. So, it is actually the main location for digestion and absorption, but we'll actually talk about how long it is in a minute. In the small and large intestine, the large intestine is A, where B12 is absorbed, B, the final organ of the luminal gut, or C, the largest part of the luminal gut, and therefore where most nutrient absorption takes place. Ah. Okay. So, answers are a little bit divided on this one, so we'll talk about it. It is the final organ of the luminal gut, and we'll talk more about the roles that it plays. So, as Brooke went over, this is the GI tract. You know, it's essentially a long hollow tube from the mouth all the way down to the anus, with several organs attached to it that help with digestion, detoxification, and nutrient absorption. Let's talk about the small intestine. So, the small intestine is actually 20 to 25 feet long. Probably does vary a little bit on how tall you are. Like Brooke talked about, the stomach contents are digested. They're broken into smaller bits, and then they're emptied into the small intestine. So, the small intestine is really where we get most of the final phase of digestion of our food, as well as absorption of nutrients. We have a long, long small intestine, so it's a bit redundant, but a large surface area over which we can do this. And it's sort of interesting when you think about, you know, sort of our ancestors where food maybe wasn't as available, so they had to develop really efficient ways of absorbing nutrients, and perhaps maybe that's why our small intestine is as long as it is. But of course, nowadays, where food is plentiful, you know, maybe this is also another reason why it's so easy to put on weight. So, the small intestine is divided into three parts. The duodenum, so that's the first part of the small intestine. Then you have the jejunum, the second part, and the ileum. And it's interesting because, you know, we always think about these as three parts, but there's no specific one place where these are all divided, except between the duodenum and the jejunum. So, let's talk about the duodenum. So, this is, again, right where the stomach empties into the small intestine. And definitely also an area that we can reach with a scope. So, typically when we're doing an endoscopy, what I'll tell our patients is, you know, we look at the esophagus, the stomach, and then the first part of the small intestine called the duodenum. And essentially, what we're looking at here most commonly are the bulb and the descending or second portion of the duodenum. There is, there are two more parts, though. The horizontal or third part of the duodenum, and then the ascending part of the duodenum or D4. Essentially, the demarcation between the duodenum and the jejunum is something called the ligament of trites, which sits right between D4 and the jejunum. So, this is the shortest part of the small intestine, about 12 inches in length. Also part of the duodenum is where we see the bile ducts exit into the small intestine. So, you can see that the pancreas is nicely opposed to the duodenum. And through the pancreas, you have the pancreatic duct and the common bile duct. And then, of course, the common bile duct, as I will tell us in a little bit, is connected to the gallbladder, the liver. And essentially, these are ways that our pancreatic enzymes, our digestive enzymes, and our bile acids can all exit into the duodenum and allow for further nutrient breakdown. Okay. So, once things are broken down, essentially, then we start to see different areas of absorption along the small intestine. But essentially, iron is really the main element that's absorbed in the small intestine at the proximal end. And that's also why, in certain conditions where, you know, we'll talk about this later, but where we might see small intestinal ulcers or conditions like celiac, where iron deficiency anemia might be the most common presentation of that. So now we get to the jejunum. This is considered the mid-portion of the small intestine. This is about 8 to 10 feet in length, so much longer than the duodenum. And again, as we move further south, there's no clear demarcation between the jejunum and then the last part of the small intestine, which is the ileum. This is the part of the intestine, as well as the proximal part of the ileum, that we can't see with the standard endoscope or colonoscope. And so ways in which we might visualize this, if we need to, are certainly on cross-sectional imaging, but also something like a video capsule endoscopy, single balloon or double balloon enteroscopy. And we can do those anterograde, retrograde, so we can look at them from different locations. So let's talk about the jejunum. So the jejunum's always been fascinating to me, because across the length of the jejunum, there's a lot of mixing going on. And essentially, you're breaking down those macromolecules into micromolecules and things that can actually be absorbed across the cellular lining. So essentially, across our jejunum, there's about 9 to 10 liters of fluid that gets secreted and reabsorbed, secreted and reabsorbed. And essentially, this is all in an effort to absorb the nutrients that we've taken in. And this includes carbohydrates, fats, proteins, minerals, and some more vitamins. But what you can see is a highly efficient system that's been designed to allow for this. So essentially, in the small intestine, you have what are called villi, so little tiny protrusions into the lumen, as well as crypts, which allow for more efficient water absorption. And essentially, in each of these villi, you can see systems of blood vessels, so arterial venous blood systems, which essentially go in and absorb all of the nutrients that cross the cellular layer. Now, it's interesting, you know, how do we actually absorb the nutrients? It's not just passive absorption. So passive absorption is typically with things like water, which will end up following the electrolytes. But most of the nutrients we absorb are absorbed actively, meaning we have cellular transport systems on the cells that actually allow for transport into the blood vessels. So if you're doing a scope, this is actually what the small intestine might look like. Lots and lots of villi. My husband, who's a urologist, always tells me it looks sort of like a shag rug or like our bathroom mat. But I think of it as more like sea anemones. I think it's really beautiful, and it floats in the water. And essentially, it always, again, amazes me how our bodies are designed to really facilitate the absorption of nutrients for our body. So you only see villi in the small intestine. And the large surface area really helps us to absorb things efficiently. And interestingly enough, if it were stretched out, it would be the surface area of a tennis court. Finally, the last part of the small intestine is the ileum. So it's the last 8 to 15 feet of small intestine. And what you can see on the diagram is the very end of the small intestine, which is called the terminal ileum, is connected to the beginning of the colon or the cecum. And that connection occurs at something called the ileocecal valve. We have lots of valves and sphincters throughout our body. The function of the ileocecal valve is to really regulate flow into the colon, but also to prevent backwash from the colon into the small intestine. And so essentially, in the ileum, we continue the absorption of nutrients. In the ileum, we also see a little bit more specialization as far as what we absorb. So we absorb bile salts or bile acids. That's a process called enteropatic recirculation, where essentially, a lot of the bile acids that are spit out by the liver into the gallbladder, emptied into the small intestine to help us digest our fats, then get reabsorbed in the terminal ileum. And then you sort of have that enteropatic recirculation. B12 is also absorbed here. So as we'll talk about later, if people have resections of this end part of the small intestine, they can be more prone to B12 deficiency. And then here's the large intestine. So this is also called the colon. It's the last part of the GI tract, about 4 to 5 feet in length, and definitely varies based on height, from my experience. But this is much shorter than the small intestine. And the function here is not so much in nutrient absorption, but more in absorption of water and electrolytes. Through a process of peristalsis and segmentation, it also helps to absorb the fluids, but it also helps to desiccate the stool or make the stool more formed. The colon actually is involved in reabsorbing anywhere between 1 and 3 liters of fluid, of water. And so for people who've lost a part of their colon, or even their entire colon, they may be more prone to more frequent stools or looser stools. Now, interestingly, the colon is something that we can live without. But of course, we have to take into account that physiology. So the colon is something that we'll see during a colonoscopy. But essentially, if you start on the right side, right where the terminal ileum enters the colon, you can see the appendiceal orifice and the appendix right there in the cecum. This part is called the cecum. Sorry, the mouse doesn't work very well. This part is called the cecum. Then you go up to the top right, called the ascending colon. You go across, transverse colon. Then you go down on the left side, descending, followed by the sigmoid or S-shaped colon. And then you get down to the rectum and then the anus. So motility relies on peristalsis. As Brooke mentioned, you know, this occurs throughout the GI tract in different ways and really relies on the muscular layers in the muscular layer of our GI tract. So this is important for mixing and propelling our stool downstream in the colon. And essentially, what you see in the colon are these nice coordinated contractions and relaxations of smooth muscle with the haustral folds. And so you see these nice pockets kind of moving things along, but also allowing for more efficient fluid reabsorption. Okay. So this is exactly how it goes. In the right colon is actually where most of the fluid gets reabsorbed. On the left colon is essentially more of a conduit for moving things through, essentially moving the stool or mass through peristalsis. And then let's get down to the very end, so the rectum. So the anatomy of the rectum, you can see here, comes down into the anus. So the anus is comprised of two different muscular layers, the external anal sphincter, which is on the outside, and then the internal anal sphincter. And those essentially are contiguous with the muscle layers of the GI tract. Now dividing the anus and the rectum is the dentate line. And really, that just refers to a change from the columnar epithelium to a more stratified epithelium. So again, where we might see different functions rather than absorption and storage, we might see more roles in defecation and sensory and allowance of sensation as well. So let's talk about kind of how we defecate. Always exciting for a morning. So the rectum retains stool until it's the appropriate time to release. And essentially, what happens is stool moves down our left colon, and then we have stretch receptors in our rectum that allow stool to be held in that area. We have the internal anal sphincter, which is under involuntary control, meaning you cannot consciously control how the sphincter works, but it relaxes and contracts in relation to what's going on physiologically. Defecation of the internal anal sphincter does allow greater capacity of the rectum, but you don't actually defecate until you relax the external anal sphincter, which is under your control or voluntary control. So let's talk about what happens. As stool moves through down to the distal rectum, it sort of hangs out in a sack, sort of in the rectum. And that sack is created by the puberectalis muscles. So you can see that right here, which is connected to the pubis bone. And essentially, that acts sort of like a sling to hold things, to hold onto the stool. But when stool fills up the rectal vault, the internal anal sphincter relaxes. It sends a signal to your brain saying, it's time to poop. And then it's kind of up to you whether or not you're going to do that or not. And that's actually learned behavior from when we were really young, and we probably don't even remember when we learned that. Oops, sorry. But essentially, once you're ready to have a bowel movement, you're sitting on the toilet, you relax your external anal sphincter. But what also happens in that process is you relax the puberectalis muscle. And relaxation of that changes the anorectal angle and allows descent of the pelvic floor and then allows stool to evacuate more easily. So that's also the rationale behind things like squatty potties and raising your legs up when you're defecating. The idea is to straighten out that anorectal angle because maybe it makes it easier to defecate. Now, interestingly, like I know this is totally gross, but like when you pass gas, you don't actually relax the puberectalis. And that's a good thing because then when you pass gas, stool doesn't come out with it. And that's it.

small intestine

large intestine

functions

anatomy

nutrient digestion