Good morning, everyone. I'm really excited to be back here at ARIA. I just absolutely love this course. I think it's really fun. It's exciting. It's interactive. Every time I do it, I kind of learn something new, either in content or thought process. Okay. So we're going to start with a few of these polling questions. So get ready with your Slido. The first question asks, in the esophagus and stomach, the esophagus, A, moves food from the mouth to the stomach. B, begins digesting proteins and fats in the food before arriving in the stomach. C, is normally lined by the same columnar mucosa as the rest of the GI tract. Or D, A, B, and C. Put your answers in. And yeah, you guys were not fooled by that one. So the esophagus moves food from the mouth to the stomach. That was fast. Okay. And the second question is, the stomach is, A, the main location of food digestion, B, the main location of nutrient absorption, or C, where food is ground down into small pieces to increase digestive and absorptive surface. Okay. You guys are all set here. Maybe there's something that you can still learn, though, from this course. We'll see. So yeah. So the answer is C, where food is ground down in small pieces to increase digestive and absorptive surface area. Okay. So moving on. So this is going to be our foundational GI system schematic. The GI system is really an incredibly effective and organized process by which its sole purpose really is to turn food into nutrients and energy that the body can then use to function and survive. And it does this through a process called digestion. The main organs of the GI system include the digestive tract, also sometimes called the alimentary tract, if you hear that word. And it's a long, twisty, and hollow tube that starts at the mouth, continues through the esophagus, stomach, small intestine, large intestine, also called the colon, and then ends at the anus. And then you have the liver, gallbladder, and pancreas, which are the solid organs of the digestive system that help with digestion along the way, producing and excreting various enzymes and compounds. So what is digestion? So digestion is to break down food into smaller components, where the larger, insoluble food molecules in the form of proteins and fats and starch are broken down into their smaller molecules, such as amino acids, fatty acids, and glucose, which then can be absorbed through the GI tract into the bloodstream, where then they're delivered to the other organs that depend on those nutrients and minerals to function. And this process is essential for the human body to grow and metabolize and reproduce and then basically just survive. So digestion is often divided into two processes based on how food is broken down, mechanical and chemical digestion. The term mechanical really refers to the physical breakdown of the food, of the large pieces into smaller pieces, which then can be accessed by the digestive enzymes. And it's the chemical digestion where these enzymes actually break down the smaller pieces into small molecules that the body can then actually use. The mechanical digestion involves mastication, which is that technical term for the act of chewing, and then the two types of muscular movements, peristalsis and segmentation, that mix the food and move the food. And we'll talk about that in a minute. And then the chemical digestion, it refers to the saliva, the gastric acid, the pancreatic enzymes, and the bile. Okay, so once food is ingested, the process of digestion really relies heavily on the ability of that food bowl to actually move through the GI tract through propulsion. In fact, the act of swallowing is the last voluntary act that we have of the digestive process until we defecate. So propulsion refers to the movement of the food through the digestive tract and includes the voluntary process of swallowing and then the involuntary process of motility, which is really generated by coordinated contractions and relaxation of the smooth muscle of the GI tract. And the patterns of GI contraction as a whole can be divided into two distinct types of muscular movements. We have peristalsis and segmentation. So peristalsis is that sequential alternating waves of relaxation of the circular smooth muscles of the GI tract, and it propels the food forward. And peristalsis is actually so powerful that it can overcome gravity. If you were doing a headstand, peristalsis would actually still move that food bowl forward and it wouldn't fall out the other way. And then segmentation really occurs by this rhythmic contractions of the muscles of the GI tract, which leads to mixing of the food with the digestive enzymes, and it kind of maintains a uniform composition and ensures contact with the surface area of the wall for proper absorption. So this is a really good diagram to show the difference between peristalsis and segmentation. Peristalsis, I kind of think of like, you know, those pictures, I've never seen those, but like that really big snake that ingests a mouse and you see like that mouse kind of move down the body of the snake. I don't know if that actually happens, but I feel like you see that in cartoons or something. So that's why I think of like peristalsis, like the forward motility. And then the segmentation occurs like right after you eat a meal, and it's like it occurs within like short segments of the intestine and the process is carried out by the longitudinal muscles of the muscle layer kind of moving back and forth. And so you can kind of see that the food bowl is being mixed and it's becoming this homogenous ball of nutrients that then kind of be absorbed through the small intestine. Okay, so besides motility, the other parts of digestion include secretion and absorption. So secretion involves that release of acid enzymes to mix the food into smaller components that can be absorbed by the GI tract. And then food that's broken down, though, really has no value, right, unless it can be absorbed into the bloodstream and its nutrients can be put to work. And so that is the process of absorption. Okay, so with this background of information, I'm going to start leading you on this journey of digestion. I'm going to talk to you about the mouth, the esophagus, and the stomach. Gauri will take over, talk about small intestine and the colon, and Ayo has really the daunting task of talking to you about all the accessory organs, the liver, the gallbladder, and the pancreas. Okay, so it all starts with the mouth. And the mouth is the beginning of the digestive tract, but actually digestion actually kind of even starts before you put food in your mouth. So just like thinking about food, smelling food, tasting food, it activates your salivary glands and kind of gets those enzymes started. But as you eat, you're like masticating or chewing your food, breaking it down, mixing it with the saliva, forming that food bolus, which then, with the help of the tongue, is able to move back into the end of the oropharynx and awaits to be swallowed. And chewing really increases the surface area of the food so it can mix properly with the saliva, which is produced by the salivary glands. So you have three, the parotid, the sublingual, and the submandibular gland. And saliva like moistens the food, assists in swallowing, and it contains the digestive enzymes. So salivary amylase begins breaking down carbohydrates, and lingual lipase aids in the lipid digestion. And saliva actually has a little bit of an antibacterial effect as well, so it helps a little bit with your oral hygiene. And saliva is really important, actually, for your sense of taste. OK, so once the food is swallowed, it then enters the esophagus. And the esophagus is that hollow muscular tube that connects the mouth to the stomach. It's a variable length. Generally, it's like between 18 to 26 centimeters. And its purpose really is to deliver that food bolus into the stomach. And of note, I don't think the length of the esophagus really corresponds so much with your height. I don't think so. I don't really know if that's true. So anyway, the esophagus is located behind your trachea in front of your spine. And epiglottis is like that small flap that prevents the food bolus from going into your trachea when you swallow, going into the wrong tube. And by all sense, the esophagus really is the transport organ. OK, so moving on, we're going to just really quickly review the wall layers of the GI tract. So no matter where you are in the GI tract, the esophagus or the colon, you're going to have wall layers. It's important to remember that the wall is described in terms of its component layers. But they're all connected together by connective tissue and nerves and blood vessels and such. And I don't really love this diagram. I don't know if you guys all love this diagram, but it's OK. But it shows the segments of the GI tract divided into four layers. But see, it's like the mucosa layer, which is on the inside. And that includes epithelium, the lamina propria, and then the muscular mucosa. And then you have a submucosa layer. And then you have your muscle layer, which is the muscularis propria, which then contains the circular muscle and the longitudinal muscle. And then you also have an outer layer, which is called the serosa, which is like the casein on sausages, for what I'm going to call it. So the mucosa layer is a layer that lines the lumen. On the esophagus, it's the inside of the lumen, so the site that's closest to the food. It contains the stratified squamous epithelium, or flattened epithelium. It provides protection against the mechanical stress, the chemical abrasions. And it has the ability for continuous self-renewal. If you think about it, it always needs to keep itself healthy. So it has a really high rate of cell turnover. And it still is trying to maintain that protective barrier. And this is important, because we are constantly exposing the esophagus. The esophagus is, of all the parts of the GI system, the part of the GI system that really hits all the, that feels everything that you're putting into it. So hot coffee, or hot tea, or crusty bread, all of that. And the muscle layer that's responsible for the movements of the esophagus, the muscularis propria layer, contains that inner circular muscle, and then the outer longitudinal muscle. And again, the circular muscle contracts and waves, and helps propel that food down towards the stomach. And then the longitudinal muscle contracts to shorten the esophagus to help that circular muscle move, so it doesn't have to work as fast, or as hard, I should say. And then the esophagus is unique, because unlike really any other of the digestive organs or in the digestive tract, it's composed of two different types of muscle. So the upper third of the esophagus has this skeletal muscle, which is like the, quote, voluntary muscle. And the two thirds of, the lower two thirds of the esophagus is made up of the smooth muscle, or the involuntary muscle. So this kind of makes sense, because swallowing is that last act of voluntary contribution to your digestion. And so it's aided by this help of the skeletal muscle. And then once the food gets into the esophagus, the digestive process takes over, and we don't have to think about it. But this is really important, differentiating problems with the esophagus and dysphagia, which is the difficulty in swallowing, to know if it's kind of from the initiation, or if it's after the food is already in the esophagus. Is that where it's getting hung up? The transition between the esophagus and the stomach is called the gastroesophageal junction. And we talk about this all the time. It's characterized by the change in the cells that line the esophagus, and the transition between those cells that line the esophagus and the cells that line the stomach. So the transition is usually really very well demarcated. The squamous cells of the esophagus change to the more columnar or square cells of the stomach. And this is sometimes also called in the squamo-columnar junction. I like my ear stem popped yet from the plane, so I cannot tell, like I'm in a fog right now. Am I like yelling, or am I talking okay? You're good. Okay. I feel a little bit like tongue-tied too, and I have holes inside my mouth, so sorry about that. Anyhow. Blah, blah, blah. The distinction is really important in differentiating problems with the Z-line or the squamo-columnar junction. And we'll talk about that in a minute. Okay. So what's important about the Z-line or the GE junction is that the lower esophageal sphincter is located in this region. And this sphincter has been really classically, like historically really important and interesting to study. Because unlike other sphincters, the area, the muscle in this area is not really thickened so much, at least endoscopically, but yet it still functions as a sphincter. So it's comprised entirely of smooth muscle, and it functions like a muscular ring. And it works in conjunction with the crew of the diaphragm actually, or we think it does, which are two tendinous structures that together kind of pinch in that same location and aid to its function. So with this mechanism, the LAS relaxes to let food pass into the stomach, and then contracts after food passes to try to prevent anything from the stomach from refluxing back up into the esophagus. And again, we'll talk about that in a couple hours. And it's not a valve, but it is a sphincter. Okay. So this is a really good endoscopic view of that GE junction. So you can see that really nice pale mucosa of the esophagus, the clear demarcation leading to the columnar cells of the stomach, which is a little bit more red salmon colored. And it's a pretty clear demarcation. And this is kind of where we look when we start talking about Barrett's esophagus or problems at the GE junction. And then on the right is a histological view of this GE junction, which is so impressive, I think, showing clear demarcation between that squamous mucosa of the esophagus and then the columnar mucosa of the stomach. Okay. We made it to the stomach, and I don't even know how I'm doing with time. I think I might be okay. But so the stomach is the reservoir, the J-shaped reservoir of our GI tract. It's a muscular hollow organ. It acts like a container, holds the food while it's being mixed with the stomach enzymes, which continue that process of digestion. And as the food is broken down and mixed with acid, it forms chyme, which is that semi-fluid aggregate of partly digested food that's been broken down mechanically and starting to be worked on by the stomach enzymes. And this is the anatomy of the stomach. So after you see the GE junction or the lower esophageal sphincter right there, and then you have the cardia, the fundus, the body, the antrum, and the pylorus. And in the absence of food, the stomach deflates inward, and its mucosa and semicosa kind of fall into these large folds called rugae. Excuse me. The rugae are really important because they provide the elasticity of the stomach, meaning that it allows for a really increased surface area of the stomach to expand in response to your food ingestion. It accommodates that food without really affecting the pressure that the stomach is feeling. And this is a really busy slide, but it shows the wall layers of the stomach, which are comprised of a lot of different things. But there are a few things that make the stomach really unique in what is kind of housed in those mucosal lining. So the stomach's mucosal epithelial lining consists of surface mucus cells that secrete a protective coat of alkaline mucus. A large number of these gastric pits dot the surface of the epithelium and mark the entry for each gastric gland, which is really magnified here. And the cells located within the gastric glands secretes a really complex digestive juice, or gastric juice, I'm just going to call it. And we're going to dive into the gastric glands in a second. But the other major difference to note in the gastric wall layers is the muscular's proprio layer. There is a third muscle layer, you guys, besides the circular and the longitudinal, but we also have an oblique layer of muscle that also aids in the mixing and the propulsion of the food. And this is a great picture. I like this one. This is a picture of the muscle layers of the stomach. And you actually see an EUS image of this, which kind of clearly delineates the layers of the stomach walls. The mucosa, the submucosa, the muscular's proprio, and then the outer layer, the serosa. And this is just a histological view of that same thing, showing the wall layers. And then again, pointing out that you have that circular layer, the longitudinal layer, and then that third oblique layer, muscle layer, that's unique to the stomach. Okay, and finally, without getting into too much detail, we're going to review stomach acid physiology. So this is a response to eating, which signals the need for the digestive enzymes to be released. And in a really simplified view, when you eat, there is gastric distension that stimulates the G-cells that are located in the antrum. The G-cells release gastrin. And then gastrin goes on to stimulate the parietal cells. Those are located in the body in the fundus to release the hydrochloric acid that is needed for the acid digestion. It also stimulates the interchromaffin-like cells, the ECL cells, to produce histamine, which then also stimulates the parietal cells to release the acid. And then there is, you know, once you have all the acid production, there's actually a negative feedback mechanism that goes into the duodenum, stimulates these G-cells, then to release somastatin that then kind of tries to tell the stomach to stop producing acid that they're good. So although simplified, you can see that the secretion of this gastric juice is really controlled by nerves and hormones from various stimuli in the brain and the stomach and the small intestine through a really kind of fascinating process. And I think that's it for me. So that was a lot, but I'm really excited to hand it over to Gauri to talk about small intestine.

gastrointestinal system

digestion process

esophagus

stomach

mechanical digestion