are inviting me to talk today. I am a pediatric gastroenterologist, so a little bit of what I'll be doing is talking tomorrow about pediatric GI. But kids have stomachs, too, so we'll be talking about the GI tract today. A great pleasure to open this thing up to talk about the GI tract, which is also sometimes known as the alimentary tract or the digestive tract, and it's T-R-A-C-T, not T-R-A-C-K. This is really a continuous tube that begins in your mouth and ends at the anus. And I am from Massachusetts, and we like a lot of donuts. So the way I explain this to my patients is think of yourself like a donut. We all have a skin on the outside, and we all have a hole that goes from our mouth to our bum, and it's just another skin. And yes, with endoscopy, we get a chance to look at that all the time. We're very aware of it. But this is really just an entry into that hole, important hole in the middle of your donuts. It's a long hole. It measures about 30 feet in the average adult, which is kind of extraordinary. And it has three functions, so digestion, basically, of food into nutrients, absorption of nutrients into the bloodstream, and elimination of solid wastes. So this is a schema that you're going to see throughout the course. And I'll be focused on the esophagus, and it enters into the stomach. And then my colleagues are going to take you through the rest of the 30 feet and a couple of extra organs. So it's really a complex and wondrous system. It is a field, I think, that many of us knew about before we went to medical school. I'll ask my colleagues later about that. But you get into medical school, you start realizing that the GI tract is pretty extraordinary. And like it or not, it's basically perfectly designed to acquire every calorie that you eat out of every meal. So if it's working right, it's something you're sort of always contending with throughout lifetime. And really, the esophagus and the stomach, which I'm focused on now, is designed to get food into the system and start the process of digestion. So there's really no absorption or nutrition happening at this part. It's just the entry point. So the journey through digestion is really the mouth to the esophagus into the stomach, and then all of digestion is really happening in the small intestine, and then elimination in the large intestine. Digestion, though, does begin in the mouth. And that's because you need to chew. So mechanically, you have to start to break down food. And then we do have saliva in the mouth, which is part of digestion. You also have a tongue. Your tongue is actually remarkably important. And you guys are going to be thinking a lot about dysphagia. Some of what we're doing in GI, when someone says, I have trouble swallowing, is trying to sort out, is it in the mouth, or is it in the esophagus? But the tongue is very important for arranging food boluses for swallowing. Saliva is all about moistening the food. There's a lot of glands that produce it. But it also includes amylase. And that's a piece of carbohydrate digestion. So that is being started, basically, as soon as you put food in your mouth and it starts getting moist. The other thing saliva has is some antibacterial effects. And I think we've become increasingly aware of all the bacteria that live in our gastrointestinal tract, starting in our mouth, but throughout the GI tract over the past 10 or 15 years. So food has to move into the esophagus. That's really the narrowest part of the GI tract, with the exception of the appendix, which is this skinny little thing that we'll talk about later. And there's two really narrow parts. So the first part is, actually, I think if I do it here, can you guys see that? Yes. OK. So if the narrowest part is at the upper part, that's the upper esophageal sphincter. And then down at the bottom, where it passes through the diaphragm, that's the lower esophageal sphincter. And then in the chest, it's vertically oriented. It's slightly to the left of median. And it's basically passing alongside. So it sort of does this little dance with the aortic arch and descends into the posterior mediastinum along the right side of the aorta. And then in the diaphragm, it's actually passing a little bit in front and a little to the left of the aorta. And as I'll talk about in a second, it's a remarkably thin organ. So we are very aware of the heart in the chest. I don't really notice it as much anymore. But my fellows, that's one of the very first things they'll notice is the esophagus kind of pumping. And it is, because the heart is in there, too. Just some terminology around swallowing. So when we talk about feeding, that's actually the placement of food in the mouth, the manipulation of food in the oral cavity prior to swallowing. And that might be chewing, so mastication. And it might be the oral stage of the swallowing, where, again, the tongue works to push the food bolus backwards. Swallowing is moving the food in the mouth and then basically through the pharynx and into the esophagus through that upper cricopharyngeal juncture, that upper esophageal sphincter. And then there's a word deglutition, which is basically all about swallowing. The phases of swallowing are quick. You don't even think about it for the most part. The oral phase, which basically gets it to the back of the mouth, is about one second. Then pharyngeal is about one second. And that's got a lot of complicated movements of muscles in the back of your mouth that keeps the food from going up into your nose. And then we're starting a process that will continue throughout the GI tract called peristalsis, which is basically the movement, muscular movements, through the GI tract. And all of this stuff, again, happens without you even thinking about it, and it takes about a second. And it is, unfortunately, as people get older, one of the things that can start to fall apart. And then I'm in pediatrics, so in the beginning, infants have a very different swallowing experience from after 6 to 12 months. I know Alexis has a one-year-old, so she's just been through this. Once the food is in the esophagus, it takes about 8 to 20 seconds to move from the upper esophagus down to the GE junction. And really, it's all about digestion. So we are trying to get stuff into the rest of the GI tract, and that's because that's how people grow. But it's also, once you've grown as much as you're going to grow, it's all about metabolism, body maintenance, and reproduction. So in terms of digestion, again, a little bit of this is mechanical. That's the chewing or mastication, the peristalsis, moving stuff throughout the GI tract, and then segmentation. And I'll talk about the difference between peristalsis and segmentation in a second. There's also a lot of chemicals, though, involved. So we have saliva. Once you're into the stomach, it's all about hydrochloric acid, a very strong acid that's basically a big piece of digestion. And then further down, there are enzymes that are secreted, some by the pancreas, and then also the liver creates bile, which is important. A lot of this is about movement. If anything gets stuck at any point, that's a problem. And so really, digestion is a process of moving things through the GI tract. And the motility of the GI tract really involves a lot of muscle movements. And those two muscle movements are broken down into peristalsis, which is propelling the food forward, and segmentation, which is sort of shaking it up and moving it together with simultaneous contractions. And then also part of digestion is all these different enzymes and whatnot being secreted. And then later on will be absorption. So motility is really a fascinating piece of GI, although I dare say most of us in the room probably don't get that excited by it. But there's a whole group of gastroenterologists out there that are really motilists at this point, as they call themselves. And they are very, very interested in how the GI tract works. And it's really all about muscle movement. That's got two goals. So there's propulsion, and there's mixing. And I'll show you these differences. So this is peristalsis, which is basically the concept that the muscle is the muscles, because I'll show you there's two muscles in the esophagus, are moving things down. And then segmentation is it's actually mixing stuff as it goes, so going sort of back and forth so that it's mixing it up. All right, so getting a little bit further into the esophagus. So the esophagus is that first tube. It's a hollow muscular tube that connects the mouth to the stomach. It's collapsed at rest. So we are going to be doing endoscopy later. We do endoscopy. I'm used to thinking of it as a tube, but actually it's quite collapsed. And it's flat in the upper 2 3rds of the chest and then rounded in the lower third. And as I said before, it's very thin walled. So it's about 2 millimeters thick. And you're going to see that when we do endoscopy in the pigs, but we won't be doing it in each other. But it's very, very thin. It has four distinct layers, which I'll take us through. So the mucosa, the submucosa, a muscular layer, and then there's an outer fibrous layer. And what the esophagus does not have is what's called a serosa. So it doesn't have an outer layer, like a lining on the outside. In terms of its length, it is an organ that grows as you grow. So it's about 8 to 10 centimeters at birth. By the time you're 5 years old, you've grown considerably. And it's now 16 centimeters. At about 15 years of age, it's going to be close to adult size. And adults, it really all depends on how tall you are in terms of how long it is. So somewhere between 18 to 26 centimeters. I'll usually say 30 to 35, even. Some very tall people have a very long esophagus. The diameter really is going to vary based on whether the bolus of food or fluid is passing through. And it can be quite flat, or it can be sort of expanded out. But it is technically about 5 millimeters at birth. And then again, diameter is growing as the organ gets bigger. So it's 15 millimeters at 5 years of age. And then we do consider normal diameter in adults, at rest, to be 20 millimeters. But it can stretch. So it can take that bolus and stretch out to about 30 millimeters. And really, the esophagus, again, what it can handle all varies by age. So a lot of people in my world ask me, why is my baby spinning up all the time? And the answer is because they have a very short esophagus. And this is the adult with their nice, long esophagus. All of us are refluxing all the time. Anyone who's drinking coffee right now, trust me, you are refluxing. You don't necessarily think about it. But it's all about transient relaxations of the lower esophageal sphincter, which we'll talk more about. Babies are also always refluxing. They also lie on their backs a lot, don't have gravity. And their esophagus is 10 centimeters. So an adult comes up 10 centimeters. You don't notice. A baby comes up 10 centimeters, and it's in their mouth. Here's esophageal wall layers. So you have the most inner lining is the mucosa. And then you have the muscularis mucosa, the tiny little muscular layer. Then you have the submucosa. And then what's really important is we have two muscles. We have a circular muscle, and we have a longitudinal muscle, all in that 2 millimeters thickness. And the other thing that's fascinating about the esophagus is its stratified squamous epithelium. It's actually very similar to your skin's epithelium. And that's different from later on in the GI tract, as soon as you get to the stomach. In terms of the muscles, you have the upper third of the esophagus is skeletal muscle, which is voluntary muscle. And then the lower 2 thirds is involuntary. So I don't know that I can control my esophagus. But technically, it's skeletal muscle up at the top that's a little bit more. You have some control over it. And then, again, the way these two muscle layers are working is the inner circular muscle is contracting the esophagus and essentially moving the food down. So that's the peristalsis. And then the outer longitudinal muscle is important for shortening it and also helping to move, also helping with segmentation. And all of this, not just in the esophagus, but moving throughout the GI tract is relying on something called the enteric nervous system, which is not connected to your spinal cord. And it's a rather extraordinary nervous system that is huge. And so I think as we're starting to get into it and understand it better, we can understand some of these motility issues. The last pieces to go over is as we move into the stomach, that happens through the gastroesophageal junction. And there is a moment when the esophagus hits the stomach. So you have the esophagus hitting the stomach at what we call the squamo-columnar junction, or the Z line. And that's because the type of lining, the type of cells down here is columnar cells, as opposed to the stratified squamous epithelium of the esophagus. And all of this is happening right around where the diaphragmatic ligaments are coming in. So we can talk more about the lower esophageal sphincter, which people think of as a muscle, but a lot of that is actually about how the diaphragmatic ligaments are coming in on both sides, or all around the esophagus, really. And then here's your stomach. So that lower esophageal sphincter, which is the diaphragmatic ligaments, are sometimes called the crural ligaments. And they create this little angle of hiss. All of that is what's keeping you from refluxing all the time. And it's your antireflux barrier. The lower esophageal sphincter, by design, relaxes. It needs to relax, or else you wouldn't be able to get stuff into your stomach. So you don't want it tight all the time. But ideally, it's contracting to allow a lot of backflow of stomach acid and food into the esophagus. But it's all about these transient lower esophageal relaxations that will determine just how much you reflux. Again, all of us reflux a little bit. You have to after you eat. And it's very uncomfortable if you can't reflux. But you want it to be able to mostly stay closed when it needs to stay closed, and not let too much stuff come back up. OK, looking at it endoscopically, this is the GE junction, the gastroesophageal junction. So you have this nice salmon pink of the esophagus, and then a darker pink of the stomach, literally where they're meeting. And if you look at it under a microscope, you will see the stratified squamous epithelium hitting the columnar epithelium of the stomach. The stomach is a J-shaped, or sometimes I say kidney bean, reservoir. And it is where, basically, you're putting in a lot of food. And now you need to start a new process where you're really going to break it down. So you've masticated and chewed up stuff mechanically. But now we need to really start the acid that will really help to get the food softer and more broken down, and then also some enzymes. And it's really on the stomach's responsibility not to dump all that food into the small intestine. So it has to be pacing you and releasing small amounts of partially digested food into the small intestine in a measured way so your body has a chance to absorb what's coming. So when we think about the stomach, you go through the lower esophageal sphincter. And you can either go along the greater curvature, or you'll see what we call the lesser curvature. And you have these different parts of the stomach. So up here, near the heart, is what we call the cardia, because it's near the heart. The fundus, this is all considered the body of the stomach. And then the drain of the stomach is called the antrum, or I would say the pre-drain part. And then it actually drains out the pylorus. And this would be a picture of the antrum looking down. So you've got your lesser curvature here, your greater curvature here. You're heading down. And then this is actually where retroflex. So you'll be able to do this in the pig. You're going to look up. And you can actually see the scope coming through the lower esophageal sphincter. And you really get a sense of the cardia and the fundus. The stomach looks remarkably different from the esophagus. It takes one endoscopy to realize that it's a completely different place from the esophagus. So it's got all these rugae in it, which are folds. And that is, again, all about digestion. A lot about digestion is trying to maximize your absorptive spaces. But the rugae can be flattened out. So you can add a lot of air into the stomach and flatten them out. But they're there. And again, the stomach also has a lot of layers. So it has a mucosa. It has a muscularis mucosa, a submucosa, a muscularis propria. And then it has a serosa. So the stomach has a serosa. And those columnar cells have a lot of different glands in them. And frankly, depending on which part of the stomach you're in, you'll have different cells. So I tend to send all of my biopsies for the stomach. I'll just send them in the same vial because the pathologist knows whether they're in the cardia or the antrum based on which cells they're seeing. There's another way also to look at all these layers, which is called endoscopic ultrasound. And this is the transducer in the middle. And you're seeing the different layers of the stomach. And again, corresponding to those different layers. This is a picture. So here you've got your rugae. But each of those rugae is lined with these very columnar cells. And you can see even they have more folds to them. So lots and lots of absorptive capacity throughout the stomach and into the small intestine. This is your columnar epithelium. And then the stomach has three muscle layers. So the esophagus had two. The stomach has three. It has a longitudinal, a circular, and then what we call an oblique layer. And your stomach is a remarkably large muscle is a remarkably muscular organ, which most of us know, unfortunately, when we vomit, it will feel very, very strong. And that is because your stomach has a lot of muscle in it. So it can really do a lot of contracting and doing things. And then I'll end with just this concept that the other thing going on in the stomach, which in the world of EOE we think about, but we don't have to think about too much, is there's a lot of hydrochloric acid in the stomach. And it's an interesting feedback system where you think about food. And the G cell here down near the antrum will secrete gastrin. And that tells this enterochromatin-like cell to start making histamine. And histamine is definitely what triggers the parietal cells, which tend to be higher up in the fundus. They're saying, go ahead and make the acid. And then once you've got enough acid, you actually turn off the acid with something called the D cells. So it is a feedback mechanism that you say, we need more acid and you can stop making acid now. And that gets me to the end of this talk. Thank you.

pediatric gastroenterologist

gastrointestinal tract

digestion

absorption

esophagus

stomach