Okay, so I'm going to talk today in the first part now on the esophagus and stomach. You can see the whole GI tract here. So we have the esophagus, stomach, and then you'll hear from my co-speakers, small intestine, large intestine, and then we go to the digestive organs, which are associated with the digestive tracts, liver, gallbladder, and pancreas. So let's move on. So what's digestion? So it's basically breakdown of food into smaller components, allows the body to absorb, and when I say body, it's the GI tract here, absorb the nutrients and minerals, and remember also nutrients includes vitamins. It's necessary for growth, metabolism, body maintenance, and reproduction. Now there are two fundamental mechanisms of digestion and the process of digestion. One is a mechanical process that includes mastication, which is mixing, peristalsis, and segmentation. I'll go over in the following slides the difference between peristalsis and segmentation. On the other side, we have the chemical process where obviously saliva mixes with the food and then gastric acid and gastric juice, pancreatic juice, or pancreatic enzymes, and bile, they all assist in the process of digestion. The whole process also very much depends on the motility of the movement of the GI tract. It's basically muscular movements, and you'll hear later it's mostly these are smooth muscles. Peristalsis, as I mentioned, it propels the food forward, more towards the small intestine from stomach, say for example, via a process of contraction and relaxation. So one segment contracts, moves the bolus down, the other segment relaxes, so you have to have this kind of very coordinated movements, contraction, relaxations, whereas in segmentation it basically helps in mixing the food with simultaneous contraction of the segments. And this is what it looks like. So you can see how the food mixes. In the peristalsis, the bolus moves forward, in the segmentation is mostly the mixing process going on, but eventually the bolus will move with peristalsis. So as we mentioned, the process again, the motility, we talked about the motility part, then the secretion, that's a release of these acid enzymes to mix with the food, and then finally you have the absorption. Most of the absorption, again, will be taught by the next speakers in the following slides. Now the journey through digestion. So you have bolus from the mouth going into the esophagus, we call it, basically it's a hollow food tube where the food bolus moves to into the stomach, and then from there into small intestine where basically there are other organs like pancreas, liver, and gallbladder secreting their juices, mixing and helping the digestion process, and absorption, and finally into the large intestine. Okay, let's quickly go through the mouth. So basically mechanical chewing breaks down the food, and the food then stimulates saliva production, and tongue also helps to arrange the food bolus for swallowing. Saliva is produced by the salivary glands, and you can see parotid, sublingual, and submandibular glands. Obviously these glands secrete juices, and they contain amylase, which initiates carbohydrate digestion. Saliva also contains a little bit of lipase, which also helps in breaking down, initial breaking down of some of the fat. It also helps in moistening food, because very important, it has lots of mucus, and that's the first line of defense, by the way, antibacterial effect. Now it's the hollow tube I was talking about. It's a muscular tube. I'll show you the muscles in a second. This is esophagus, which is basically connecting the oral cavity, a mouth, to the stomach. It's roughly 18 to 26 centimeter in length, and as you see, it transports saliva and food. These are the layers of the esophagus. You have the inner lining called the mucosa, then followed by the muscularis mucosa, or lamina propria, then comes the submucosal layer, and then you have the inner circular muscle and the outer longitudinal muscle. And this is the mucosa. It's basically stratified squamous epithelium, very similar to your skin, but it doesn't have the keratin. Okay, let me point this out. Very important to know. It's a stratified squamous epithelium without the keratin, but this, as you can see, we'll talk about it later, I guess in the afternoon, how this layer changes in certain conditions. The musculature, you have the inner circular, which helps in the contraction of the esophagus and moves the bolus down, and you have the outer longitudinal muscle, which basically helps in kind of shortening of the esophagus, that shortening of the length. That also helps in propulsion of the food bolus. Now this is another thing important to remember. The upper one third of the esophagus, it's under voluntary control because it's a skeletal muscle, just like the muscles in your hands, fingers, legs. The lower two third of the esophagus is under involuntary control. It's actually aligned by the smooth muscles. So the inner circular, outer longitudinal, these are all smooth muscles. Now we come to the G-junction or the gastroesophageal or esophagogastric junction, however you want to say that. The lining, as you saw, it changes, the inner lining, the mucosa, it's from the stratified squamous epithelium, it becomes columnar, and it light pink color to more orangish, as you can see. Now the junction is otherwise known as the Z-line, the esophagogastric or the squamo-columnar junction. Now the reason it's a Z, it stands for zigzag. The line is actually zigzag, it's irregular to start with, that's why the Z-line is. And then you can see there the diaphragmatic prura, which basically holds this particular junction in place. And there is also a particular angle, I don't know if I can point this out here, but maybe I can. So there is this particular angle here, which also helps in maintaining this particular sphincter, it's called the lower esophageal sphincter, we'll come back to it later, it's called the angle of his, and this is the stomach. So this is the, again, talking about the lower esophageal sphincter, as you can see, it's right at the Z-line, or the zigzag line, where the esophagus meets the stomach. It relaxes with swallows. Now remember, there is always a transient, this is normal actually, lower esophageal sphincter relaxation. You'll hear the word T-L-E-S-R, but that's basically it, it's normal, and you get a little bit of a reflux of stomach contents into the G-junction, Z-line, that's why it is irregular. But that's normal, that's physiological. And this sphincter is important because it prevents backflow of the stomach acid and food back into the esophagus, because this sphincter needs to maintain its integrity, and we'll talk about when that loss happens, and what happens after that. But suffice to know right now, that this is a, there's a kind of a group of muscles at the lower end of the esophagus, this muscular layer here, which helps in maintaining this junction. So this is what it looks like during endoscopy, you can see the change in color from light pink to more orangish, that's the, this is the Z-line, see? And it's slightly irregular. And this is what the pathology shows, from the stratified squamous, non-characterizing epithelium, to more columnar. So stomach is a J-shaped preserver, you can see this, food mixes, and small amounts of partially digested food then goes into the intestine, these are the different parts of the stomach, you can see this is the fundus, this is the, we call it the anatomic cardia, this is where the Z-line was, and that's the sphincter, this is the body, the greater curve, this is the lesser curve, this is the angle where the, on the lesser curve the body meets the antrum, it's called the incisor, you might hear that word, this is the antrum, part of the stomach, and this is where the pylorus starts. This is just the mucosa, you can see this nice gastric folds, or the gastrogae, we call it, and the columnar line. So this is the mucosa, that's what it looks like, you can see these, these are called the gastric pits, and these are the gastric glands, and this whole thing is the surface epithelium with the lamina propria, that's the mucosa, and it's columnar epithelium. And this is where the muscularis mucosa lies, this is the submucosa, and now for the stomach, the muscularis propria, the proper muscle layer, there are three, instead of two as you saw in esophagus, here you have an oblique layer, then you have a circular, and then the longitudinal. The gastric, the whole, the gland part, this is what it looks like, you have the parietal cells, this is the important cells that makes the acid, you have this endocrine cells, these endocrine cells secrete neuropeptides, so the hormones, we call it, and the chief cells are the ones that also secrete enzyme to digest protein, pepsin, for example. So endoscopic ultrasound is one of the tools that you are familiar with, most of you are familiar with, where we basically use the sound wave to see the layers, we send it through water, actually you can then visualize different layers of the, whether it's esophagus or stomach. Most of the time we call it five layers, and the way it works is that you can see the mucosa, submucosa, muscularis propria, serosa, and actually in the mucosa we have just not only the mucosa and the muscularis, so these are the five layers you see. So this is the way it works, so first you see this layer here, which is hyperechoic, and then you have the muscularis mucosa, which will be hypoechoic, then the submucosa, which will be again hyperechoic, the muscularis propria will be hypoechoic, and then the serosa, which will be hyperechoic. And when we say these echo intensities, we usually compare that with the liver or spleen, and we say whether they're isoechoic, meaning the same echo intensity, or hyper or hypo. Okay, so that's in brief, you may see more and more of these endoscopic ultrasound pictures later on. We talked about the columnar epithelial mucosa, we talked about the muscle layers, so this is key for the stomach, because one of the key functions is the production of acid. Now the way it works is the G cells, which are some of these specialized cells in the stomach, which secrete this one of these hormones called gastrin, very important. Now G cells are stimulated by the nerves, actually, acetylcholine receptors are present, so this acetylcholinergic input comes in, activates, that's where we are thinking that you're eating, right, that whole stimulation starts. G cells make gastrin, gastrin now then goes and binds on the gastrin receptors, which is usually the CCK2 receptor on the enterochromaffin cells. So these are cells which basically make histamine, you know, one of these transmitters, neurotransmitters, and this histamine, which is also important in allergic reactions, as you know, but this one particularly binds to the H2 receptors, not the H1 for allergy, and this then helps in the secretion of acid. Now acid then plays a negative role, basically, when you have too much acid production, then the gastrin production shuts off. So that's a nice physiologic control, that's the way it works. Any questions on this very basic introduction of esophagus and stomach? Anything? I mean, I know you guys did already the pre-test questions, and anything from this pre-test that you feel that you need any clarifications on, or everything is crystal clear, pretty much, on the basic anatomy and physiology.

esophagus

stomach

digestion

mechanical processes

chemical processes