All right. In this video, we're going to cover the gross anatomy and microscopic anatomy of the stomach. So, the stomach is located in the upper left quadrant of the abdominal pelvic region. And contrary to common belief, the role of the stomach as a digestive organ, it is not the main region where digestion occurs. Uh its main function is as a temporary storage and there is a little bit of digestion that occurs with the bolus um as you mix it with gastric juices stomach acids and we now call the mixture not a bolus but once it mixes with that stomach acid we call that mixture kime. So if we look at the gross structure, the esophagus feeds into the stomach in the region called the cardia or the cardial part. This is fairly close to the heart, hence the name cardia. And as the food goes through the cardiac sphincter, it deposits into the stomach. This dome-shaped region here, uh, this lies just beneath the diaphragm. This is known as the fundus. And then you have this middle portion of the stomach known as the body. Uh and then lastly, you have this region here called the pyloris. So there are several parts to the pyloris. There's the antrum, which is the more superior part. And you'll notice that in this antrum, there are these little folds called rug. And that's going to be important when we look at the stomach when it is not full. It has the capacity of about 50 mls. Uh but it allows the stomach to stretch. So the stomach itself can hold once full can hold up to about four lers of food. So that's a little over a gallon of food um due to the compliance of the stomach tissue. So as we continue down the pyloric antrum, we have this narrowed region here called the pyloric canal. And at the very end of that pyloric canal is where we have another uh circular smooth muscle that will act as a sphincter. So this is a valve that sort of separates this part of the stomach from the dadum which is the first section of the small intestine. So at a time usually uh when we deposit material and we'll talk about this in the context of digestion we do not dump all of the stomach contents into the deadnum. The rhythmic contraction of the stomach and the temporary loosening of the smooth muscle in the pyloric sphincter will allow for sort of 3 ml bits and pieces to go into the dadum. This is uh very important because the stomach is an environment where the pH is really really really low while in the dadum for those digestive enzymes to work the pH is going to be closer to uh 7. So if we continue our analysis of the gross anatomy of the stomach uh I'm going to flash back to the previous slide. We have the convex lateral surface otherwise known as the greater curvature shown over here of the stomach and we could also see this concave uh region here. This is known as the lesser curvature of the stomach. So the mezzentaries are actually associated with these curvatures. So the lesser momentum is associated with the lesser curvature. So that is going to go from the liver to the lesser curvature to sort of anchor as we see in this particular figure right here. It allows us to sort of anchor the stomach to other elements of the GI tract. And here we see an image of a cadaavver with the lesser curvature associated. uh you don't really see the lesser momentum because it has been sort of removed to see the various parts of the GI tract. Then we have the greater momentum and the greater momentum is associated with the greater curvature. So it's going to go over the small intestines, the spleen, uh the part of the colon known as the transverse colon and it sort of fuses with the messylon. So the greater um will have lots of lymph nodes uh just to uh police any potential pathogens in the peritineal region uh and it contains lots of fat deposits. So here we see that greater um sort of attached to the transverse colon. So that's the part of the colon that goes sort of parallel to your diaphragm. Uh all right. So then we have the autonomic nervous system stimuli the blood supply. I wouldn't worry about this particular slide uh too much. I just kind of incorporated here uh just for completion. So there are sympathetic stimuli as well as parasympathetic stimuli. the same parasympathetic stimuli that we've talked about through the vagus nerve uh is going to go to elements of uh your GI tract while the sympathetic is going to go down a branch called the splangic nerve through the ciliac plexus network. With regards to the blood supply, all the arteries are going to come from the ciliac trunk. specific branches are the gastric and the spleenic branches while the veins are emanating from the hpatic portal system. So again I wouldn't wouldn't worry too much about this particular slide. Just understand that there are autonomic uh inputs onto the stomach. What is of very high importance is understanding the structure within the stomach. Specifically, we're going to talk about the gastric glands on the next slide. So, similar to other elements of the elementary canal, there are four basic tunics in the stomach. Those are the four basic layers. We talked about the muscularis, the mucosa, the sarosa, um, and such. However, in the stomach, two of those layers are going to be extensively modified. So first and foremost when we look at the musculature the muscularis externa ordinarily there are two layers there's the longitudinal layer uh and then there's the circular layer but in the stomach we actually have a third layer which is an oblique layer and so this is going to be of special importance in the function of the stomach. The function of the stomach is not just to move material down to the next step, right? Down to the next element of the GI tract. That's where the longitudinal and the circular smooth muscles came in. What the oblique layer does is it allows the stomach, and again, I'm going to flash back over here. We can bend the stomach sort of along that curvature. So, it sort of jack knives. And this jacknifing sort of facilitates the mechanical breakdown of food. So this is involving the physical breakdown of food into smaller morsels to increase surface area for enzymes to more efficiently break down material. So that is a huge difference with other elements of the alometric canal. The second main difference is when we look at the mucosa. So the mucosa does have simple columnular cells. Uh and this layer of mucus uh mucous cells excuse me will secrete a two-layer coat of very alkaline mucus. So there's a lot of bicarbonate ion that is secreted into the mucus to make it very alkaline. So this bicarbonate rich surface is sort of protecting the underlying layers from the you know the very high acidity of the gastric juices that we'll talk about shortly. The other crucial difference is that the mucosa is going to be dotted by these regions called gastric pits. So, we're going to focus on these gastric pits because these gastric pits are where we produce that very acidic gastric juice. So, if we dive a little bit deeper into those gastric pits, again, I'm going to sort of flash back between this slide and the next, you're going to have a bunch of mucosal cells. Okay? So, when we looked at the fundus and the body, you have glands there that are producing a lot of gastric juice in the mucus neck. So if we go to these cells here, we have our gastric pit. These epithelial cells are producing of course a lot of mucus to protect the stomach. Then you have these sort of I I want to say transitional cells, but that's kind of misleading. Basically, these cells are flanking the gastric pit region and the gastric gland region. So these mucus neck cells, if we go back here, these mucus neck cells are actually secretreting a different type of mucus. This mucus instead of being bicarbonate rich is actually a little bit acidic and we really don't know the function that it serves in the digestive tract. We just know that this is what happens through these cells. But as we dive deeper into this gastric gland, the next cell that we're going to come across are the parietal cells. So these parietal cells are very important. If we actually observe them with a light microscope, they kind of look uh sort of circular, but with higher refined or higher resolution microscopes, they actually look like little fuzzy trident. Uh so there's a huge amount of surface area here. And so what these parietal cells do, they are the ones that are going to be actively secretreting hydrochloric acid. And this hydrochloric acid, keep in mind the pH is like anywhere from 1.5 to 3.5. So very acidic. that's not going to break down proteins into amino acids, but it does destroy the three-dimensional structure. So, it denatures those proteins. It disrupts the hydrogen bonding of those proteins. So, it might make them o more open up to facilitate enzyatic breakdown. The acid serves a second function other than just denaturing proteins. The chief cells, which we'll talk about shortly, they actually secrete an inactive enzyme called pepsinogen. So this acidic environment activates pepsinogen into its active form which we call pepsin and that will facilitate the breakdown of protein. And then the third function is that this acidic environment helps to break down the sugarbased plant cell wall. So when we're eating a lot of um leafy material, uh the cell walls are very hard. We don't have enzymes to break it down. So the acidic environment sort of facilitates that process. And then lastly, the acidic environment really prevents a lot of bacteria from infecting us. Uh there are a couple of exceptions uh which we'll talk about uh momentarily when we discuss ulcers. So going back to how do we produce hydrochloric acid? Well, there's a very important chemical reaction that occurs in your red blood cells, and it actually occurs all over the place, but it's basically water and carbon dioxide. And there's an enzyme called carbonic and hydrates. and it converts water and carbon dioxide into a very weak acid called carbonic acid. And it's a weak acid because it stays mostly in this molecular form, but it can dissociate into a hydrogen ion and the bicarbonate ion. And so this is the hydrogen ion that forms the hydro part. And then what happens is within the cell we will antiport. So we'll exchange a bicarbonate ion for a chloride ion. So in the end we deposit the hydrogen here as well as the chloride ion. Hence the hydrochloric acid. So we mentioned the chief cells momentarily. So the chief cells are a little bit more basil to uh the gastric pit uh excuse me to the parietal cells. So they do produce and secrete pepsinogen uh and then pepsinogen not only does it get activated in the acidic environment but it also has a positive feedback mechanism. So pepsin will help to activate other pepsinogen enzymes and the chief cells do secrete some lipaces and in the past this was sort of uh we thought was negligible but in fact it does uh serve to help digest some of the lipids that we consume. So lastly, when we get to the very bottom of this gastric pit, we have a group of entroendocrine cells. So these cells are going to be secretreting various chemical messengers into the laminaropria and some of these signals are going to be paracrine, which means they're going to be very local signals. So, for example, serotonin, which is also a neurotransmitter, and histamin, which you've probably heard of, involved in inflammation, both of these are going to be involved in signaling for an increase in acid production. Acetylcholine is actually a third compound um released by the nervous system, the parasympathetic nervous system that also promotes acid production. And then there are a couple of hormones, somatin and gastrin, which we'll talk about later, but gastrin more so is involved in sort of motility and movement of material through the GI tract. So we mentioned that that mucosal layer is of is of utmost importance because you have such low pH such harsh conditions in the stomach to facilitate digestion you could damage stomach tissue. So this mucosal layer this very thick mucosal layer that is bicarbonate rich that ensures that lower layers are not going to be damaged. The second sort of adaptation that we have are the epithelial cells have complete tight junctions between them to prevent any kind of leakage that would damage underlying tissues. The third sort of adaptation that we have is those cells that are in the surface epithelium here. these mucosal cells next to them you have a bunch of undifferentiated stem cells and so any damaged epithelia is quickly replaced by the division. In fact those surface cells are sort of replaced every 3 to 6 days uh to mitigate any potential leakage of stomach acid into the lower tissues. Now of course there are instances where there are damages to the stomach. Gastritis is an inflammation caused by anything that is going to breach the mucosal layer. So under produ under production of mucus, over production of acid, these tend to be rarer issues. um more or less uh what's going to usually be the cause of these imbalances is a specific bacterial infection by a bacteria called heliccobactor polylori and these are a type of bacteria known as acidtoiles meaning they love low pH environments and so they bore into the epithelium so they are disrupting that mucosal layer which allows stomach acids there to damage the underlying tissues. Now, in cases where bacteria aren't the cause, overuse of non-steroidal um anti-inflammatory drugs, uh like ibuprofen, for example, can also cause this sort of damage to that epithelial or to that mucosal layer that leads to these sort of lesions.