hello and welcome to part one of the chapter on the gastrointestinal tract this first part will cover just the normal anatomy and physiology um as always uh the very beginning of the chapter I'd like to Super simplify the system that we're interested in and let's talk about the function of the gastrointestinal tract I will sometimes refer to this as the GI tract if you think of it as any business a hospital a clinic a university there's a department that's known as the receiving department basically uh gets the orders of materials that have been written out from different departments that need food or they need IV bags or they need sheets or they you know whatever is needed gets ordered and then brought in and somebody's job it is to receive I mean they absorb get that stuff into the building that is the GI tract um it's receiving all different kinds of materials um but it's it's in um and so some of the the three main things that get received are water really importantly um hugely important for the circulatory system and for the functioning of the respiratory system for all of our systems we have to have water and that's the one way in um then we're going to divide the other things that get received into macronutrients and micronutrients it's a pretty arbitrary division but it basically means macronutrients are the things that we receive in large quantities uh large amounts and we're going to break those down into four categories the macronutrients this is what's covered in that bonus um review lecture if you haven't had a chance to take a look at that um and you're not sure what these are go ahead and take some time to watch that review lecture so we're talking about the carbs proteins nucleic acids and lipids it's three of these carbs proteins and lipids that people usually think about when they talk about the macros in their diets excuse me and these are the nutrients that are needed um for a couple of different things we use them for energy you think about in terms of the calories that they contain uh we burn that stuff and we use the the energy from that to make ATP and we use ATP to pay for everything else but the other thing um that we use these items for is to actually build ourselves the stuff that we are made out of is the stuff that we take in through our digestive system and if you don't think that's weird I mean just think about your car for a moment um imagine going uh to the gas station and instead of putting gasoline in your tank you just start funneling car parts into there and then say you know buddy you figure it out you know I know you're going to need some of this you know to to get us to move along the road and some of it you're going to need to like rebuild yourself you know change tires and do all the stuff that you need to do but I know that you can take care of that that is what our digestive system does is figure out okay what do I need for fueling and what do I need to build stuff with and it's the same material wow okay and then the other stuff micronutrients these are things that we need in smaller amounts not huge honking amounts that we need the macronutrients in and these include the electrolyte sodium potassium chloride calcium magnesium etc etc uh the minerals we'll talk primarily about iron um but there are others you know Trace miscellaneous a bunch of different things that we'd even tiny amounts and then vitamins these are multi-atom uh molecules some of them are lip or lipid soluble some of them are water soluble we'll talk about a few examples all right so that's the big overview that's what we're taking in the gastrointestinal tract um just a quick building it up again if you haven't already and you don't know what these things are carbs proteins nucleic acids and lipids and what they're made out of go ahead and see the bonus lecture all right so let's quick review the anatomy of the gastrointestinal techno what we're talking about it is one big long Hollow tube start at the mouth ends at the anus so it includes the mouth the tube that connects from the mouth the esophagus going down to the stomach attached to the stomach is the small intestine and it's called small because it's small in diameter it's actually much longer than the large intestine um the the small intestine is roughly broken into these three parts duodenum jejunum and Ilium the way I remember it is DJs people who play music DJI I don't know I think of a logo it's got an eyeball it's stupid but I remember it so that's that's not stupid if it helps me remember all right so DJI uh duodenum jejuna minilium and different things get absorbed better in different parts of the small intestine okay then what we call the large intestine is also known as the colon uh there are different parts we're not going to go over in a huge amount of detail in this class but there is um you've probably heard the terms ascending transverse and descending colon and sigmoid colon and oh my gosh which one's which where is that the easiest way to remember it is it's roughly shaped like a question mark so I'm gonna try to do this correctly for you so ascending up across transverse that's about where the belly button is you know umbilicus um and then descending question mark and then the little squiggle part at the bottom that's the sigmoid and then when it straightens out that's where we get to the rectum wreck that this rect where that comes from is the Latin for Straight Like erect direct something um so rectum is the straightaway at the end and then the anus is where the sphincters are to control um when we evacuate uh the rectum all right one big long tube there are some accessory organs there's accessory glands that kind of hang off the side of this big long tube but it's the big long tube that's the GI tract layered structure means there are different kinds of tissues so there's an innermost epithelial layer and then we'll go out from there and we'll cover that um and then uh who tells the gastrointestinal tract what to do some some coordination is enteric that means it's um within the gastrointestinal tract there are neurons that belong just to the GI tract and uh I don't know how to describe it other than like internal memos you know sending information from one part to the other just of the GI tract you know don't need the brain or the spinal cord involvement anyway but we can make things go faster or slow things down with the autonomic nervous system kind of like with the heart you know it had that signaling system signaling pathway within the heart but then we could you know make it go faster and make it go slower and then there are hormones and local mediators that they're molecules that are being produced for signaling nearby or actually some of it is even put into the bloodstream but the targets are also part of the GI tract and their immune immune cells that play a role and we'll cover all of this and then physiology it's all about moving things through we have secretions that get added by glands like the salivary glands and organs notable two notable ones the liver which we'll cover next week and the pancreas which we'll be covering today so the different segments have different jobs the first part of the GI tract is about like smashing things up um you know receiving um in the mouth chewing um pulverizing mixing and this is about getting the end some of the enzymes that need to access the molecules mixing that stuff up so that the enzymes can get to this to the molecules that they need to get to um in the in the stomach it's also very acidic in there so there's some like antimicrobial action of this very low ph and then we get to the small intestine there's some more mixing for digestion and adding in some digestive enzymes um and then this is where we really start to absorb the nutrients and so there's an insane amount of surface area way more than you would predict on a little tube if it's that thin there's just a lot of ridges and then within the ridges there are more ridges and then if you look even closer on the cells there are ridges on the surfaces of the cells it's an insane amount of of surface area for all this absorption that has to happen then we get to the large intestine as we've said also known as the colon what's largely Happening Here is water reabsorption we've we've gotten most of the nutrients that we can possibly get out of this slurry we've already gotten them out and now we just need to reclaim uh the water and some of the bile salt some of the things that that we put into the the food to mix it up and get it to move through the digestive system now we want that stuff back and finally the stool formation and then the rectum and the anus are about uh sensing when the tank is full and then going up and going ahead and opening up and allowing evacuation to happen excretion to happen the accessory structures I've alluded to these already um so the the GI tract is the hollow organ the hollow tube that goes all the way through us and in case this hasn't become clear this hollow part it's inside us but it's not inside us inside us and and um I think I've used this analogy before when you eat something you get it into your digestive system that's inside you I'll get to my belly you know but it's in the same sense that you know when you put your finger in the middle of a donut hole your fingers in The Donut but it's not in the donut in the donut so kind of like that so um that so the the gastrointestinal tract is the hollow part and then we've got these um accessory structures they're more solid and they kind of leak fluids into that Hollow tube and those include the salivary glands there are some enzymes actually so digestion actually starts in the mouth um and some of the enzymes are for digesting starch and for digesting fat molecules now interestingly there are no enzymes for digesting proteins and that's not really an accident I mean because if you had it already in your mouth you don't have a lot of protection here and so the protein digestion will start like eating away at your tissues just not good okay the other next organ is the liver we're going to cover it in chapter 14 so we'll go into it in some detail um but the part that we're going to mention here is that the liver produces bile that's absolutely required for being able to access the fat and help to digest the fat that that we ingest and then the third structure we mentioned is the pancreas the pancreas it's hard to see but it's this kind of tongue-shaped structure it's behind the stomach um we'll see we'll see a close-up but it it produces some fluids that get pumped into the very first part of the small intestine so as things just just come out of the stomach the liver and the pancreas are already giving their contributions right there and one of the most important things that the pancreas has to do is neutralize the acid that's coming out of the stomach because the stomach has a huge amount of mucus and has great protection against all of that acid but this the small intestine doesn't and so we need to neutralize the acid and that's what this bicarbonate Rich mucus is about this bicarbonate Rich fluid and then very importantly we'll get to this in more detail are the digestive enzymes that are produced by the pancreas here we're calling them pro-enzymes because they're not activated yet um remember I said the reason we don't have proteases in the mouth the enzymes digest protein is going to start digesting the mouth same goes in the pancreas like we have these enzymes they're going to be for breaking apart these molecules but we don't want them activated yet until it hits the the small intestine then okay then you can start digesting the stuff that's in there but not before otherwise they would start digesting the pancreas which actually happens as we will see later in this lecture all right here's that layered structure of the GI tract um I know I have it written from outside in over on the left but take a quick look from the inside that's where the epithelial layers are this is where the the absorption can start to happen um and then we've got different kinds of tissues going to the outside so we'll follow this format here the outer layer of this tube is referred to as the serosa this is just like remember around the the lungs we said there was this pleura and around the heart we said there was pericardium around the digestive tract we have serosa it's all the same stuff it's all epithelial tissue and it's there to secrete a little bit of fluid to have that um easily movement without a lot friction every time the digestive system moves which it does why it's called different things in different places is different scientists studied them and so they named them different things but essentially we're talking about exactly the same kind of tissue it's all epithelial tissue covering the surface and then there's a parietal layer that's on the peritone is on the body wall like on the inside of the abdominal muscles um and that's you know for the easy smooth movement so you don't have to scrape against the sides okay then we've got a couple of muscle layers uh quick and dirty um you know those Chinese finger puzzles at least that's what we used to call them when we were kids we'd put our fingers in it and it would get real short and squat you'd put your fingers in and then go to pull them out and that would get narrow and then your fingers were stuck in there that's kind of what these muscle layers are like so there's an internal one um that contracts this way and that makes the the Lumen skinny and stretches the thing out then the outer layer the muscles are arranged this way and so when that contracts this gets short and back and open and they take turns Contracting and that's how this thing changes shape and moves things around and they coordinate movements so they can get peristalsis to happen then we have the nerve plexuses I can't say that word um plural of plexus um the internal nervous system you know for communication here of the of the digestive system there's a submucosa with lots of glands again producing the mucus and producing the the fluids that will help materials Move Along and finally we get to the innermost layer that's the absorptive layer the epithelial tissue it's going to be able to like once things are digested um pull those molecules in fantastic all right GI tract innervation this is way more detail than we need but I wanted to point out a few things here and then I'll simplify it on the next slide so innervation there's the enteric nervous system um these are the the neurons that can communicate or create communication within the digestive system um that can independently control motility those muscle contractions that we were just talking about as well as secretions from some of the glands and they can act independent of the rest of the nervous system but as we mentioned there's parasympathetic and sympathetic innervation that can help speed things up or slow things down um it's parasympathetic like always along the vagus nerve this last little bit the sacral cord this is kind of the end of the digestive system so this this is the part that controls like is it time to go poop now and then sympathetic let me go ahead and move to the next slide because there's less stuff on it and it's easier to concentrate implied this is a terrible diagram and I say terrible just because everything's abbreviated so you don't know what anything stands for if you don't know what all the words are um but what I really want you to see is the autonomic nervous system has something to say to the GI tract and we don't have to worry about what all the abbreviations stand for what I want you to see is um this x that's the cranial nerve 10 the vagus nerve um telling that the GI tract to do stuff and then the sympathetic nervous system and this is backwards from what we've said before um the parasympathetic is the excitatory system here because we always say the parasympathetic is rest and digest so rest we think of is just relax but the digest part that's what this this organ system is about this is when it gets to work um and then the sympathetic nervous system fight or flight so you think of it as being very active but when you're busy fighting and fleeing you need your digestive system to chill you do not need it to be digesting at that time and you know that blood flow to go elsewhere and so this is why the the what's excitatory and what's inhibitory it seems to be backwards um so parasympathetic is excitatory sympathetic is inhibitory if you need a way to think about this um if you think about uh when you're uh you've got some time off you're not working and you say oh I'm too tired I want to cook let's go out to eat um and so you're going to take a break and you're going to go to a restaurant well there's a whole bunch of people at the restaurant that are not relaxing they are busy that's when they are working that's when they're doing their job and that's what the digestive system is about um so when it's parasympathetic it's time to quote rest and digest that is when the GI tract is the most excited it's when it's doing the most oops sorry it's doing the most activity um and sympathetic huh this is kind of rough but um if you've ever gotten really bad news you may have had this experience where everything was okay and then you got the very very very bad news and I don't mean a little bit stressful I mean like awful just awful news there are times when the sympathetic nervous system stimulation of the digestive system causes us to vomit I mean it it just like this is not the time to be digesting things and the the nervous system or the sympathetic nervous system basically evacuates the digestive systems we are not going to be busy doing that right now and you've not had that experience I'm very glad for you okay um a little more uh communication this is hormonal control I'm going to give you a few examples um so there's lots and lots of different hormones lots and lots of different signals that happen uh to coordinate the different parts of the digestive system or the GI tract so examples include over here I have this cell that makes the signal then in the next column I put this the name of the signal what cells have receptors for that signal in other words the target cell and then what effect this target cell expresses all right so in the stomach there are some cells called G cells they make something called gastrin uh the the targets for gastrin or other cells also in the stomach but in a different place these are parietal cells and we've mentioned these before because we talked about intrinsic factor do you remember way back when we were talking about um uh B12 and the effect uh the B12 has on making red blood cells okay these are those cells parietal cells they make the intrinsic factor they also make gastric acid hydrochloric acid and so when they receive the signal of gastrin that means okay food's coming I need some hydrochloric acid and I need some intrinsic factors so make it and pump it out now and the parietal cells respond then as things are moving along um let's see some things got mushed around in the stomach and then passed along into the small intestine there are some cells of the small intestine that detect that food has arrived especially uh despite they detect food uh including fatty food has arrived and they produce something called cholecystokinin um you know cholecysts are going to be communicating with the liver and the gallbladder um kind of an activation remember that kinetic so cck the target cells here the the so the cells that have receptors for this signal are in the gallbladder and in the pancreas what are they going to do well the gallbladder is going to start Contracting to push out some of the bile to push it into the small intestine and the pancreas is going to do a similar thing it's going to secrete some digestive enzymes these haven't been activated yet but they'll get activated once they get into the small intestine but wait there's more then as the food is the time this mixture that comes from the stomach as that moves along the small intestine it eventually arrives at the the last part of the small intestine the ilium and some of the cells in the ilium produce glp1 glucagon-like peptide um and this one now we're going to communicate backwards we're going to communicate back to the stomach and we're going to communicate forward to the pancreas see if you see if this makes sense so the glp-1 the stomach cells when they get that signal they're being told chill decrease motility you don't have to work so hard anymore the food's already out of there and already down here and then the pancreas cells um where before we were talking about the exocrine cells of the pancreas that make the the end digestive enzymes now we're going to talk about the Endocrine cells of the pancreas that will secrete insulin into the bloodstream for dealing with the carbohydrates the sugars that might start showing up in the blood all right so I think about it this way um the ilium is basically saying look these back back with the restaurant analogy all right these people are done with their meal so you can start cleaning up the dining room you can clear the table but we need the cashier because they're going to want to pay and they're going to want to get ready to go so backwards planning like clean up the stuff that was happening before and then what's going to be happening next we're going to have to prepare for that too so this is what I mean by there's coordination of all these different parts in the GI tract okay so some more signaling and and this actually has a significance for our patients every every day um the enteric nervous system we've mentioned this um it's communication here in the the GI tract it has uh opioid peptides and receptors for these opioid peptides and when they get stimulated that slows things down reduced motility well the consequence this has uh for that it's of nursing significance is constipation the slowing down of movement of the digestive system that's an enormously important side effect when we treat with opioid analgesics you know so when patients are receiving opioids everything slows down and it's harder for them to have a bowel movement because everything is slowing down um and then we can use this to our advantage um because if our patient has the problem they might not be in pain we can just have diarrhea we can give them opioids or opioid analogs we don't want to get them high but we want to take advantage of that slow everything down uh part of this like for example loperamide it's not going to cross the blood-brain barrier so they're not going to get high from it but it can tell the digestive system slow down we need to reabsorb some water before you just keep dumping things out of the anus very clever okay a little bit more about immune activity so the immune system involvement already starts in the mouth now think about this I've said this is on the still the outside of the body if you will uh you know even if it goes inside the mouth and in the esophagus and in the stomach we know that we're bringing in things from the outside and there's the possibility of getting some microorganisms or some toxins or something that's not supposed to be there and so we have some immunoglobulins waiting and you might remember remember Iggs and igms those were important in the bloodstream but iga's aqueous you said those were in the solutions and the secretions so AGA is largely what the the immunoglobulin that you find in in the GI tract it you starts as early as the saliva but it goes throughout the the GI tract all the way through to the end okay and then um there's more immune system involvement in the small and large intestines as we'll see in a moment so here's the small intestine immune components we have a how to describe it um there's the possibility of having some microorganisms in the small intestine there's not nearly as much in the small intestines in the large but you still have some microbes in here and so we have these areas where there's a lot of gut immune activity they're called pyre's patches you may have heard of them referred to as gut Associated lymphatic tissue Galt g-a-l-t gut Associated lymphatic tissues into Piers patches this is the the other name for them and continuing on in the colon in the large intestine in other words that's where the my the majority of our microorganisms are we refer to it as the microbiota it's just a fancy way of saying all the microbes that are commensal that that either don't harm us or are actually beneficial to us and contribute to digestion and we have an immune function and immune tolerance um we know we're not trying to kill these microbes all the time in other words but we do have lymphocytes part of the adaptive immunity and macrophages part of the innate immunity cruising around just in case because we know there's a lot of microbes in here and it's just to make sure things um make sure that the microbes continue to behave and that is the end of part one see you in part two