Hi how are you doing I'm going to give an introduction to the gastrointestinal tract the gut the GI tract the alimentary canal uh I've got a bit of a playlist of introductions to systems and I'll probably add introductions to regions of the body I guess because those are two ways you can approach Anatomy but that means we're going to work our way from knife to anus we're going to follow the tube all the way through mostly focus on the anatomy we're always going to talk a little bit about function because structure and function are related and we'll work our way along the tube and add on a bit of nervous system a bit of blood supply at the end all right it's not going to be in massive detail because this is an introduction so I'm going to leave things out but you'll find I have plenty of other videos of far too much detail if you'd like more [Music] so the gastrointestinal tract uh starts up here it starts in the embryo as a very simple tube it's one of the first shapes to form it's a tube that runs through us and in that respect the the inside of the tube is an external surface inside of us which is an important idea medically clinically anatomically um and it has a number of layers I mean it has a mucosa on the inside does various things but we can think of it as a muscular tube there's the esophagus there and two of the layers are muscle and some of the muscle is running longitudinally along the length of the muscle and some of the muscle is running in a circular fashion around the tube and together those use peristalsis to push the contents the stuff that you eat along that tube which is several meters in length right so peristalsis the circular muscle will squeeze the contents of the tube and then the longitudinal muscle can make it shorter so it can push the things along the contents of the tube and inside that external surface within us we find many many many many many many many many many bacteria and the bacteria are living in there and helping us with our digestive processes because that's the purpose of the gastrointestinal tract right is to eat food and drink and break that down to molecules and solubilize those molecules and then take those molecules across the mucosa so from that internal external surface enter across the cells and into the blood and then once it's in the blood it's in our body properly and then we can make use of it right food that is not digested is not absorbed continues through the GI tract and is excreted through the hole at the other end the anal aperture the anus the anal opening and also waste products produced by the body get put into the gastrointestinal tract so that those get removed as well um the the bile from the gallbladder is very green we have components in there that are being excreted and that's what actually gives feces their characteristic brown color I'm just taking some organs out so we can see the organs that we're interested in okay so the start of the gastrointestinal tract is the oral cavity in there hmm here is a um here's a half head right uh there's the oral cavity in there we find teeth we find a tongue we find these muscles of elastication that move the jaw and we find salivary glands there's a salivary gland and there are salivary glands under there so digestion starts when we put food in our mouths we start to break it down by chewing it saliva starts to enzymatically break it down so that has a whole bunch of other jobs as well the tongue moves things around the mouth to help with chewing but taste and smell are both important in getting that food in the first place in wanting to eat and wanting to eat it and knowing that it's good to eat and that sort of thing and then once the food has got has been broken down and we form a bolus the tongue and these structures back here push the bolus of food to the back of the oral cavity towards the pharynx and then swallowing occurs so the trick here is that we have to put food and water and air through the same passageway through the same tube which can be a little bit risky if some food gets stuck in the wrong place you cannot breathe you kind of need to breathe so then you need to get that food out but ideally food will then be passed into the pharynx and then down here that there is the esophagus the start of the esophagus this is the airway the larynx and the trachea this is held open because we're breathing all the time the esophagus is normally collapsed because we're not eating and drinking all the time and when food gets pushed down here the esophagus opens as that food gets pushed into it peristalsis begins and the food and drink moves down the esophagus the esophagus then runs through the neck it's posterior to the airway and it's very posterior it's up against the vertebral bodies back there so the esophagus is a tube connecting the oral cavity with the stomach here's the stomach here and it does not rely on gravity to get things to your stomach you can eat in zero gravity in space you can eat and drink upside down or lead down probably I don't recommend it but it still works because the muscular tube pushes fingers along and that's the structure for the whole tube those those layers of muscle doing that same sort of function this is the diaphragm so the esophagus then passes through the thorax because in the thorax we have organs of the circuitry system the heart moves blood around the body and organs of respiration where we bring oxygen in and push CO2 out we have the lungs on either side so it's kind of jumping that Gap whereas the abdomen the abdominal cavity in here is where we find most of the uh the the gastrointestinal tract most of the organs of digestion so the esophagus passes through the diaphragm it has a it has a special hole for it and then here's the liver take the liver so we can see a little bit more here's the stomach here now the stomach is a muscular bag um it's an expandable bag you can you can store a whole meal in here um and again it has those layers of muscle in fact it has layers of muscle running in different directions and it has a very strong sphincter here at this end the pyloric sphinct is like a it's a really thick muscle and that keeps that end of the stomach tightly closed so that in here the stomach can tune all the food around that you've chewed up more enzymes get added to the mix and we have some mechanical digestion and some enzymatic digestion going on in the stomach the famous thing about the stomach is that it produces acid now the the main purpose of that is that to break down proteins um the stomach secretes an inactivated a safe form of pepsin so a safe form of a proteinase because well the body's made of protein so if you weren't careful you could produce enzymes that would break down your own body from the inside it's going to be a bad thing to do so um this this this version of a proteinase a protein enzyme an enzyme that breaks down proteins to amino acids as a version is secreted with a safety on and then with the hydrochloric acid with the low PH that enzyme becomes activated and then it can get to work on the proteins that are within the stomach as in the ones you've eaten not the ones the stomach is made of and the stomach is also lined with mucus to help protect it from that um the stomach then squeezes allows a little bit of chyme just a tiny amount of its content into the duodenum just a little bit at a time it doesn't all go through at once and then from the stomach the contents the claim gets passed into the duodenum the duodenum is the first part of the small bowel um now it's hidden away here if I okay the spleen's gonna fall out isn't it if I take this out so there's the duodenum there that's where the stomach has passed its content into the duodenum when I turn this around you can see that it forms this C shape here so the duodenum is pretty well fixed to the posterior abdominal wall which is why I've had to turn this around and in that c shape we find the pancreas so there's the pancreas there so the duodenum is first part is adapted to cope with the stomach acid and it also receives secretions from the pancreas so enzymes that are going to further Aid digestion and it also receives bile that's the Green Tube there from the liver and the gallbladder the gallbladder is just kind of like a store it's liver that makes the bile and the bile contains waste products from the body which are going to get excreted eventually but it also emulsifies fats to Aid in the digestion of lipids right the duodenum curves around Rises up a little bit it's about 25 centimeters long and then from this point on it becomes the next part of the small intestine which is what we're seeing over here the jejunum now it's the jejunum where I think it's safe to say most absorption of nutrients occurs inside the jejunum we see massive modifications to maximize surface area we see the Pre-K circularis we see circular folds on the internal surface of the jejunum giving more surface area right and then we see villi finger light projections from the from the wall which actually gives it a very soft velvety touch if you ever get the chance to stroke the inside of the jejunum sorry too far I teach Anatomy um the Villi give them more surface area and then the cells that are on the surfaces of the Villi have their own projections of cell membrane which are microvilli so there's just surfs Anatomy sorry there's surface area plus surface area plus surface area it's already Wiggly right so this is the jejunum is about two and a half meters long if you were to dissect this out and stretch it out and it it gradually becomes the ilium which is also small intestines the small intestine has three parts duodenum jejunum and then ilium and the island is about three meters long and it's slightly different but it's still a site where lots of absorption of nutrients is happening but we're seeing absorption of the bile salts so a bit of recycling of materials here the absorption of vitamin B12 and things like that so as the stuff you're eating passes along the small intestine most of it is getting digested and absorbed in the first part to do duodenum jejunum and then ilium and then the ilium here this is the last part it passes into the large bowel uh this is the ileocecal valve which ensures that the contents of the small bowel pass into the large bowel and not the other way the cecum then is the start of the large bowel the start of the large colon the large intestine and it's a blind ending pouch and this is where we would find the appendix have you got an appendix I can't remember I bet you have right oh there it is a big obvious one there's the appendix there's the appendix is an eight pouching from the cecum it's an eight paging of bowel in there is um well it's essentially you've got the bacteria throughout the gut right so you've also got the bacteria of the gut in there if somebody has a gastrointestinal illness and the contents of the bowels get flushed out with diarrhea and vomiting the bacteria that Aid digestion gets flushed out as well so the good gets chucked out with the bad right recovering from that illness supposedly there is then a collection of those good bacteria still within the appendix they've survived survived that flushing out process so you can repopulate the bacteria of your gut quicker from those bacteria that stayed in the appendix right uh okay large bowel is then sensibly named so we have the cecum and then we have the ascending colon this is fixed in place so as the small bowel has got a mesentry which is the connective tissue the peritoneum covers and aligns all this one of my first ever videos was about the peritoneum I did it on a phone recorded it on a phone balanced up against some books go seek that one out if you want to know more it's quite complex but it means that the small bowel is attached to the posterior abdominal wall That's How blood vessels and nerves get to it but it can also move around which is important when you're pushing things along through the inside of it right so the ascending colon doesn't have that it's fixed in place over here we have the liver so where the angle changes uh we call this the hepatic flexure or the right colic flexure and then we have the transverse colon which actually has a mesentry here so it can move around it often hangs down here and then on this side we have the spleen so this angle change gets called the splenic flexure or the left colic flexure and then we have the descending colon which is also fixed in place and then we start to make a bit of a wiggle so that s shape this wiggle here is the sigmoid colon and the sigmoid codon then continues as the rectum and the rectum is the last part of the colon well the last part of the large bowel and it descends through the pelvis to the anal canal now the jobs of the large bowel are largely water resorption and salt resorption so you get a lot of water from your food you don't need to drink like two liters of water a day that's just a made-up number everybody's different and it depends how much anyway you get most of your water through your food or you certainly get a lot of water through your food and the large bowel is resorbing that water and it also absorbs a few nutrients special nutrients vitamins and that sort of thing so you can imagine then that if the water is being resolved to get taken away from the contents in here the contents are being packed down they're getting dried out and the large bowel is forming feces those feces then get stored in the rectum and the rectum has stretch receptors in its wall and when it starts to get stretched it lets you know that it would like to be emptied please the job of the anal Canal is there are four mechanisms in there to stop it emptying when it's inconvenient or inappropriate to do so so here we can see in the pelvis there's the rectum becoming the anal canal I've got a whole video on fecal continence it's honestly it's surprisingly interesting Anatomy if I take this apart um there's the rectum and there's the anal Canal here like textbooks this model is showing that this is open but of course it's closed almost all the time and what we see here is we see smooth muscle within the gut so that autonomic muscle that muscle you don't have any control over those layers of muscle of the uh of the intestinal wall propelling things along they make us sphincter so they help close the anal aperture then we have skeletal muscle because we're getting to the skin again here and this skeletal muscle you have control over so you have control over one sphincter that's the one you can hold on to but you don't have control over the other sphincter that's like autonomically kept closed and some other bits and Bobs and that is replicated up here in the esophagus so when the esophagus begins it is mostly made of skeletal muscle tool you're aware of swallowing you can do swallowing you can choose the swallow it's an autonomic series of events but you're aware of it right but then as the food passes down the esophagus the muscle down here is smooth muscle muscle you're not aware of not in control of it's automatically controlled so as we start near the skin we have skeletal muscle and then it becomes smooth muscle and then at this end we have smooth muscle and then we start to see skeletal muscle getting involved as we get close to the skin again how cool is that right so there's a lot going on here and I I nodded towards some of the organs associated with the gastrointestinal tract like the pancreas and like the liver and all of this has to be managed and controlled and has to work together right and it's often said that the gastrointestinal tract has its own brain the enteric nervous system there are about half a billion neurons associated with just this there are a lot of regulating feedback loops and if you disconnected from the brain it seems that it will just regulate and do its own thing so there are nerves controlling all these processes switching on glands and increasing blood flow and telling muscles to contract and glance to squeeze and that sort of thing but there are also hormones so there are hormone Loops um driving all of these processes switching them on and off when food is in here and when it's passed through there and that sort of thing and those hormones are associated with Sensations that we aren't aware of like hunger sometimes right but this is also connected to the brain and the spinal cord and these are organs so they're under autonomic control like I said you're not really aware of this happening most of the time you can't think about it or control it so we see the sympathetic and the parasympathetic divisions of the nervous system tying into this and we see visceral afferent fibers that is Sensory neurons carrying information back to the brain so this can be modulated at different levels and hormones from elsewhere um the parasympathetic division of the nervous system is usually associated with rest and digest functions driving the gastrointestinal tract and it's the vagus nerve that carries parasympathetic fibers down to this system sympathetic fibers come out of the spinal cord and also drive this so they have effects on smooth muscle on blood flow to different regions because you don't have blood going at full whack to every part of your body at one your body sends blood to the regions that need blood at the times they need blood right so sympathetic comparison pathetic Innovation help regulate the flow to the organs that need it yes that's kind of the The Innovation vagus sympathetic and then the sensory nerves find their way back through those same Roots I'm sure I've talked about them elsewhere the other interesting thing is the blood supply did you notice the whacking great big artery next to the GI tract so here's the heart that's the aorta coming out of the left ventricle that is then going to send blood around pretty much the entire body and the aorta it's right next to the esophagus so the aorta is sending branches to the GI tract straight away um and if we look inside the abdomen we see that the aorta has three major anterior branches here and those anterior branches well what's anterior of the aorta is the gastrointestinal tract in these Associated organs so we see the Celiac trunk is the first branch that appears just after the diaphragm and the Celiac trunk will supply blood to this stomach the esophagus half the duodenum and then the second branch is the superior mesenteric artery which will supply blood to the rest of the duodenum the whole of the small bowel ascending colon most of the transverse colon and then down here we have the inferior mesenteric artery which will supply blood to the rest of the transverse colon descending colon sigmoid colon upper part of the rectum and what we're seeing here is can you imagine how the embryo has a simple R3 running down here can you imagine how there was a simple tube running down here and that simple tube actually had a bit of a loop here but it had those three branches supplying that tube and as that tube got longer and longer and folded and moved around it took branches with it but if you hear people referring to forgot which you often will in clinical medicine related to the gastrointestinal tract these arteries Define the forger and the midgar and the hindgut those are embryological ideas but the arteries went with the structures that formed from those bits of tubes so if we're worried about the blood flow through the Celiac trunk we worry about the parts of the foregut so we think esophagus stomach um half the duodenum and so on all right so that is the anatomy of the gastrointestinal tract the alimentary canal the tube that runs through us which is actually an external surface inside us that we pass external substances through break them down to molecules solubilize them and absorb them into our blood and make use of them it follows a similar layer all the way through in terms of being layers of muscle and mucosa but each part has a slightly different job I mean it's about it was seven meters long it's a really long tube that's an overview it's always fun talking about a system as a whole if you would like more information more detail I've made far too many videos about the anatomy of this sort of thing if you're interested in that sort of thing use YouTube's search function use my name put in whatever topic you're interested in it'll show you if I've made a video about it I hope Google is quite good at search anyway I hope that was useful I really hope it was interesting see you next week