The Digestive System - Overview and Functions

so this is the digestive system the digestive system is a very large complicated system and the entire purpose of the digestive system is to take nutrients in and to break those nutrients down physically and chemically into something small enough that the body can actually absorb these nutrients and then these nutrients are going to be used for energy as building blocks for building various bodily components and so in this process we&#39;re going to be talking about the GI system the GI tract which is this very long tube which runs from the mouth to the anus through the body and we&#39;re also going to have to deal with some accessory organs and accessory glands which are associated with um putting fluids into this tube this two will absorb most of those fluids as we&#39;ll see uh going through and um so we have to talk about you know how the nutrients well how the food stuffs get in how we break down uh proteins carbohydrates and fats the three main nutrients in the body how those nutrients get absorbed because each one gets absorbed in a different fashion we&#39;re going to have to see how those uh factors once they get absorbed get distributed throughout the body that&#39;s not always um the same for all of these um components we&#39;re going to have to look at the function of all these different glands what they do where they are and you know look at their role in um the digestive process we&#39;ll have to look at how materials get moved through this tube and uh what happens to the material which isn&#39;t absorb right that&#39;s um you know has to be eliminated from the body and we&#39;ll have to talk about that also so there&#39;s a lot to cover in digestion and uh let&#39;s get into it so we&#39;re going to cover the anatomy first because there&#39;s a lot of it and we need to understand this Anatomy before we can understand the physiology of this material so we have the digestive tract and all these accessory organs uh accessory glands so the GI tract the gastrointestinal tube Elementary canal basically it runs from the mouth to the anus and it includes the oral cavity the ferx and the throat the esophagus the tube which runs through the thoracic cavity the take material which is swallowed and deliver it to the stomach and in the stomach will process materials and pass it on to the small intestine uh the small intestine will absorb the nutrients it will then uh also uh well the stomach and the intestine uh will do uh especially in the small int the majority of the digestion process and then material from the small intestine is passed down into the large intestine which includes the sigmoid colon the rectum and then finally the anus for the accessory glands we have things like um the tongue the uh and accessory organs I should say right we have the tongue the teeth salivary glands the liver which is you know a fairly large organ we have the gallbladder pancreas and we also have um any glands which secrete into the GI tra and as we&#39;ll see these glands can reside not only in the stomach in the lining mucosal lining of the stomach but also in the mucosal lining of the small intestine and the large intestine actually does some secretion also but mostly of mucus to um allow for uh lubrication of the materials as they pass down this tube so here we can see this GI system laid out for the digestive system we can see all these uh materials up here in the uh mouth area in the bual cavity and all the major organs are listed on the left hand side and on the right we have the accessory digestive organs so we&#39;ve got the oral cavity or the mouth then we&#39;ve got the ferin and we can see this long esophagus here now secreting into the mouth we&#39;ve got the salivary glands the first of these um accessory glands that we&#39;ll need to deal with now down in the abdominal cavity which is where most of the digestive organs are we&#39;ve got the stomach here which will then lead into this mass of tissue down here which is going to be the small intestine which then leads into the large intestine here and we&#39;ve got the ascending colon let&#39;s go with why I can&#39;t I yet there&#39;s green ascending colon transverse colon and then into the descending colon the sigmo colon is here leading down into the anus and then the rectum uh down here so in the way of other accessory glands and organs we need to talk about we&#39;ve got the liver we have the gallbladder which is just a storage organ for the bile it does not secrete bile it is not a gland it is a storage organ much like your urinary bladder Only Stores the urine it doesn&#39;t make the urine the urine&#39;s actually made in the kidneys as we&#39;ll find out uh and we&#39;ll have to deal with the pancreas an incredibly important gland in the body so the functions of the digestive system so we have to ingest the food basically and then we have to perform mastication mastication is just chewing and so ingestion is just putting the food into the mouth chewing then we chew it up and then we have to swallow that material so the chewing process mastication is a physical process we are breaking that material down into smaller chunks by physically grinding it or cutting it which is the uh function of the teeth another function that we&#39;re going to have to talk about is propulsion we need to know how the GI system moves this material through this tube we also use the same Bas basic mechanism to mix the material that&#39;s in there because we have all these secretions coming in all these um digestive enzymes and these need to be uh efficiently introduced and mixed with the material uh in the tube so that we can chemically break down proteins into amino acids we can break down complex carbohydrates into simple um monosaccharides and we need to break um triglycerides the main d dietary fat into simple fatty acids in order to get these materials absorbed so propulsion involves swallowing or detination which is basically moving the food which at this point is called a Bolis U basically from the oral cavity into the esophagus uh peristalsis refers to um a coordinated effort between two layers of muscle in most of the GI system to move material down to move this Bolis of material down um to the next section of the uh GI tract so we get um as we&#39;ll see we&#39;ll need relaxation ahead of this material of that muscle so the circular smooth muscle will actually relax ahead of it and um and then we get a wave of uh contraction of this smooth circular smooth muscle behind the Bolis to move that material downward and so where we need a fair number of neurons in the GI tract in order to coordinate this sort of balance between this Con relaxation of smooth muscle ahead of the Bolis and a wave of contraction behind it and these two waves will move can move this material um quite efficiently in quite a large distance down the GI track we will have to discuss mass movements which is basically the process of defecation the expulsion of non-digestible material uh from the GI track uh from the uh rectum through the anus now propulsion also includes mixing so again we use this U contraction of the smooth muscle in the GI tract to uh mix the uh ingested materials with uh the secretions of the GI track and all its Associated glands in order to chemically break down this material now we also have to deal with a number of secretions uh in this um organ system so um these secretions have multiple purposes we can we need lubrication um some of the material that we swallow is can be quite abrasive and um with some of the material at the very bottom of this GI track can also be quite abrasive and so along the entire system the entire tract we&#39;re going to need lubrication uh we have to liquefy a lot of this material and we have to digest this material and by digest I mean chemically break down so for lubrication we make mucus mucus is a polysaccharide material it&#39;s got a lot of water in it and it&#39;s very slippery and it&#39;s made along the entire length of the Elementary Canal it lubricates the passage of the food down uh the the uh pathway um and basically it protects um the GI tract from uh being getting well basically getting scraped up uh and also we need protection from stomach acids and the digestive enzymes which are being secreted to break down the food materials we don&#39;t want this material these enzymes and the acid eating holes in our tissue and you know poking holes into the GI tract we don&#39;t want to self uh digest the GI system so we need U this mucus to help with that we secrete liters of water into the GI tract in order to liquefy the material to help move this material Downstream and to make it easier to digest and the enzymes which is we secrete and we&#39;ll secrete a variety of enzymes to help break down the proteins the will break we&#39;ll make another whole set of enzymes to break down the carbohydrates and another whole set of enzymes to break down the fats it&#39;s not just a digestive enzyme we have to be very specific about this because enzymes are incredibly specific a proteas like Tron will only cut in very particular places in a protein sequence it won&#39;t cut just anywhere it has it&#39;s very picky very specific about where it will cut now we&#39;ve gotten well digestion right so digestion involves not only this chemical breakdown through the enzymes and the stomach acid but it&#39;s also mechanical we&#39;ve already spoken about mechanical breakdown which happens in the teeth but there&#39;s also mechanical breakdown in the stomach where we can actually pulverize the food just due to the large amount of um muscle in the walls of the stomach now once we have the material broken down we now have to absorb the nutrients we now have to get these nutrients from this the inside of this tube which is still technically outside the body it&#39;s um you know not technically technically anywhere where it it is of use to anything in the body yet not until the cells of the small intestine take this material into the cell and then pass it through the cell and then deliver it to the basil membr bra and put it out into the interstitial area uh below these cells where then the bloodstream and the lymphatics can now pick up these nutrients for distribution to the body and then we have elimination basically this is um removal of the feces from the body and the feces is just the undigested material that you&#39;ve uh consumed um along with some bacteria which have been in there we&#39;ve got billions of bacteria living in our guts um and they have have uh positive and potentially negative benefits uh to digestion as we&#39;ll see and then defecation is the process of uh removal of this material this FAL material from the [Music] body all right let&#39;s look in at peristalsis in a little bit of detail so what we&#39;re looking at here is we&#39;ve got this nice diagram of a u section of the G tract with a Bolis of food here and what has to happen is we have to have relaxation of the muscle in front of this Bolis because every this Bolis is traveling left to right and so in order for that to happen we need to relax the area of the GI track in front of this bolus and then we get a uh area of contraction behind it and then when that smooth muscle contracts that uh smooth muscle contraction behind the Bolis will move the Bolis forward into this area which due to the relaxation is has a slightly larger diameter to its Lumen now this can be a continuous process because we get this you know along a long section of the say small intestine we can get this wave of relaxation to pass down a long length of the GI tra a long length of the small intestine and then right behind it this wave of contraction right behind this Bolis so that this bolus of food moves down the entire length of this um piece of the intestine so we get peristalsis we can get that uh in the esophagus um right we can get material down from the esophagus into the stomach in about 10 seconds where it so it has to move through the entire um thoracic cavity um now peristalsis in the large intestine small intestines generally is only a fairly short distance but as we can see in the esophagus can you know be quite a long distance uh over a fairly um you know short time frame 10 seconds to move from well uh your throat down into your stomach now segmental contractions also involve contraction of this muscle in the um GI tract and this can happen in a couple different places in the GI tract so now we&#39;re dealing with K which is material which is either in the stomach or in the um large intestines right it&#39;s no longer considered a bolus basically due to its more fluidic nature at this point so what happens is we get a wave of contraction which will U imp well impact the uh bolus and uh just basically move this bolus back and forth within a very small area the Bolis gets diluted out uh the bolus spreads out in both directions and in this process we are mixing in the secretions from the G GI tract so those secretions will contain lots of water digestive enzymes bile um and uh materials like [Music] that so here we can see a slide where we&#39;ve got a listing of everything that happens in various places in the GI track U we&#39;ll come back to this slide a couple of times uh throughout the course of the um well our discussions of digestion so you know have to worry about what happens in oral cavity what happens in the ferx uh in the esophagus stomach small intestine and large intestine so this is you know within the elementary Canal the GI tract uh but you know in this we&#39;ll have to talk about where all these accessory glands um and other organs uh perform their [Music] function so let&#39;s look at the hystology of the GI tractor as we move down through this system so there are four layers in the GI system and it&#39;s very consistent throughout most of the GI system um there&#39;s a u mucosal layer sub mucosal layer a muscular external layer and then the Sosa or the yentia on the outside we&#39;ll be going through these uh individually coming up in the next couple of slides so the mucosa is on the is the innermost layer it&#39;s uh surrounding the Lumen the hole in the middle excuse me of this GI track uh it&#39;s uh can be a um well it&#39;s line with epithelial cells but the nature of the epithelial cells will change as we go along the system uh in the upper stretches of the GI system it&#39;s a non-keratinized stratified squamous epithelium basically the the same um sort of material which is found on the skin only this is non- Caren ised it doesn&#39;t have that hard keratin fiber uh protein fiber in it and the skin is a dry membrane while what we&#39;re dealing with in the oral cavity down through the esophagus basically are wet membranes these are referred to as mucosal membranes just due to the mucus which is there so uh again we&#39;re going to find this non-ar I stratified Squam epithelium in the mouth the oral fering uh the esophagus um and basically in the anal Canal the very end of the system where we&#39;re again going to be switching over uh very quickly uh to well the skin again now in the remainder of the G TR in the stomach the small intestine the large intestine uh we have a simple columnar epithelial cell so what this means is is that for a large portion of the GI tract there&#39;s only one cell between all these digestive enzymes and your tissues in the stomach there are digestive enzymes in there uh there&#39;s a proteas in there there&#39;s a lipase in there and it&#39;s at ph1 one to2 we don&#39;t want those materials eating away um and getting you know destroying the uh intestinal walls or the stomach walls right but there&#39;s only one cell there I mean it&#39;s a columnar cell it&#39;s a tall cell but it&#39;s still only one cell so these cells whether they&#39;re the ep the squamous epithelial cells or these colar cells they&#39;re sitting on a connective tissue a lamin appropria which is a fairly loose open connective tissue and we have a little bit of smooth muscle associated with the mucosa the muscularis mucosa and this is all smooth muscle and um the muscularis mucosa helps move the um and support the mucosal uh layer of the U GI tract now we have um some intestinal glands or pits so U this epithelial layer uh especially in the stomach and the intestines both large and small will have these invaginations in it then these are referred to as glands or CPS these are there to increase the surface area and we have a number of specialized cells that we can find in very vious areas of the GI tract some of these are mechanical receptors which are part of the U nervous system uh to help with the control of uh peristalsis we have chemo receptors in there to uh monitor the chemical digestion we even have some uh endocrine type cells which actually secrete hormones uh which help coordinate the digestive process we need both uh input from the nervous system and from the uh endocrine system to control this digestive process because it&#39;s a very complicated [Music] process all right the next layer of the four layers that we need to talk about is the sub mucosa so this is directly underneath and supports the mucosal layer so it&#39;s got a fairly thick layer of connective tissue with it we&#39;ve got nerves in here we have blood vessels we have lymphatic uh vessels and we also have a number of small glands in here uh we have a layer of neurons here the submucosal plexus also an older name for this is the Meisner plexus and U these are neurons and G cells of what&#39;s known as the inic nervous system inic referring to um digestive functions and basically these guys are uh responsible for controlling intestinal gland secretions uh we don&#39;t find this plexus in the stomach and the esophagus right but that means in the um intestines right large and small intestines we&#39;re going to have this and this is an incredibly important um control system for the uh GI track now the third of the three layers we need to wear about is referred to as the muscularis now in most places in the uh GI tra it&#39;s going to be two layers but there are some places where there are three layers um and there&#39;s always going to be a circular layer and a longitudinal layer of muscle this muscularis will contain the myeric plexus again another network of neurons uh in the GI track also known as Arbok plexus and this is resides between the these uh circular layer of uh muscle cells in a longitudinal layer and basically this uh network of nervous cells controls the contraction of these muscle cells now uh and within the mic plexus we&#39;re going to have these interstitial cells as they&#39;re referred to and these guys are little pacemakers and they promote rhythmic contractions which are a Hallmark of the motion of the uh GI system and uh we have this whole inic uh nervous system which is comprised of that submucosal and this myenteric plexus so uh in some ways and what&#39;s actually kind of surprising is that there are more neurons associated with the gut than there are with the spinal cord I mean you know spinal cord is part of the central nervous system but it&#39;s incredibly important but we have a lot of neurons in the GI tract now the outermost layer is can can be referred to as the Sosa or the adven tial layer it&#39;s basically connective tissue so we&#39;ve got some uh visceral perenium again this is directly on the GI organ this can also be called Sosa and um if it&#39;s derived from uh adjacent connective tissue it&#39;s referred to as an adentia type of um [Music] situation so here we can see a nice diagram of a representative section of the GI system so here we&#39;ve got the Lumen of the GI system so this is where cheeseburgers and milkshakes reside right as they&#39;re passing down we&#39;ve got this mucosal layer here so we&#39;ve got the in this mucosal layer we&#39;ve got the epithelium we&#39;ve got a lamin appropriate and we&#39;ve got a little bit of smooth muscle right so the smooth muscle is this brown material here and we&#39;re also going to see see these green little spheres there they are lymphatic nodules they are collections of lymphocytes which reside in the mucosal layer of the gut because remember this area in the Lumen right here that is really just you know this hollow part of a pipe which runs from the mouth to the anus and if you were to swallow say you know a very large plastic bead say you know a qu in diameter your GI system is incapable of you know digesting that it&#39;s just going to pass down through the system never really enter your body is just passing through this tube and eventually you know um when you know especially young children swallow this type of thing right Mom and Dad have to check their um fecal material to make sure that they&#39;ve passed this material so that&#39;s the mucosal layer now in the sub mucosa we can see some glands here so here&#39;s a gland here&#39;s a gland we also have a large number of blood vessels in this area uh for this particular gland we can see that you know it&#39;s uh making some sort of secretion and it has a duct which is sending that material into the Lumen of the GI tract um we can start to see this uh submucosal plexus right of nerves these yellow structures in this area here so that&#39;s the submucosal plexus um and um helps with the um coordination of the secretion from glands in this area now up next we&#39;ve got the muscularis and most for most of the GI tract we&#39;re going to have an inner circular layer of muscle and then the outer longitudinal layer of muscle so this is the interner circular this is the outer longitude now what I&#39;m referring to there is so these are smooth muscles uh for the most part uh the stomach small intestine large intestine have a large uh quantity of smooth muscle and it&#39;s all smooth muscle there in the esophagus it starts off as uh skeletal muscle in the upper third the middle third is going to be a mix of smooth muscle and skeletal muscle and the bottom third is going to be SK uh smooth muscle again uh but so this inner LED inner circular layer what happens there is the smooth muscle cells right nice long spindle shaped cells those that inter circular layer those run around the Lumen of the um GI tra in that area so those cells are basically running around the Lumen around the periphery uh the circumference of that um piece of tubing the outer longitudinal there the long axis is long the smooth muscle cell is aligned with the um long axis of the GI tra right so um those cells are uh basically pointing in the direction that the uh GI TR is pointing now between the inter circular and the outer longitudinal layer we&#39;ve got the myeric plexus that&#39;s a layer of nerve cells which helps control the contraction of those two layers to help control the coordination of peristalsis and the U mixing process uh that can happen with this smooth muscle now on the outside of all this then the fourth layer we&#39;ve got the Sosa which is a connective tissue layer and it&#39;s going to have a little bit of a simple squamous epithelial cell on the surface right so here&#39;s going to be you know a bunch of epithelial cells but underneath that&#39;s going to be this fairly heavy connective tissue now in some areas of the GI system we get a structure known as a mesentary and what happens there is we&#39;ve got the back of the abdominal cavity wall is back here the posterior side here and this mesentary will start out here it will be this layer of um connective tissue with those endothelial cells on it it&#39;ll uh come up come up on one side of this GI tube and then wrap around this material and then come up here and go back to the abdominal wall again so uh let&#39;s go so what will show up let&#39;s go Blue so what I&#39;m tracing right now is one of those layers of connective tissue and then there&#39;s another layer here and so in between these two layers we have a space and that within that space we can run nerves we can run lymphatics and we can run arteries and veins and this is the pathway that a lot of these support structures the vascular structures lymphatic structures the nerves themselves which uh you know Supply the uh plexuses within the gut right they have to get there and uh they&#39;re uh through these mesenteries which is this double layer of connective tissue and the mesenteries will also help anchor uh everything into the back abdominal wall so how do we regulate this entire system so the nervous regulation of this so we&#39;ve got locally we&#39;ve got the anic nervous system so we&#39;ve got uh Sensory neurons we have motor neurons and we have inter neurons in there right sounds like a pretty complete uh nervous system right so this coordinates uh peristalsis and will regulate local reflexes to help move material down uh stream um so so this nervous system is uh coordinates with the central nervous system uh this uh anic nervous nervous system might initiate some reflexes um due to uh information coming in through um sensory organs like you know the eyes or the nose or actually the ALF faction t uh and the sense of taste uh parasympathetic activity primarily with will uh occur in this area and parasympathetic activity increases the ability of these things to do their job uh parasympathetic activation uh of the GI tract will increase secretion it will increase the motility uh increase peristalsis things like that sympathetic input as we would expect would have the opposite effect it will inhibit contraction will cut down on the amount of secretion and the volume of secretions and it can actually decrease blood flow into the uh digestive tract chemical regulation for all this so we&#39;re going to have neurotransmitters involved well because where we&#39;ve got a major uh chunk of the nervous system down there um and so uh we&#39;ve got a lot of acetal choline from the uh parasympathetic and norepinephrine from the sympathetic which is going to inhibit activity serotonin is an incredibly important neurotransmitter for digestion it stimulates digestive um motility and 95% of all your serotonin is found in the digestive tract and uh there are a large number of hormones that we&#39;re going to have to discuss later on down the line including uh gastrin secret secreton cckk uh and histamine and a number of other compounds are considered uh paracrine in nature they act locally and uh these guys will help in local uh control of reflexes and um help with you know um the pH levels within various sections of the digestive system so the parium right we need to talk about that again so it&#39;s a double layer of continuous serious membrane and we can have visceral perenium which is again visceral always refers to that connective tissue covering the organs and we have parietal which is going to cover the interior surface of the body while in this case for the what we&#39;re for a large part of it it&#39;s aligning the abdominal cavity now we do have organs which are known to be in what&#39;s known as a retr peronal uh placement what this means is they don&#39;t have mesenteries Associated when they don&#39;t have this double layer of U membrane associated with with them they are covered by peritan but only on one side and uh basically they&#39;re not uh part of this peronal uh system so examples of this include the kidneys which is a urinary not a digestive organ uh the pancreas and the dadum the dadum is referring to the first of three sections of the small intestine the dadum is actually not in this area uh sections of the large intestine pop in and out of the um mentaries the sigmoid colon and the anus are retrop peronal um couple of disease conditions real quickly peritonitis is an inflammatory condition of the perenium this is you know an inflammation of the uh this connective tissue on the outside of the GI tract and uh aites is um an accumulation of fluid within the peronal cavity and um you know this can lead to fairly large um volume displacements actually leading to a fairly large increase in waste size in some disease conditions so a little bit more detail about the mesenteries so it&#39;s a double layer of mesothelium uh basically we&#39;ve got these two layers uh surrounding this entire organ it&#39;s a piece of loose connective tissue this big sheet of connective tissue and uh basically it is a can be considered an organ in of itself uh in some ways so we&#39;ve got this mesenteric root this is where the mesentary is attached into the back wall of the abdominal cavity and the space between these two layers right so we&#39;ve got this one layer coming off of the back wall we&#39;ve got this other layer of mesentary coming off the back wall and in there we can run we got an artery we&#39;ll make a vein in green we can have a let&#39;s make nerves orange and we&#39;ll go with a lymphatic vessel in purple right so all those run through this cavity with the back abdominal wall being here so in the adult we can break the mesenteries down into six different regions we&#39;ve just have the mesentary of the small intestine uh and basically this is for the junam the middle section of the um small intestine and the ilum the third and final section into the posterior abdominal wall we have a misoc colon on the right side which is going to anchor the right side of the colon or the large intestine we&#39;ve got a transverse mesocolon between the hepatic and the splin fix flexures a left misoon for the left side of the colon a miso sigmoid which is sort of uh mobile and fix to attach to the sigmoid colon and then the Miso rectum attaches the rectum uh into the system and uh we&#39;ve got a mesaric domain visceral which is basically the visceral perenium and the non- mesenteric which is the parietal so that&#39;s just different names for uh visceral and perianal essentially now we&#39;ve gotal organs that or allal segments and ligaments that are also uh supportive and structures we need to know about in discussing the anatomy here so we&#39;ve got a greater momentum so this is basically we&#39;re dealing with mesentary but it&#39;s this this double fold of mesentary so it&#39;s a piece of mesentary which comes off of the stomach the greater curvature of the stomach the bottom portion of the stomach and will drop down and then come back up and attach to the transverse colon and so what is coming off of the greater curvage of the stomach is a mesentary it&#39;s that double layer but then it will drop down and it will come back up U folding back over itself essentially and so we now have this double layer of mesentary um we&#39;ve got the Lesser Elum which is going to connect the uh the more concave curvature the Lesser curvature of the stomach to uh the proximal part of the dadum uh and to the liver and the diaphram we&#39;ve got the momental Bersa which is basically between the two layers of the mesentary the greater momentum and it&#39;s just a space that&#39;s there we have ligaments associated with the liver we have the coronary running between the liver and the diaphragm and the falia form between the liver and the anterior abdominal wall these um ligaments help support the liver and um help keep it in place so here we can see uh some diagrams of the abdominal cavity so here we can see the stomach the liver sort of been pulled up here and we can just see the Lesser momentum here connecting this lesser curvature of the stomach up to the uh the uh liver up here now this structure this all this yellow material that I&#39;m outlining here that is going to be the greater momentum and again it comes from it it basically starts off here at this greater curvature of the stomach will come down and then folds back up on itself and attaches to the transverse colon this section of the colon right here so on this right hand diagram here we can you can see this uh the greater momentum has been basically pulled up so we can see what&#39;s going on underneath it because this uh greater momentum often just overlies all of the small intestines and some of the large intestines down there so here we can see the ascending colon transverse colon and then the descending colon here we can see this uh meil colon here you know hanging off of the transverse colon and running downwards and we can just see the standard mesentary uh ping the small intestine this uh squiggle of lines there into the back abdominal wall and um at the bottom diagram right we can see uh some of the um right misoc colon and some more of this uh small intestine mesentary in a little bit more detail all right uh go going to the opening of the GI system the oral cavity so uh unsurprisingly this is a a fairly familiar piece of anatomy uh because it&#39;s fairly easily seen right we it&#39;s uh basically uh starts at the lips on the anterior uh aspect and runs to what&#39;s known as the uh Fales basically the opening into the fairings on the back side or posterior side so basically that&#39;s to here the vestibule is basically between the cheek and gum and uh you know this is where we keep all of our teeth the oral Capal the oral cavity excuse me is basically medial to the aviol processes those little spaces where we keep the teeth so basically this area here is going to be the oral cavity proper and we&#39;ve got this fairly moist non- caroteno which is continuous with the keratinized Squam uh uh stratified squamous epithelium of the skin right because your skin comes up comes up over the lips and the lips we sort of switch over from that keratinized to uh the non- Caren once we get uh into the oral cavity and uh and so on so in here we also have you know the tongue hanging down here with the lingual lingual is referring to the tongue the frenulum that little piece of connective tissue uh anchoring the tongue in place Place we&#39;ve got the uh uula uh cheeks Etc um yeah so we&#39;ve got gingiva which is uh you know part of the gums uh covering the uh roots of the teeth and and so on um the labial frenum is basically uh a frenum for the upper lip as opposed to the one on the bottom of the tongue so so a little bit about teeth so here we can see in cross-section a tooth um we&#39;ve got a couple of different layers to the tooth and then we&#39;ve got a hollow center to it so the crown is the portion of the tooth which we can actually uh you know see when we look in the oral cavity right that&#39;s covered with enamel and enamel is the hardest material that we make it&#39;s uh very calcified type of tissue um the points are referred to as cusps and uh the neck is basically the transition between the crown which is the visible portion and the root that portion of the tooth which is buried uh in the bone of uh the scull either the Maxa or the mandibles so the roots will anchor the tooth in place the an animal is U non-living tissue it&#39;s virtually a it&#39;s it&#39;s not virtually it is a cellular it&#39;s protect active in function but since it&#39;s acellular if there&#39;s any damage to it it can&#39;t repair itself so that&#39;s why we get oral cavities cavities are caused by bacterial uh basically uh growing and releasing acids in the and on the surface of the teeth and those acids eat into the the teeth causing cavities now underneath the enamel this white portion here is this brown material that&#39;s going to be the Dentin uh is living tissue there are cells in there and it is calcified um now further on down Dentin gets covered up by a bone-like structure which helps hold the tooth in the socket but the tooth is not composed of bone basically because we don&#39;t have osteocytes or osteoblasts and we don&#39;t have osteons um but it is calcified tissue uh now the pulp cavity is the center cavity and that&#39;s filled with blood vessels and nerves and those uh blood vessels and nerves enter this pulp cavity through the apic fan epical meaning bottom fan meaning hole and for this particular tooth which is going to be one of the larger ones there&#39;s actually two Roots so there&#39;ll be an apical framan associated with each of them some of the larger molers the bigger teeth in the back they might have up to four roots Maybe even five in some cases in each root will have a fan associated with it now we&#39;ve got uh perodontal ligaments these are connective tissues very tough connective tissues which anchor that tooth into the little avolar socket that it&#39;s sitting in and the gingiv is basically a very dense very fibrous connective tissue um around um the tooth and basically has a stratified squamous epitheliums around it and it is a again we&#39;re in the oral cavity so it&#39;s going to be a moist and wet non- caroen type of epithelial cell tissue moving on to salivary glands and so there are multiple cellary glands uh and basically there are three pairs of these things and these are multicellular glands these are all exocrine in nature so we have a duct that&#39;s going to take this material from made in the gland into the oral cavity so the pared glands these are the largest they m mostly make a Cirus type of secretions right so it&#39;s going to be very watery these guys are just in front of the ears and a little bit below it now the paric duct is the duct which brings this seirus material into the oral cavity crosses over the Mader muscle penetrates the buchin nator and enters the oral cavity next to the second upper Moler so it&#39;s going to come in so here&#39;s pared gland here we can see the paried duct is labeled and we see the little green guys just emptying into the top portion of the oral cavity submandibular uh glands are mixed by that they um make more of a cirrus than a mucousy type of secretion but there is mucus in there the posterior half uh is on the inferior border of the mandible and so um so Mand gland is going to be here is going to be basically this gland right sitting right down there so you know it&#39;s associated with the mandible and the duct in are all cing on either side of the lingual frenum so basically right in the front so we can see this green line running this way to enter to put that M uh saliva just in front of the tongue sublingual is the smallest of all the glands and these are again mixed but these guys primarily make mucusy type of secretions there are 10 to 12 ducks and these guys enter the oral floor basically on the sides basically underneath the tongue so these guys well the sub lingual gland is going to be this one and if you look you can see little tiny Ducks leading up into the oral cavity from these guys so there are other small uh coil tubular glands located throughout the oral cavity these guys are considered um salivary glands some of these are lingual they&#39;re deep to the tongue epithelium they&#39;re palentine in nature that which means they&#39;re in the pallet on the upper uh borders of the upper surfaces of the oral cavity bucal glands are in the cheeks and labial glands are basically in the lips now I should say that it&#39;s these smaller glands which make most of the saliva on the you know throughout the course of the day it&#39;s the main salivary glands which really kick in uh you know when we&#39;re eating because there&#39;s an increase in U saliva production when we start to chew when we put food in our mouth and um that can happen in anticipation of eating you know sometimes you walk into the house and it just someone&#39;s cooking something and it just smells so good that your mouth starts to water that&#39;s an in in reaction in anticipation of you actually consuming some of that stuff so we&#39;re getting them uh the major salivary glands the pared the sub lingal and sub maxillary to start secreting because we&#39;re going we are anticipating that we&#39;re going to need this saliva but what keeps your mouth moist you know in the middle of the night are going to be these uh accessory or these smaller uh glands on this slide so salivary glands are compound a Senor type of glands and they produce a solution known as saliva now saliva has multiple functions within the digestive system so it contains an enzyme the first uh of the digestive enzymes that we need to worry about is known as is salivary amalaye and so an amalay is going to be a a enzyme which breaks down carbohydrates and again uh enzymes are incredibly incredibly particular about what they break down amales will only break down carbohydrates they will do nothing to fats they will do nothing to proteins they will do nothing to DNA or RNA there&#39;s also a lingual liase in the saliva now a lipase will break down lipids so it will break down TR glycerides saliva also helps form the Bolis it helps um moisten the food which is can be a fairly major issue for some things right you know if you are trying to swallow Jello-O you&#39;re not going to need that much help from saliva to moisten that food it&#39;s pretty wet to begin with but you know um you know crackers or Brad rice can be fairly dry they&#39;re still quite tasty but we need a lot of saliva to moisten that material to help form the Bolis to help and as um salival will also help lubricate because remember some of the secretions from the salivary glands are mucousy in nature that will help lubricate uh this Bolis as it&#39;s passed down the esophagus in the swallowing uh reflexes so uh these glands are going to get parasympathetic innervation through the facial and glossop Fingal uh nerves and uh this parasympathetic activity will increase increase saliva production and um you know your higher brain centers just can respond to um the smell of food thinking about food can do this or even the site of food can do this so on the right hand side here we can see a diagram of what this looks like so we&#39;ve got these ASI these little areas which are going to be where the U material is actually produced and then we&#39;ve got the duct work to bring that material into the oral cavity so we&#39;ve got some mixed aseni we&#39;ve got some uh fully mucousy uh secreting aseni and we&#39;ve got some Cirrus aseni so Cirrus is are the blue cells in the mixed areas we see the blue and the white cells the white cells are you know um creating the mucus for us in the actual um histogram here we can see this very large duct with this very nice uh cuboidal almost columnar simple EP e thelial lining it but you know we can see nice ASI here and here each one of those circular structures is one of these ASI and th ASI are connected to a duct somewhere we&#39;re just not seeing those connections in this particular slice of tissue so the functions of saliva right so large volumes of saliva will help uh prevent bacterial infections in the mouth because we&#39;re continuously washing those bacteria into the stomach and the stomach is not a very hospitable uh environment for most bacteria we will secrete bicarbonate ions H3 in this saliva that will help neutralize any acids produced by the oral BAC bacteria this will help cut down on cavity formation which is always a good thing uh saliva also contains Lymes in the imunoglobulin a molecule lme it&#39;s an enzyme it has a fairly weak antibacterial action imunoglobulin a is one of the antibody type of of molecules and if those antibody molecules are directed against the bacteria well that will help prevent bacterial infections by basically interfering with the bacteria to do uh what they need to do to stay alive uh and the mucus in the cab also helps the GI tract from well self-destruction through self-digestion and basically uh because uh this mucus will help lubricate anything which is abrasive that we happen to be swallow um if you are not producing enough saliva right you can run the risk for ulceration and infection of the mucosa you&#39;ll get an increase in cavities in the teeth and just all kinds of [Music] problems so here&#39;s this uh diagram again so we&#39;ve just gone over basically the oral cavity right so enzymes produced here we can actually start digestion here uh chemical digestion we do physical digestion here by chewing uh basically grinding and cutting the food particles between our teeth um we produced saliva here we just went over all the major functions of saliva and the importance of saliva for the digestive process now we need to talk a little bit about swallowing so here we need to talk about the fings that uh basically that area behind the nasal passages behind the oral cavity and uh down towards the larynx and so the posterior walls of the Oro fings and the L lingo fings contain Fingal constrictor muscles which uh help with swallowing the epiglottis is this little flap of tiue tissue um which covers the larynx uh to keep food and drink from entering the esophagus right we don&#39;t want um those solids and those liquids entering the lungs that&#39;s always a bad thing and the epiglottis will make sure that everything that we swallow goes into the esophagus a muscular tube which runs from the ferx down to the stomach and it has to go through the diaphragm and that opening is known as the esophageal Hiatus and basically ends down at the stomach um a hiatal hernia is a widening of the Hiatus it just gets bigger than what it should be and there&#39;s going to be sphincters there&#39;s going to be an upper sphincter which is going to be striated by that we mean it&#39;s striated muscle it&#39;s going to it&#39;s skeletal type of muscle the lower is going to be smooth muscle and the mucosa here is moist squamous epithelium it&#39;s a stratified epithelium they&#39;re multiple layers of cells here because these cells aren&#39;t very thick they&#39;re squamous we um scrape these things off on a pretty regular basis in spite of the fact that you know the saliva is doing its job and um the esophagus itself will produce a fairly thick layer of mucus to help protect this moist um stratified squamous epithelial cell layer which aligns its [Music] mucosa so there a couple different phases of swallowing so there&#39;s voluntary which means if it&#39;s voluntary and it&#39;s muscular it has to be skeletal muscle and basically the uh bols of food is moved by the Tongue uh from the oral cavity basically into the fairings and the tongue there&#39;s a lot of uh skeletal muscle in the tongue it runs in a lot of different directions so that we can move the food around we can make sure that we don&#39;t bite the tongue although that does occasionally happen and um all this muscular activity in the tongue is also very important for Speed because we um by changing the size and the shape of the oral cavity we can change this the nature of the sound that we are able to produce the next phase of swalling is the fenal phase this is reflexive so this is not actively controlled by the uh conscious nervous system right it&#39;s it&#39;s all reflexive and we&#39;ve got swallowing centers in the medulla which uh coordinate all this for us so the soft pallet is the furthest uh portion of the pallet there&#39;s no bone uh unlying U supporting this uh that&#39;s going to elevate the upper esophageal sphincter will actually relax the ferx will open up into the esophagus the food goes down the esophagus by the uh propelled by the feral constrictors and we get uh contractions going from the superior to to the inferior along the fairings the iglus is going to be tipped down U posteriorly to the uh bis coming down the larynx that gets elevated to prevent the food from passing down into the larynx that should actually be the epicus um we&#39;ll do that and then um the last phase is again reflexes it&#39;s esophageal this is just peristal say through the um esophagus in that that&#39;s uh kicked off by stretching of the esophagus to um start peristalsis to move this material down and then this Bolis is delivered into the stomach

so this is the digestive system the digestive system is a very large complicated system and the entire purpose of the digestive system is to take nutrients in and to break those nutrients down physically and chemically into something small enough that the body can actually absorb these nutrients and then these nutrients are going to be used for energy as building blocks for building various bodily components and so in this process we&amp;#39;re going to be talking about the GI system the GI tract which is this very long tube which runs from the mouth to the anus through the body and we&amp;#39;re also going to have to deal with some accessory organs and accessory glands which are associated with um putting fluids into this tube this two will absorb most of those fluids as we&amp;#39;ll see uh going through and um so we have to talk about you know how the nutrients well how the food stuffs get in how we break down uh proteins carbohydrates and fats the three main nutrients in the body how those nutrients get absorbed because each one gets absorbed in a different fashion we&amp;#39;re going to have to see how those uh factors once they get absorbed get distributed throughout the body that&amp;#39;s not always um the same for all of these um components we&amp;#39;re going to have to look at the function of all these different glands what they do where they are and you know look at their role in um the digestive process we&amp;#39;ll have to look at how materials get moved through this tube and uh what happens to the material which isn&amp;#39;t absorb right that&amp;#39;s um you know has to be eliminated from the body and we&amp;#39;ll have to talk about that also so there&amp;#39;s a lot to cover in digestion and uh let&amp;#39;s get into it so we&amp;#39;re going to cover the anatomy first because there&amp;#39;s a lot of it and we need to understand this Anatomy before we can understand the physiology of this material so we have the digestive tract and all these accessory organs uh accessory glands so the GI tract the gastrointestinal tube Elementary canal basically it runs from the mouth to the anus and it includes the oral cavity the ferx and the throat the esophagus the tube which runs through the thoracic cavity the take material which is swallowed and deliver it to the stomach and in the stomach will process materials and pass it on to the small intestine uh the small intestine will absorb the nutrients it will then uh also uh well the stomach and the intestine uh will do uh especially in the small int the majority of the digestion process and then material from the small intestine is passed down into the large intestine which includes the sigmoid colon the rectum and then finally the anus for the accessory glands we have things like um the tongue the uh and accessory organs I should say right we have the tongue the teeth salivary glands the liver which is you know a fairly large organ we have the gallbladder pancreas and we also have um any glands which secrete into the GI tra and as we&amp;#39;ll see these glands can reside not only in the stomach in the lining mucosal lining of the stomach but also in the mucosal lining of the small intestine and the large intestine actually does some secretion also but mostly of mucus to um allow for uh lubrication of the materials as they pass down this tube so here we can see this GI system laid out for the digestive system we can see all these uh materials up here in the uh mouth area in the bual cavity and all the major organs are listed on the left hand side and on the right we have the accessory digestive organs so we&amp;#39;ve got the oral cavity or the mouth then we&amp;#39;ve got the ferin and we can see this long esophagus here now secreting into the mouth we&amp;#39;ve got the salivary glands the first of these um accessory glands that we&amp;#39;ll need to deal with now down in the abdominal cavity which is where most of the digestive organs are we&amp;#39;ve got the stomach here which will then lead into this mass of tissue down here which is going to be the small intestine which then leads into the large intestine here and we&amp;#39;ve got the ascending colon let&amp;#39;s go with why I can&amp;#39;t I yet there&amp;#39;s green ascending colon transverse colon and then into the descending colon the sigmo colon is here leading down into the anus and then the rectum uh down here so in the way of other accessory glands and organs we need to talk about we&amp;#39;ve got the liver we have the gallbladder which is just a storage organ for the bile it does not secrete bile it is not a gland it is a storage organ much like your urinary bladder Only Stores the urine it doesn&amp;#39;t make the urine the urine&amp;#39;s actually made in the kidneys as we&amp;#39;ll find out uh and we&amp;#39;ll have to deal with the pancreas an incredibly important gland in the body so the functions of the digestive system so we have to ingest the food basically and then we have to perform mastication mastication is just chewing and so ingestion is just putting the food into the mouth chewing then we chew it up and then we have to swallow that material so the chewing process mastication is a physical process we are breaking that material down into smaller chunks by physically grinding it or cutting it which is the uh function of the teeth another function that we&amp;#39;re going to have to talk about is propulsion we need to know how the GI system moves this material through this tube we also use the same Bas basic mechanism to mix the material that&amp;#39;s in there because we have all these secretions coming in all these um digestive enzymes and these need to be uh efficiently introduced and mixed with the material uh in the tube so that we can chemically break down proteins into amino acids we can break down complex carbohydrates into simple um monosaccharides and we need to break um triglycerides the main d dietary fat into simple fatty acids in order to get these materials absorbed so propulsion involves swallowing or detination which is basically moving the food which at this point is called a Bolis U basically from the oral cavity into the esophagus uh peristalsis refers to um a coordinated effort between two layers of muscle in most of the GI system to move material down to move this Bolis of material down um to the next section of the uh GI tract so we get um as we&amp;#39;ll see we&amp;#39;ll need relaxation ahead of this material of that muscle so the circular smooth muscle will actually relax ahead of it and um and then we get a wave of uh contraction of this smooth circular smooth muscle behind the Bolis to move that material downward and so where we need a fair number of neurons in the GI tract in order to coordinate this sort of balance between this Con relaxation of smooth muscle ahead of the Bolis and a wave of contraction behind it and these two waves will move can move this material um quite efficiently in quite a large distance down the GI track we will have to discuss mass movements which is basically the process of defecation the expulsion of non-digestible material uh from the GI track uh from the uh rectum through the anus now propulsion also includes mixing so again we use this U contraction of the smooth muscle in the GI tract to uh mix the uh ingested materials with uh the secretions of the GI track and all its Associated glands in order to chemically break down this material now we also have to deal with a number of secretions uh in this um organ system so um these secretions have multiple purposes we can we need lubrication um some of the material that we swallow is can be quite abrasive and um with some of the material at the very bottom of this GI track can also be quite abrasive and so along the entire system the entire tract we&amp;#39;re going to need lubrication uh we have to liquefy a lot of this material and we have to digest this material and by digest I mean chemically break down so for lubrication we make mucus mucus is a polysaccharide material it&amp;#39;s got a lot of water in it and it&amp;#39;s very slippery and it&amp;#39;s made along the entire length of the Elementary Canal it lubricates the passage of the food down uh the the uh pathway um and basically it protects um the GI tract from uh being getting well basically getting scraped up uh and also we need protection from stomach acids and the digestive enzymes which are being secreted to break down the food materials we don&amp;#39;t want this material these enzymes and the acid eating holes in our tissue and you know poking holes into the GI tract we don&amp;#39;t want to self uh digest the GI system so we need U this mucus to help with that we secrete liters of water into the GI tract in order to liquefy the material to help move this material Downstream and to make it easier to digest and the enzymes which is we secrete and we&amp;#39;ll secrete a variety of enzymes to help break down the proteins the will break we&amp;#39;ll make another whole set of enzymes to break down the carbohydrates and another whole set of enzymes to break down the fats it&amp;#39;s not just a digestive enzyme we have to be very specific about this because enzymes are incredibly specific a proteas like Tron will only cut in very particular places in a protein sequence it won&amp;#39;t cut just anywhere it has it&amp;#39;s very picky very specific about where it will cut now we&amp;#39;ve gotten well digestion right so digestion involves not only this chemical breakdown through the enzymes and the stomach acid but it&amp;#39;s also mechanical we&amp;#39;ve already spoken about mechanical breakdown which happens in the teeth but there&amp;#39;s also mechanical breakdown in the stomach where we can actually pulverize the food just due to the large amount of um muscle in the walls of the stomach now once we have the material broken down we now have to absorb the nutrients we now have to get these nutrients from this the inside of this tube which is still technically outside the body it&amp;#39;s um you know not technically technically anywhere where it it is of use to anything in the body yet not until the cells of the small intestine take this material into the cell and then pass it through the cell and then deliver it to the basil membr bra and put it out into the interstitial area uh below these cells where then the bloodstream and the lymphatics can now pick up these nutrients for distribution to the body and then we have elimination basically this is um removal of the feces from the body and the feces is just the undigested material that you&amp;#39;ve uh consumed um along with some bacteria which have been in there we&amp;#39;ve got billions of bacteria living in our guts um and they have have uh positive and potentially negative benefits uh to digestion as we&amp;#39;ll see and then defecation is the process of uh removal of this material this FAL material from the [Music] body all right let&amp;#39;s look in at peristalsis in a little bit of detail so what we&amp;#39;re looking at here is we&amp;#39;ve got this nice diagram of a u section of the G tract with a Bolis of food here and what has to happen is we have to have relaxation of the muscle in front of this Bolis because every this Bolis is traveling left to right and so in order for that to happen we need to relax the area of the GI track in front of this bolus and then we get a uh area of contraction behind it and then when that smooth muscle contracts that uh smooth muscle contraction behind the Bolis will move the Bolis forward into this area which due to the relaxation is has a slightly larger diameter to its Lumen now this can be a continuous process because we get this you know along a long section of the say small intestine we can get this wave of relaxation to pass down a long length of the GI tra a long length of the small intestine and then right behind it this wave of contraction right behind this Bolis so that this bolus of food moves down the entire length of this um piece of the intestine so we get peristalsis we can get that uh in the esophagus um right we can get material down from the esophagus into the stomach in about 10 seconds where it so it has to move through the entire um thoracic cavity um now peristalsis in the large intestine small intestines generally is only a fairly short distance but as we can see in the esophagus can you know be quite a long distance uh over a fairly um you know short time frame 10 seconds to move from well uh your throat down into your stomach now segmental contractions also involve contraction of this muscle in the um GI tract and this can happen in a couple different places in the GI tract so now we&amp;#39;re dealing with K which is material which is either in the stomach or in the um large intestines right it&amp;#39;s no longer considered a bolus basically due to its more fluidic nature at this point so what happens is we get a wave of contraction which will U imp well impact the uh bolus and uh just basically move this bolus back and forth within a very small area the Bolis gets diluted out uh the bolus spreads out in both directions and in this process we are mixing in the secretions from the G GI tract so those secretions will contain lots of water digestive enzymes bile um and uh materials like [Music] that so here we can see a slide where we&amp;#39;ve got a listing of everything that happens in various places in the GI track U we&amp;#39;ll come back to this slide a couple of times uh throughout the course of the um well our discussions of digestion so you know have to worry about what happens in oral cavity what happens in the ferx uh in the esophagus stomach small intestine and large intestine so this is you know within the elementary Canal the GI tract uh but you know in this we&amp;#39;ll have to talk about where all these accessory glands um and other organs uh perform their [Music] function so let&amp;#39;s look at the hystology of the GI tractor as we move down through this system so there are four layers in the GI system and it&amp;#39;s very consistent throughout most of the GI system um there&amp;#39;s a u mucosal layer sub mucosal layer a muscular external layer and then the Sosa or the yentia on the outside we&amp;#39;ll be going through these uh individually coming up in the next couple of slides so the mucosa is on the is the innermost layer it&amp;#39;s uh surrounding the Lumen the hole in the middle excuse me of this GI track uh it&amp;#39;s uh can be a um well it&amp;#39;s line with epithelial cells but the nature of the epithelial cells will change as we go along the system uh in the upper stretches of the GI system it&amp;#39;s a non-keratinized stratified squamous epithelium basically the the same um sort of material which is found on the skin only this is non- Caren ised it doesn&amp;#39;t have that hard keratin fiber uh protein fiber in it and the skin is a dry membrane while what we&amp;#39;re dealing with in the oral cavity down through the esophagus basically are wet membranes these are referred to as mucosal membranes just due to the mucus which is there so uh again we&amp;#39;re going to find this non-ar I stratified Squam epithelium in the mouth the oral fering uh the esophagus um and basically in the anal Canal the very end of the system where we&amp;#39;re again going to be switching over uh very quickly uh to well the skin again now in the remainder of the G TR in the stomach the small intestine the large intestine uh we have a simple columnar epithelial cell so what this means is is that for a large portion of the GI tract there&amp;#39;s only one cell between all these digestive enzymes and your tissues in the stomach there are digestive enzymes in there uh there&amp;#39;s a proteas in there there&amp;#39;s a lipase in there and it&amp;#39;s at ph1 one to2 we don&amp;#39;t want those materials eating away um and getting you know destroying the uh intestinal walls or the stomach walls right but there&amp;#39;s only one cell there I mean it&amp;#39;s a columnar cell it&amp;#39;s a tall cell but it&amp;#39;s still only one cell so these cells whether they&amp;#39;re the ep the squamous epithelial cells or these colar cells they&amp;#39;re sitting on a connective tissue a lamin appropria which is a fairly loose open connective tissue and we have a little bit of smooth muscle associated with the mucosa the muscularis mucosa and this is all smooth muscle and um the muscularis mucosa helps move the um and support the mucosal uh layer of the U GI tract now we have um some intestinal glands or pits so U this epithelial layer uh especially in the stomach and the intestines both large and small will have these invaginations in it then these are referred to as glands or CPS these are there to increase the surface area and we have a number of specialized cells that we can find in very vious areas of the GI tract some of these are mechanical receptors which are part of the U nervous system uh to help with the control of uh peristalsis we have chemo receptors in there to uh monitor the chemical digestion we even have some uh endocrine type cells which actually secrete hormones uh which help coordinate the digestive process we need both uh input from the nervous system and from the uh endocrine system to control this digestive process because it&amp;#39;s a very complicated [Music] process all right the next layer of the four layers that we need to talk about is the sub mucosa so this is directly underneath and supports the mucosal layer so it&amp;#39;s got a fairly thick layer of connective tissue with it we&amp;#39;ve got nerves in here we have blood vessels we have lymphatic uh vessels and we also have a number of small glands in here uh we have a layer of neurons here the submucosal plexus also an older name for this is the Meisner plexus and U these are neurons and G cells of what&amp;#39;s known as the inic nervous system inic referring to um digestive functions and basically these guys are uh responsible for controlling intestinal gland secretions uh we don&amp;#39;t find this plexus in the stomach and the esophagus right but that means in the um intestines right large and small intestines we&amp;#39;re going to have this and this is an incredibly important um control system for the uh GI track now the third of the three layers we need to wear about is referred to as the muscularis now in most places in the uh GI tra it&amp;#39;s going to be two layers but there are some places where there are three layers um and there&amp;#39;s always going to be a circular layer and a longitudinal layer of muscle this muscularis will contain the myeric plexus again another network of neurons uh in the GI track also known as Arbok plexus and this is resides between the these uh circular layer of uh muscle cells in a longitudinal layer and basically this uh network of nervous cells controls the contraction of these muscle cells now uh and within the mic plexus we&amp;#39;re going to have these interstitial cells as they&amp;#39;re referred to and these guys are little pacemakers and they promote rhythmic contractions which are a Hallmark of the motion of the uh GI system and uh we have this whole inic uh nervous system which is comprised of that submucosal and this myenteric plexus so uh in some ways and what&amp;#39;s actually kind of surprising is that there are more neurons associated with the gut than there are with the spinal cord I mean you know spinal cord is part of the central nervous system but it&amp;#39;s incredibly important but we have a lot of neurons in the GI tract now the outermost layer is can can be referred to as the Sosa or the adven tial layer it&amp;#39;s basically connective tissue so we&amp;#39;ve got some uh visceral perenium again this is directly on the GI organ this can also be called Sosa and um if it&amp;#39;s derived from uh adjacent connective tissue it&amp;#39;s referred to as an adentia type of um [Music] situation so here we can see a nice diagram of a representative section of the GI system so here we&amp;#39;ve got the Lumen of the GI system so this is where cheeseburgers and milkshakes reside right as they&amp;#39;re passing down we&amp;#39;ve got this mucosal layer here so we&amp;#39;ve got the in this mucosal layer we&amp;#39;ve got the epithelium we&amp;#39;ve got a lamin appropriate and we&amp;#39;ve got a little bit of smooth muscle right so the smooth muscle is this brown material here and we&amp;#39;re also going to see see these green little spheres there they are lymphatic nodules they are collections of lymphocytes which reside in the mucosal layer of the gut because remember this area in the Lumen right here that is really just you know this hollow part of a pipe which runs from the mouth to the anus and if you were to swallow say you know a very large plastic bead say you know a qu in diameter your GI system is incapable of you know digesting that it&amp;#39;s just going to pass down through the system never really enter your body is just passing through this tube and eventually you know um when you know especially young children swallow this type of thing right Mom and Dad have to check their um fecal material to make sure that they&amp;#39;ve passed this material so that&amp;#39;s the mucosal layer now in the sub mucosa we can see some glands here so here&amp;#39;s a gland here&amp;#39;s a gland we also have a large number of blood vessels in this area uh for this particular gland we can see that you know it&amp;#39;s uh making some sort of secretion and it has a duct which is sending that material into the Lumen of the GI tract um we can start to see this uh submucosal plexus right of nerves these yellow structures in this area here so that&amp;#39;s the submucosal plexus um and um helps with the um coordination of the secretion from glands in this area now up next we&amp;#39;ve got the muscularis and most for most of the GI tract we&amp;#39;re going to have an inner circular layer of muscle and then the outer longitudinal layer of muscle so this is the interner circular this is the outer longitude now what I&amp;#39;m referring to there is so these are smooth muscles uh for the most part uh the stomach small intestine large intestine have a large uh quantity of smooth muscle and it&amp;#39;s all smooth muscle there in the esophagus it starts off as uh skeletal muscle in the upper third the middle third is going to be a mix of smooth muscle and skeletal muscle and the bottom third is going to be SK uh smooth muscle again uh but so this inner LED inner circular layer what happens there is the smooth muscle cells right nice long spindle shaped cells those that inter circular layer those run around the Lumen of the um GI tra in that area so those cells are basically running around the Lumen around the periphery uh the circumference of that um piece of tubing the outer longitudinal there the long axis is long the smooth muscle cell is aligned with the um long axis of the GI tra right so um those cells are uh basically pointing in the direction that the uh GI TR is pointing now between the inter circular and the outer longitudinal layer we&amp;#39;ve got the myeric plexus that&amp;#39;s a layer of nerve cells which helps control the contraction of those two layers to help control the coordination of peristalsis and the U mixing process uh that can happen with this smooth muscle now on the outside of all this then the fourth layer we&amp;#39;ve got the Sosa which is a connective tissue layer and it&amp;#39;s going to have a little bit of a simple squamous epithelial cell on the surface right so here&amp;#39;s going to be you know a bunch of epithelial cells but underneath that&amp;#39;s going to be this fairly heavy connective tissue now in some areas of the GI system we get a structure known as a mesentary and what happens there is we&amp;#39;ve got the back of the abdominal cavity wall is back here the posterior side here and this mesentary will start out here it will be this layer of um connective tissue with those endothelial cells on it it&amp;#39;ll uh come up come up on one side of this GI tube and then wrap around this material and then come up here and go back to the abdominal wall again so uh let&amp;#39;s go so what will show up let&amp;#39;s go Blue so what I&amp;#39;m tracing right now is one of those layers of connective tissue and then there&amp;#39;s another layer here and so in between these two layers we have a space and that within that space we can run nerves we can run lymphatics and we can run arteries and veins and this is the pathway that a lot of these support structures the vascular structures lymphatic structures the nerves themselves which uh you know Supply the uh plexuses within the gut right they have to get there and uh they&amp;#39;re uh through these mesenteries which is this double layer of connective tissue and the mesenteries will also help anchor uh everything into the back abdominal wall so how do we regulate this entire system so the nervous regulation of this so we&amp;#39;ve got locally we&amp;#39;ve got the anic nervous system so we&amp;#39;ve got uh Sensory neurons we have motor neurons and we have inter neurons in there right sounds like a pretty complete uh nervous system right so this coordinates uh peristalsis and will regulate local reflexes to help move material down uh stream um so so this nervous system is uh coordinates with the central nervous system uh this uh anic nervous nervous system might initiate some reflexes um due to uh information coming in through um sensory organs like you know the eyes or the nose or actually the ALF faction t uh and the sense of taste uh parasympathetic activity primarily with will uh occur in this area and parasympathetic activity increases the ability of these things to do their job uh parasympathetic activation uh of the GI tract will increase secretion it will increase the motility uh increase peristalsis things like that sympathetic input as we would expect would have the opposite effect it will inhibit contraction will cut down on the amount of secretion and the volume of secretions and it can actually decrease blood flow into the uh digestive tract chemical regulation for all this so we&amp;#39;re going to have neurotransmitters involved well because where we&amp;#39;ve got a major uh chunk of the nervous system down there um and so uh we&amp;#39;ve got a lot of acetal choline from the uh parasympathetic and norepinephrine from the sympathetic which is going to inhibit activity serotonin is an incredibly important neurotransmitter for digestion it stimulates digestive um motility and 95% of all your serotonin is found in the digestive tract and uh there are a large number of hormones that we&amp;#39;re going to have to discuss later on down the line including uh gastrin secret secreton cckk uh and histamine and a number of other compounds are considered uh paracrine in nature they act locally and uh these guys will help in local uh control of reflexes and um help with you know um the pH levels within various sections of the digestive system so the parium right we need to talk about that again so it&amp;#39;s a double layer of continuous serious membrane and we can have visceral perenium which is again visceral always refers to that connective tissue covering the organs and we have parietal which is going to cover the interior surface of the body while in this case for the what we&amp;#39;re for a large part of it it&amp;#39;s aligning the abdominal cavity now we do have organs which are known to be in what&amp;#39;s known as a retr peronal uh placement what this means is they don&amp;#39;t have mesenteries Associated when they don&amp;#39;t have this double layer of U membrane associated with with them they are covered by peritan but only on one side and uh basically they&amp;#39;re not uh part of this peronal uh system so examples of this include the kidneys which is a urinary not a digestive organ uh the pancreas and the dadum the dadum is referring to the first of three sections of the small intestine the dadum is actually not in this area uh sections of the large intestine pop in and out of the um mentaries the sigmoid colon and the anus are retrop peronal um couple of disease conditions real quickly peritonitis is an inflammatory condition of the perenium this is you know an inflammation of the uh this connective tissue on the outside of the GI tract and uh aites is um an accumulation of fluid within the peronal cavity and um you know this can lead to fairly large um volume displacements actually leading to a fairly large increase in waste size in some disease conditions so a little bit more detail about the mesenteries so it&amp;#39;s a double layer of mesothelium uh basically we&amp;#39;ve got these two layers uh surrounding this entire organ it&amp;#39;s a piece of loose connective tissue this big sheet of connective tissue and uh basically it is a can be considered an organ in of itself uh in some ways so we&amp;#39;ve got this mesenteric root this is where the mesentary is attached into the back wall of the abdominal cavity and the space between these two layers right so we&amp;#39;ve got this one layer coming off of the back wall we&amp;#39;ve got this other layer of mesentary coming off the back wall and in there we can run we got an artery we&amp;#39;ll make a vein in green we can have a let&amp;#39;s make nerves orange and we&amp;#39;ll go with a lymphatic vessel in purple right so all those run through this cavity with the back abdominal wall being here so in the adult we can break the mesenteries down into six different regions we&amp;#39;ve just have the mesentary of the small intestine uh and basically this is for the junam the middle section of the um small intestine and the ilum the third and final section into the posterior abdominal wall we have a misoc colon on the right side which is going to anchor the right side of the colon or the large intestine we&amp;#39;ve got a transverse mesocolon between the hepatic and the splin fix flexures a left misoon for the left side of the colon a miso sigmoid which is sort of uh mobile and fix to attach to the sigmoid colon and then the Miso rectum attaches the rectum uh into the system and uh we&amp;#39;ve got a mesaric domain visceral which is basically the visceral perenium and the non- mesenteric which is the parietal so that&amp;#39;s just different names for uh visceral and perianal essentially now we&amp;#39;ve gotal organs that or allal segments and ligaments that are also uh supportive and structures we need to know about in discussing the anatomy here so we&amp;#39;ve got a greater momentum so this is basically we&amp;#39;re dealing with mesentary but it&amp;#39;s this this double fold of mesentary so it&amp;#39;s a piece of mesentary which comes off of the stomach the greater curvature of the stomach the bottom portion of the stomach and will drop down and then come back up and attach to the transverse colon and so what is coming off of the greater curvage of the stomach is a mesentary it&amp;#39;s that double layer but then it will drop down and it will come back up U folding back over itself essentially and so we now have this double layer of mesentary um we&amp;#39;ve got the Lesser Elum which is going to connect the uh the more concave curvature the Lesser curvature of the stomach to uh the proximal part of the dadum uh and to the liver and the diaphram we&amp;#39;ve got the momental Bersa which is basically between the two layers of the mesentary the greater momentum and it&amp;#39;s just a space that&amp;#39;s there we have ligaments associated with the liver we have the coronary running between the liver and the diaphragm and the falia form between the liver and the anterior abdominal wall these um ligaments help support the liver and um help keep it in place so here we can see uh some diagrams of the abdominal cavity so here we can see the stomach the liver sort of been pulled up here and we can just see the Lesser momentum here connecting this lesser curvature of the stomach up to the uh the uh liver up here now this structure this all this yellow material that I&amp;#39;m outlining here that is going to be the greater momentum and again it comes from it it basically starts off here at this greater curvature of the stomach will come down and then folds back up on itself and attaches to the transverse colon this section of the colon right here so on this right hand diagram here we can you can see this uh the greater momentum has been basically pulled up so we can see what&amp;#39;s going on underneath it because this uh greater momentum often just overlies all of the small intestines and some of the large intestines down there so here we can see the ascending colon transverse colon and then the descending colon here we can see this uh meil colon here you know hanging off of the transverse colon and running downwards and we can just see the standard mesentary uh ping the small intestine this uh squiggle of lines there into the back abdominal wall and um at the bottom diagram right we can see uh some of the um right misoc colon and some more of this uh small intestine mesentary in a little bit more detail all right uh go going to the opening of the GI system the oral cavity so uh unsurprisingly this is a a fairly familiar piece of anatomy uh because it&amp;#39;s fairly easily seen right we it&amp;#39;s uh basically uh starts at the lips on the anterior uh aspect and runs to what&amp;#39;s known as the uh Fales basically the opening into the fairings on the back side or posterior side so basically that&amp;#39;s to here the vestibule is basically between the cheek and gum and uh you know this is where we keep all of our teeth the oral Capal the oral cavity excuse me is basically medial to the aviol processes those little spaces where we keep the teeth so basically this area here is going to be the oral cavity proper and we&amp;#39;ve got this fairly moist non- caroteno which is continuous with the keratinized Squam uh uh stratified squamous epithelium of the skin right because your skin comes up comes up over the lips and the lips we sort of switch over from that keratinized to uh the non- Caren once we get uh into the oral cavity and uh and so on so in here we also have you know the tongue hanging down here with the lingual lingual is referring to the tongue the frenulum that little piece of connective tissue uh anchoring the tongue in place Place we&amp;#39;ve got the uh uula uh cheeks Etc um yeah so we&amp;#39;ve got gingiva which is uh you know part of the gums uh covering the uh roots of the teeth and and so on um the labial frenum is basically uh a frenum for the upper lip as opposed to the one on the bottom of the tongue so so a little bit about teeth so here we can see in cross-section a tooth um we&amp;#39;ve got a couple of different layers to the tooth and then we&amp;#39;ve got a hollow center to it so the crown is the portion of the tooth which we can actually uh you know see when we look in the oral cavity right that&amp;#39;s covered with enamel and enamel is the hardest material that we make it&amp;#39;s uh very calcified type of tissue um the points are referred to as cusps and uh the neck is basically the transition between the crown which is the visible portion and the root that portion of the tooth which is buried uh in the bone of uh the scull either the Maxa or the mandibles so the roots will anchor the tooth in place the an animal is U non-living tissue it&amp;#39;s virtually a it&amp;#39;s it&amp;#39;s not virtually it is a cellular it&amp;#39;s protect active in function but since it&amp;#39;s acellular if there&amp;#39;s any damage to it it can&amp;#39;t repair itself so that&amp;#39;s why we get oral cavities cavities are caused by bacterial uh basically uh growing and releasing acids in the and on the surface of the teeth and those acids eat into the the teeth causing cavities now underneath the enamel this white portion here is this brown material that&amp;#39;s going to be the Dentin uh is living tissue there are cells in there and it is calcified um now further on down Dentin gets covered up by a bone-like structure which helps hold the tooth in the socket but the tooth is not composed of bone basically because we don&amp;#39;t have osteocytes or osteoblasts and we don&amp;#39;t have osteons um but it is calcified tissue uh now the pulp cavity is the center cavity and that&amp;#39;s filled with blood vessels and nerves and those uh blood vessels and nerves enter this pulp cavity through the apic fan epical meaning bottom fan meaning hole and for this particular tooth which is going to be one of the larger ones there&amp;#39;s actually two Roots so there&amp;#39;ll be an apical framan associated with each of them some of the larger molers the bigger teeth in the back they might have up to four roots Maybe even five in some cases in each root will have a fan associated with it now we&amp;#39;ve got uh perodontal ligaments these are connective tissues very tough connective tissues which anchor that tooth into the little avolar socket that it&amp;#39;s sitting in and the gingiv is basically a very dense very fibrous connective tissue um around um the tooth and basically has a stratified squamous epitheliums around it and it is a again we&amp;#39;re in the oral cavity so it&amp;#39;s going to be a moist and wet non- caroen type of epithelial cell tissue moving on to salivary glands and so there are multiple cellary glands uh and basically there are three pairs of these things and these are multicellular glands these are all exocrine in nature so we have a duct that&amp;#39;s going to take this material from made in the gland into the oral cavity so the pared glands these are the largest they m mostly make a Cirus type of secretions right so it&amp;#39;s going to be very watery these guys are just in front of the ears and a little bit below it now the paric duct is the duct which brings this seirus material into the oral cavity crosses over the Mader muscle penetrates the buchin nator and enters the oral cavity next to the second upper Moler so it&amp;#39;s going to come in so here&amp;#39;s pared gland here we can see the paried duct is labeled and we see the little green guys just emptying into the top portion of the oral cavity submandibular uh glands are mixed by that they um make more of a cirrus than a mucousy type of secretion but there is mucus in there the posterior half uh is on the inferior border of the mandible and so um so Mand gland is going to be here is going to be basically this gland right sitting right down there so you know it&amp;#39;s associated with the mandible and the duct in are all cing on either side of the lingual frenum so basically right in the front so we can see this green line running this way to enter to put that M uh saliva just in front of the tongue sublingual is the smallest of all the glands and these are again mixed but these guys primarily make mucusy type of secretions there are 10 to 12 ducks and these guys enter the oral floor basically on the sides basically underneath the tongue so these guys well the sub lingual gland is going to be this one and if you look you can see little tiny Ducks leading up into the oral cavity from these guys so there are other small uh coil tubular glands located throughout the oral cavity these guys are considered um salivary glands some of these are lingual they&amp;#39;re deep to the tongue epithelium they&amp;#39;re palentine in nature that which means they&amp;#39;re in the pallet on the upper uh borders of the upper surfaces of the oral cavity bucal glands are in the cheeks and labial glands are basically in the lips now I should say that it&amp;#39;s these smaller glands which make most of the saliva on the you know throughout the course of the day it&amp;#39;s the main salivary glands which really kick in uh you know when we&amp;#39;re eating because there&amp;#39;s an increase in U saliva production when we start to chew when we put food in our mouth and um that can happen in anticipation of eating you know sometimes you walk into the house and it just someone&amp;#39;s cooking something and it just smells so good that your mouth starts to water that&amp;#39;s an in in reaction in anticipation of you actually consuming some of that stuff so we&amp;#39;re getting them uh the major salivary glands the pared the sub lingal and sub maxillary to start secreting because we&amp;#39;re going we are anticipating that we&amp;#39;re going to need this saliva but what keeps your mouth moist you know in the middle of the night are going to be these uh accessory or these smaller uh glands on this slide so salivary glands are compound a Senor type of glands and they produce a solution known as saliva now saliva has multiple functions within the digestive system so it contains an enzyme the first uh of the digestive enzymes that we need to worry about is known as is salivary amalaye and so an amalay is going to be a a enzyme which breaks down carbohydrates and again uh enzymes are incredibly incredibly particular about what they break down amales will only break down carbohydrates they will do nothing to fats they will do nothing to proteins they will do nothing to DNA or RNA there&amp;#39;s also a lingual liase in the saliva now a lipase will break down lipids so it will break down TR glycerides saliva also helps form the Bolis it helps um moisten the food which is can be a fairly major issue for some things right you know if you are trying to swallow Jello-O you&amp;#39;re not going to need that much help from saliva to moisten that food it&amp;#39;s pretty wet to begin with but you know um you know crackers or Brad rice can be fairly dry they&amp;#39;re still quite tasty but we need a lot of saliva to moisten that material to help form the Bolis to help and as um salival will also help lubricate because remember some of the secretions from the salivary glands are mucousy in nature that will help lubricate uh this Bolis as it&amp;#39;s passed down the esophagus in the swallowing uh reflexes so uh these glands are going to get parasympathetic innervation through the facial and glossop Fingal uh nerves and uh this parasympathetic activity will increase increase saliva production and um you know your higher brain centers just can respond to um the smell of food thinking about food can do this or even the site of food can do this so on the right hand side here we can see a diagram of what this looks like so we&amp;#39;ve got these ASI these little areas which are going to be where the U material is actually produced and then we&amp;#39;ve got the duct work to bring that material into the oral cavity so we&amp;#39;ve got some mixed aseni we&amp;#39;ve got some uh fully mucousy uh secreting aseni and we&amp;#39;ve got some Cirrus aseni so Cirrus is are the blue cells in the mixed areas we see the blue and the white cells the white cells are you know um creating the mucus for us in the actual um histogram here we can see this very large duct with this very nice uh cuboidal almost columnar simple EP e thelial lining it but you know we can see nice ASI here and here each one of those circular structures is one of these ASI and th ASI are connected to a duct somewhere we&amp;#39;re just not seeing those connections in this particular slice of tissue so the functions of saliva right so large volumes of saliva will help uh prevent bacterial infections in the mouth because we&amp;#39;re continuously washing those bacteria into the stomach and the stomach is not a very hospitable uh environment for most bacteria we will secrete bicarbonate ions H3 in this saliva that will help neutralize any acids produced by the oral BAC bacteria this will help cut down on cavity formation which is always a good thing uh saliva also contains Lymes in the imunoglobulin a molecule lme it&amp;#39;s an enzyme it has a fairly weak antibacterial action imunoglobulin a is one of the antibody type of of molecules and if those antibody molecules are directed against the bacteria well that will help prevent bacterial infections by basically interfering with the bacteria to do uh what they need to do to stay alive uh and the mucus in the cab also helps the GI tract from well self-destruction through self-digestion and basically uh because uh this mucus will help lubricate anything which is abrasive that we happen to be swallow um if you are not producing enough saliva right you can run the risk for ulceration and infection of the mucosa you&amp;#39;ll get an increase in cavities in the teeth and just all kinds of [Music] problems so here&amp;#39;s this uh diagram again so we&amp;#39;ve just gone over basically the oral cavity right so enzymes produced here we can actually start digestion here uh chemical digestion we do physical digestion here by chewing uh basically grinding and cutting the food particles between our teeth um we produced saliva here we just went over all the major functions of saliva and the importance of saliva for the digestive process now we need to talk a little bit about swallowing so here we need to talk about the fings that uh basically that area behind the nasal passages behind the oral cavity and uh down towards the larynx and so the posterior walls of the Oro fings and the L lingo fings contain Fingal constrictor muscles which uh help with swallowing the epiglottis is this little flap of tiue tissue um which covers the larynx uh to keep food and drink from entering the esophagus right we don&amp;#39;t want um those solids and those liquids entering the lungs that&amp;#39;s always a bad thing and the epiglottis will make sure that everything that we swallow goes into the esophagus a muscular tube which runs from the ferx down to the stomach and it has to go through the diaphragm and that opening is known as the esophageal Hiatus and basically ends down at the stomach um a hiatal hernia is a widening of the Hiatus it just gets bigger than what it should be and there&amp;#39;s going to be sphincters there&amp;#39;s going to be an upper sphincter which is going to be striated by that we mean it&amp;#39;s striated muscle it&amp;#39;s going to it&amp;#39;s skeletal type of muscle the lower is going to be smooth muscle and the mucosa here is moist squamous epithelium it&amp;#39;s a stratified epithelium they&amp;#39;re multiple layers of cells here because these cells aren&amp;#39;t very thick they&amp;#39;re squamous we um scrape these things off on a pretty regular basis in spite of the fact that you know the saliva is doing its job and um the esophagus itself will produce a fairly thick layer of mucus to help protect this moist um stratified squamous epithelial cell layer which aligns its [Music] mucosa so there a couple different phases of swallowing so there&amp;#39;s voluntary which means if it&amp;#39;s voluntary and it&amp;#39;s muscular it has to be skeletal muscle and basically the uh bols of food is moved by the Tongue uh from the oral cavity basically into the fairings and the tongue there&amp;#39;s a lot of uh skeletal muscle in the tongue it runs in a lot of different directions so that we can move the food around we can make sure that we don&amp;#39;t bite the tongue although that does occasionally happen and um all this muscular activity in the tongue is also very important for Speed because we um by changing the size and the shape of the oral cavity we can change this the nature of the sound that we are able to produce the next phase of swalling is the fenal phase this is reflexive so this is not actively controlled by the uh conscious nervous system right it&amp;#39;s it&amp;#39;s all reflexive and we&amp;#39;ve got swallowing centers in the medulla which uh coordinate all this for us so the soft pallet is the furthest uh portion of the pallet there&amp;#39;s no bone uh unlying U supporting this uh that&amp;#39;s going to elevate the upper esophageal sphincter will actually relax the ferx will open up into the esophagus the food goes down the esophagus by the uh propelled by the feral constrictors and we get uh contractions going from the superior to to the inferior along the fairings the iglus is going to be tipped down U posteriorly to the uh bis coming down the larynx that gets elevated to prevent the food from passing down into the larynx that should actually be the epicus um we&amp;#39;ll do that and then um the last phase is again reflexes it&amp;#39;s esophageal this is just peristal say through the um esophagus in that that&amp;#39;s uh kicked off by stretching of the esophagus to um start peristalsis to move this material down and then this Bolis is delivered into the stomach

Transcript for:The Digestive System - Overview and Functions

Transcript for:
The Digestive System - Overview and Functions