now we move on to the digestive system what we're going to do is consider what happens to a meal as it propagates through the digestive tract here we have our schematic for the body we're gonna focus on the digestive system and we're gonna follow in particular cheeseburger as it is broken up and makes its way first into the stomach the digestive system is really two major functions so the term digestion refers to the breaking up of the food into ultimately the scale of molecules but then the other major process is absorption taking those nutrients those molecules and absorbing them into the bloodstream so that those nutrients become available to all the cells of the body and then all the stuff that isn't nutrients the waste products that we might ingest then gets defecated pooped out through the digestive system so this mini lecture is focused on the first stages that is digestion itself and the question is how are your meals digested that of course begins when you stick the food into your mouth and start to chew it so let's look at somebody's in in profile and we have a bolus of food so we can think of this as the cheeseburger and the first digestive organ that i'll mention is the tongue the tongue is what we call a muscular hydrostat i'm not going to get into the details of it but just think about it there's no hard parts in your tongue yet somehow it manages to exert force on your food and move it around distribute it among your teeth so that the teeth can chew and it does that all all of the muscle i'm sorry the force transmission and muscle antagonism occurs in something that is just composed of soft tissue um so it's actually a pretty neat organ it's quite dexterous most the time you don't manage to to bite on your tongue so the teeth in your mouth as you're probably aware vary a lot in their shape and the ones in the front tend to have very small surface area that come in contact with the food those are the biting surfaces of the incisors and the canines that small surface area serves to concentrate force into um a large pressure on that small area and then what happens is once we've torn apart little bits of food is that our tongue then carries that food and moves it to the back of your biting surfaces that is the thick the teeth with the larger grinding surfaces um which ultimately are the molders and so what you have is this spatial gradient in function among your teeth from tearing in the front to grinding in the back and the grinding is also because the molars are closer to the jaw's joint then that means that there's greater leverage so if you have something like a big nut to crack then you sort of subconsciously toss that into the back of your mouth where you can get more leverage and therefore generate more total force even though it's distributed over a larger area now of course the goal here is to take the cheeseburger and ultimately make it into a slurry of molecules so when you chew you get closer because now the pieces are smaller in size but another thing that's going on in the mouth is that there are enzymes digestive enzymes and we'll see lots of digestive enzymes later on the stomach and the small intestine but even in your mouth along with the the mucus that secreted in your mouth as you start to as you start to swallow and process food there's also enzymes that are beginning the process of chemical digestion so what chewing does is it exposes more surface area of the food that you put in your mouth because the enzymes can only act on the surface they can't penetrate into the depths of the food through chemical digestion at least not on these time scales so let's just imagine a volume let's say it's a cube and when you cut that cube in half now you've exposed the surface area that the enzymes that might be acting on that cube can act upon and if you make another cut and you were to do the calculation of areas that are exposed what you would find is that the area is almost doubled with just those two cuts that is the total surface area that the enzymes might act on is double just with those two slices so what's happening in as you chew the food is not only are you making it smaller so closer to molecular scale but you're allowing the digestive enzymes to act on act on more surface of that volume and thereby begin to break down the food at a molecular scale so the salivary enzymes are pretty diverse actually so we break up starches through amylase fats through lipase and there are lysozymes which kill bacteria in your mouth as you can imagine there's a great potential for bacterial infections just through the things that we stick in our mouth and start to chew and so we have enzymes to start to combat the possibility for infection another factor as you um are processing the food is to move it down into the digestive tract and so we do that with a coordinated action that serves multiple functions so first of all we essentially are capable of rolling up the food into a bolus and because our faces are the entry points for respiration then we want to avoid blocking our respiratory tract by food that we are swallowing and so that includes the nasal passages as well as the trachea and so those two areas are blocked off with fleshy muscularized structures called the uvula and the epiglottis and so if you look in the back of your throat you can see a uvula as this little thing that dangles down that closes off your nasal passages so if you're drinking a glass of milk and you start to laugh and the milk comes up through your nose that's passing that milk would be passing through the uvula into your nasal passages so that's uh that's what the uvula tries to avoid the uvula is also supposed to block off the nasal passages if you ever vomit vomit's very acidic it's taking stomach acids and the nasal passages are very delicate and so there's the potential for damage to the nasal passages if not for the uvula doing that task and the epiglottis fails in instances where you take a drink and it falls down the wrong pipe and you start to cough violently the epiglottis is intended to avoid uh that activity so the food you go down your esophagus a muscularized portion of the gi tract the gastrointestinal tract and the trachea is where you breathe and so the way that the epiglottis closes the trachea and the uvula closes the nasal passages is through a reflex action that's triggered by a stretch receptor in the back of your throat so as you pass the food into the back of your throat it activates that stretch receptor and we're not going to map out the control system but um from our understanding of reflexes from that earlier lecture i'm sure you can unders you can imagine what kind of control must be required for this sort of action the uvula and epiglottis spring into action they close off those passageways so the uvula pinches off the nasal passages and then the trachea is covered by the epiglottis allowing the food to move down through the esophagus like that now just like in the small intestine and other regions of the gi tract the esophagus is comprised in part by layers of muscle and those muscle layers include a circumferential layer where the muscle fibers run around the circumference in the direction of these horizontal lines shown here so here's our bolus of food it's entered into the esophagus and it's propagated down the esophagus through what are called peristaltic waves you could say peristalsis moves the food and that's created in part by a pinching of those circumferential muscles so peristalsis moves the food generated by a traveling wave of contraction and that moves the food towards your stomach now if we the the schematic on the right is very rectangular and it's in its rendering if you look carefully at the shape of the smooth muscle layers you would find that it has something like an hourglass shape and indeed the circumferential muscles do pinch the tract which is essential but you also might notice that the shape of this hourglass is different on the stomach side of things then it is in the wake or the um the region behind that pinching and those differences in shape are achieved through the other muscle layer that wraps around the esophagus and those muscle fibers are on the length of the esophagus and so we call them longitudinal muscles and so longitudinal muscles when they become activated shorten the tract regionally and that changes their shape so this makes it so that the bolus of food doesn't get squeezed and move in both directions but instead favors moving downwards in this case so the esophagus is connected to the stomach and so there we have where the food is traveling on the left-hand drawing ultimately it will be dumped into the stomach and then after the stomach does its thing then the food will be deposited into the small intestine now the stomach as you're probably aware has a highly acidic environment in its interior in the lumen of the stomach and so in order to protect the epithelial lining of both the esophagus and the small intestine the stomach has a closure a muscularized seal that isolates that acidic environment in the stomach from those other regions of the gi tract okay so that co is provided by a sphincter i've mentioned sphincters in the case the urinary bladder they are either skeletal or smooth muscle um that lines a um you can think of it as a tract or a vessel and when it contracts it closes it off so it's a muscular closure that seals in this case the different regions of the gi tract there's also one up in this anterior region of the esophagus the esophageal sphincter which prevents backflow into the nasal passages and everywhere else uh up there the trachea for example so as you begin peristalsis this closes off the esophagus from those other regions and there are sphincters all along the gi tract and i'll point them as we go along okay so that brings us to the stomach so our cheeseburger has now been dumped into the stomach the stomach first of all is very muscularized there's three layers of muscle the stomach actually is quite dexterous if you don't chew your food well with your hard teeth then the stomach has to complete the job with its soft parts and the these smooth muscles are capable of doing that it can grind the food and it can churn it backwards and forwards and if you don't do a good job by chewing then it has a lot of work to do and so it will it will do that churning and grinding particularly in this portion of the stomach now the stomach is also a major site for chemical digestion and also bacterial killing occurs in the stomach as well and those two functions are achieved by that low ph within the stomach lumen the low ph is generated by hydrochloric acid it's really nasty stuff it has an incredibly low ph you don't have a ph this low anywhere else in the body and it does kill bacteria as we'll see in a moment it also activates proteases that break down the large proteins that might be in your meal like certainly a cheeseburger has a lot of that so uh to understand how that low ph is generated and how the stomach insulates itself from that acidic environment we're going to focus in on the epithelial lining of the stomach and so what we have there are these finger like projections always fingers you can think of enhancements of surface area and we have a whole bunch of epithelial cells that create a membrane this membrane is highly vascularized you'll see that a lot in the gi tract in the small intestine is for absorption also in the large intestine here it allows for hormonal controls the cells that line the epithelium uh there are three types of cells um within the epithelium that are important for this function of chemical digestion the first of the cells that are responsible for secreting the hydrochloric acid those are called parietal cells and they are responsible for that low ph environment um the second type of cell i want to point out they're called chief cells and they secrete pepsinogen now pepsinogen is a protease but it's in its benign form it's not until it's converted into pepsin that it will break down proteins so that's good for the stomach lining which is largely composed of proteins uh pepsin itself would digest those proteins so the pepsinogen is not converted until it's exposed to a low ph environment which is what occurs in the stomach lumen so pepsin like it says here breaks internal peptide bonds to break down proteins into peptides so the question here is why is it that the hydrochloric acid and the pepsin does not digest the stomach lining and the measure that i'm going to explain here is not featured in the small intestine or in the esophagus so if that acidic slurry in the stomach escapes the sphincters around the stomach then there's the potential for doing real damage and this happens in the form of ulcers on both ends of the stomach now the answer to how the stomach is protected comes in the form of mucus now we don't talk a lot about mucus in this class but mucus comes in a variety of forms it can be very dense and stiff and it can be like the mucus the phlegm that you're used to thinking of if you blow your nose or cough up uh phlegm uh which is not particularly stiff this is a more stiff mucus and you can think of it almost as being like a layer of snow on the lining of the stomach it's secreted by these mucous neck cells and it has uh buffers in it that manage to achieve a ph that is much more like the ph uh within the cells and pre and and it's a ph that uh protects the cells so um so it's that mucus that buffers the epithelial lining of the stomach from the acidic environment in the stomach lumen now the way this toxic substance hydrochloric acid and pepsinogen make their way into the gut lumen is almost like um burrowing through a thick layer of snow that is just imagine you had to tunnel through snow you're going to create this vessel that's just like a negative space in that volume where the snow is like mucus and that's created by positive pressure generated by the parietal and chief cells and so what they create are these mucus fingers that are essentially vessels through this thick mucus and that creates attract for the hydrochloric acid and the pepsinogen to travel into the interior of the gut of the stomach lumen without coming into contact with the epithelial lining of the stomach so the question that we set out to address in this short lecture is how are your meals digested we've considered what happens to a cheeseburger as it undergoes mechanical and chemical digestion up to the level of the stomach what will happen in the next lecture is the absorption of those nutrients now that they've been broken down to the level of at least large molecules then in the small intestine there's further chemical digestion into smaller molecules and ultimately the transport of the those nutrients into the bloodstream across the digestive epithelium and then finally all the things that aren't absorbed are defecated and leave the gi tract