now we continue with the digestive system and what we're concerned with here is the fate of the cheeseburger that we discussed at the um in the last lecture and so we left off with the cheeseburger having been broken down all the mechanical digestion of that meal is completed by the time this food emerges from the stomach and what's left is further chemical digestion and then the absorption of those nutrients into the bloodstream and then finally the removal of the remainder which is the waste products that are pooped out or defecated so that's the scope of the of the current lecture and the question that we're pursuing here is how are these nutrients um absorbed a secondary question is um how they are further broken down through the digestive enzymes and the small intestine now from the the food that you eat what do you get well of course food is a source of energy and um energy comes in its most dense form in in fat which is more than twice the energetic energy density as what's available in the carbohydrates and proteins building products there are nine essential amino acids that your body cannot synthesize so that must come from food fatty acids are essential for membrane structure again it must come from your food and then finally we have many many vitamins and minerals that can only come from your food so you need a balanced diet in order to get these important building materials and the vitamins and minerals that provide major functions for your physiology so let's consider the rest of the digestive anatomy and the parts that are going to be featured in this lecture so the liver plays a role you're going to see the liver for the remainder of of these lectures play a sort of recurring role in this lecture we'll see how it's really important for um the the breakdown and absorption of fat the small intestines are the most important organ for nutrient absorption the large intestine plays a smaller but yet important role because it houses a microbiome that breaks down well as a byproduct of the metabolism of that microbiome there are essential vitamins that are produced which are then absorbed in addition to water before your waste is packaged in the form of feces the pancreas plays an important role it secretes bicarbonate and digestive enzymes as we'll see and then finally the waste products are packaged in a rectum and in each of the major portions of the gi tract they can be separated from one another by these muscular valves called sphincters which close off one region from the next the first portion of the small intestine is called the duodenum and that is the site for the secretions of bile salts for fat digestion and digestive enzymes we're going to talk most about that and then the absorption that begins at the duodenum then progresses as the meal moves through the rest of the small intestines and just because there are diminishing returns on that basically there's a lot of nutrients of the meal as it emerges from the stomach and then less and less as it moves further then you can imagine that there's going to be less nutrient absorption as the meal progresses but all of what all detail applies you know to the entire length of the small intestine so back to the cheeseburger the cheeseburger when we left off was getting worked on in the stomach that's the final opportunity for mechanical digestion then um then we're going to have absorption here now a key feature of the the small intestine is the composition of its membrane which is called the gastric mucosa so that includes the epithelium connective tissue and the smooth muscles that surround it here we see the epithelial membrane it features um finger-like projections the villi and these crypts which are invaginations it's very well vascularized so you can imagine lots of capillaries and then there are also these vessels from the lymphatic system called lacteals they play a role in fat absorption and if we zoom in on this membrane in particular one of the villi then we can consider a couple of the cell types that we're going to talk about first of all there are just the absorptive cells which do play a role in chemical digestion and also the transport of nutrients into the bloodstream they feature um their own finger-like projections which are called microvilli and the apical surfaces um referred off referred to often as the brush border and it's there that you'll find some digestive enzymes that are anchored to that apical membrane and therefore aren't swept through the small intestine like some of the other enzymes there are also endocrine cells that function as sensory cells and integration centers for the release of digestive hormones and we'll feature a couple of those which aid in the digestion of the meal now the first challenge for this region of the small intestine is to neutralize the acidic and toxic through through the the pepsin that's in the the center of the stomach to neutralize those substances so that they don't burn a hole in the gastric mucosa now the food as it emerges is in this semi-liquid state and um and that's called the term that's used for that is chyme which is a rather sort of vocal vulgar word um and chyme you can think of as an acidic food slurry sounds yummy huh um now the chyme because it just emerged from the stomach has hydrochloric acid giving it a low ph that is recognized by those endocrine cells and in response they release a hormone called secretin so secretin is released in the bloodstream it's also released in the gut lumen where it can be detected by other digestive cells secretin has a number of responses first of all it inhibits acid secretion within the stomach your stomach produces hydrochloric acid releases it into the gut lumen but only at high concentrations when it's needed okay it's toxic it takes energy to produce so secretin reduces those secretions in order to neutralize the low ph or the acidic chyme then we have the pancreas playing a role and secretin has an effect on the pancreas in particular it releases bicarbonate so the pancreas in this context you can think of as a gland that secretes bicarbonate and also digestive enzymes which we'll get to in a moment the bicarbonate then acts on the chyme to neutralize the hydrochloric acid so here we have those protons that's our hydrogen chloride ions and then the bicarbonate that's released and you can just assume that sodium will be available the chemical reaction here is the production of carbonic acid and sodium chloride which will rise the the ph of the chyme considerably and then once the ph has risen then pepsin which was toxic to proteins it will break down proteins becomes neutralized at that high ph so the release of secretin largely deals with this problem of a toxic meal that emerges from the stomach now the release of the chyme also triggers this is through those endocrine cells another hormone called cold called cystokine or cck so cck is released again the bloodstream and in the gut lumen and there it acts on the smooth muscles that line the small intestine now i mentioned that when we were talking about smooth muscle that they have a capacity to autonomously generate action potentials and to essentially activate themselves to generate contractions such as peristaltic waves and churning in the gi tract cck acts on ligand-gated channels that therefore influence the contractions by the smooth muscles and it does so to reduce the activity of those muscles so essentially if you're eating a really fatty meal this is a way of adjusting how the small intestine manages that meal fats take longer to digest and therefore reducing gut motility is a strategy for enhancing the absorption of fat so cck release does that it also has a couple of other responses that are related to the absorption of fat for example the liver produces something called bile salts or you can just think of it as bile sorry we got a lot going on here so so bile salts help i'll explain in a moment how does it but they help in the ability to absorb fats and therefore it's advantageous to release bile for fatty meals and and bile is contained within something called the gall bladder okay think of it as being like a urinary bladder in that it has a smooth muscle lining and cck stimulates contraction of those smooth muscles so that squirts bile that then moves along this duct and it is released into the gut lumen okay that's where we have the bile that comes from the gallbladder excreted through this little opening here which is called the common bile duct it's an opening that provides a route not just for the bile salts to enter into the gi tract but also those digestive enzymes that come from the pancreas cck also stimulates the pancreas to release enzymes ions and water and those include a variety of enzymes including trypsin calipas amylase etc that are secreted by the pancreas okay so then what happens is your meal has now it's the kind that has been neutralized all the mechanical digestion has been achieved and there's just the matter of taking large molecules and making them smaller so that they can be absorbed as the meal progresses through the rest of the small intestine so the duodenum essentially solves the toxic chyme problem adds some enzymes and then passes off the meal to the rest of the gi tract so everything i'm going to describe follows those actions and allows for nutrient absorption so we're going to consider carbohydrates proteins and then fats so for carbs carbohydrates are polymers of um simple sugars such as glucose those could be starches glycogen for example and um i've already mentioned amylase you have amylase secreted in your mouth your you begin even as you're chewing a meal you begin the breakdown of carbohydrates and if you chew on something like a saltine cracker long enough and you have a very acute sense of taste then you might be able to pick up on the fact that that it gets sweeter in your mouth because there's a there's a very simple breakdown by amylase of a glucose polymer into dye and monosaccharides so there are um the disaccharides produced by amylase and they also amolese is also secreted by the pancreas then there's the final matter of breaking up the the disaccharides into monosaccharides and that is achieved at the brush border so there are breast border enzymes so that when disaccharides come in contact with the apical surface of these these absorptive cells then we have this conversion into monosaccharides including glucose there's also fructose galactose that are also shown schematically here you don't have to memorize any more of these just worry about the words that are written on the slides i'll mention some others along the way names of enzymes and and so forth that you don't need to worry about memorizing and we've already talked about glucose absorption the other monosaccharides are also transported across the apical membrane of the absorptive cells through secondary active transport taking advantage of the sodium gradient so i'm not going to detail how that occurs because that was a topic of an earlier lecture and therefore we can move on to proteins well what are proteins proteins have an amino terminal and then they are composed of a string of amino acids that are bound together through peptide bonds and then that's terminated with the carboxy terminal now there is a class of enzymes that act on the interior peptide bonds in these molecules those are called endopeptidases like pepsin is one of them and the product of the action of those enzymes is to is to produce smaller proteins in particular small peptides and tripeptides then the outermost peptide bonds can be acted upon by what are called exopeptidases and that produces amino acids and dipeptides okay so now we've got some pretty small molecules that can be transported let's consider how that transport works protein absorption is actually very similar to glucose absorption again there's uh taking advantage of the sodium gradient and using that via secondary active transport to move those small molecules against a concentration gradient potentially on the apical surface of the membrane so that occurs for amino acids it's then a matter of facilitated diffusion across the basal membrane and then once they have moved across that membrane then they can be transported into the circulatory system so that's how amino acids are transported it's also how tri and dye peptides are transported however instead of using sodium for secondary after transport instead protons or hydrogen hydrogen ions are the source of secondary active transport that is for tri and dipeptides like that now small peptides can be transported but they're too large for just mere secondary active transport and so instead they're essentially packaged into vesicles so that they don't actually enter into the cytoplasm of the absorptive cells but instead move across the cell through a process known as transcytosis so you can think of this small peptide is essentially moving through in a force field almost a portion of the cellular membrane is pinched off to perform a vesicle and that it is transported across the interior of the cell without actually mixing with the chemicals on its interior so that brings us to fat absorption so imagine that the fats that emerge from the stomach are mixed up in the chyme slurry and in that mixture are fat droplets now if you imagine a salad dressing as being olive oil and vinegar and if you shake it up then the olive oil will momentarily break up into these kinds of droplets now if a given time the fat droplets will find one another and form large fat droplets which is difficult because it doesn't allow for the action of enzymes on most of that volume and it's too large for absorption so here's where the bile salts come in bile salts stabilize fat droplets and prevent them from merging so as your stomach churns away and bile salts are available then the bile salts will coat coat the surface of the fat droplets and prevent them from reforming these little fat droplets are called micelles and the micelles include triglycerides and cholesterols now the micelles are small enough that when enzymes act upon them that the fats are isolated enough for them to be absorbed and the enzymes from triglycerides and cholesterols or or i should say from the triglycerides will produce monoglycerides and free fatty acids so now we're at the scale where absorption can easily occur especially because these are fats and therefore are lipid soluble so these will diffuse across the cell membrane and into the cytoplasm of those absorptive cells cholesterol requires transport and there are transporters for cholesterol and so the cholesterol can also enter into the interior of the cell now once inside of the cell the fat is handled differently from carbohydrates and proteins they fuse with the golgi apparatus and then from that point they're transported across the cell and released into the interstitial space and in particular make their way into those vessels called lacteals for the lymphatic system so that's how fats are absorbed we've talked about carbohydrates and we've talked about proteins all that remains for the meal is what occurs in the large intestine now it's called large because it's a large diameter it's actually much shorter in length we see it highlighted in green here it's not actually green and it has some sphincters the first of which separates the small from the large intestine there are a variety of functions we have a lot of water absorption occurring in the small in the large intestine this is good so we don't um we don't produce diarrhea whenever we have to defecate it also houses bacteria i mentioned the microbiome in the large intestine and how the metabolic products of that microbiome provide nutrients essential vitamins and minerals and then there's also the matter of storing and condensing the feces for getting rid of them when you want to now one little thing about defecation that i'll mention is that your anus features sphincters that are composed of smooth muscle on the interior and skeletal muscle on the exterior and we saw that there was a skeletal muscle sphincter for the urinary bladder and why do you think that is that your body has both types well uh recall that skeletal muscle is under the somatic efferent pathway and that allows for voluntary conscious control of those skeletal muscles so whenever you see a sphincter that's composed of skeletal muscles just think okay that's the opportunity to consciously control something now what happens when you have to defecate when you get that sensation is the within your rectum the feces open up the sphincter that's that's comprised of smooth muscle that's the interior sphincter and that's when you feel like you've really got to go but fortunately the external sphincter has skeletal muscles so you can control when you actually defecate and then that's finally when you actually poop out the feces and get rid of the waste okay so how are nutrients absorbed we've talked about a lot a lot of details with the digestive system in the duodenum we have first of all the low ph and presence of pepsin that are handled by the the release of secretin okay so it's a matter of the pancreas uh neutralizing that that low ph environment through the release of bicarbonate cck is also released by the fat in the meal which reduces gut motility and stimulates the release of bile salts we also have digested enzymes secreted by the pancreas as well that leaves absorption in the small intestines where we considered the fate of monosaccharides small peptides cholesterols monoglycerides and free fatty acids this is small peptides and even smaller proteins then that leaves the large intestines where we had water absorption the presence of a microbiome and this action of defecation the release of waste products