Understanding the Digestive System

Chapter 24, the digestive system. The digestive system is what we use to acquire nutrients from our environment. We use those nutrients to make essential compounds that we need in our bodies, which is a type of anabolism. We also break down things that we take in to provide energy to our cells so that we continue on with life's processes. This is an example of catabolism. The digestive tract, also known as the gastrointestinal tract, GI tract, or alimentary canal, this is a muscular tube. that extends from your oral cavity or mouth area all the way down to the anus. There will also be involved accessory organs that are helping with the process of digestion like the teeth, tongue, and various glands that will help to secrete products that will assist in digestion. So this is an overview of the main components of the digestive tract that we will be going through individually, beginning at the oral cavity, which is where we ingest, where food comes in. We then move the food through the pharynx, to the esophagus, down to the stomach, small intestine, and then large intestine. In assisting along the way, we're going to use the liver, the pancreas, and also the gallbladder to help us out of it. The main processes of the digestive system include ingestion, mechanical digestion and propulsion, chemical digestion, secretion, absorption, and finally defecation. So let's go through each of those first. Ingestion occurs when food enters your oral cavity. This is taking food into the mouth. We will then begin to mechanically digest. So mechanical digestion is different from chemical digestion in that with mechanical digestion, we are just breaking the food up into smaller, more manageable pieces so that we can actually swallow that food. We're not really doing much in the way of actual chemical breakdown at this phase. We're more just changing the size of the food. So we're going to crush it and shear it in our oral cavity. And then we will propel that food or move it down through the pharynx into the digestive tract. And once we swallow that food, it is no longer under our conscious control. Next is chemical digestion, which is when we chemically break down the food itself into small organic fragments. So, for example, if we eat protein, we would like to get that protein down to amino acids. If we had some carbs, we want to get those carbs down to individual monosaccharides. We want to get down very small because at that point we can then absorb that material across. the digestive epithelia and get benefit from it. Secretion is when the digestive tract releases water, acids, enzymes, buffers, and salts from the epithelium of the digestive tract or glands and gallbladder. And all these secretions are going to assist us in the breakdown of this food so that we can actually get nutrients from it. Absorption is probably the best payoff of this whole process. We're going to move organic molecules, electrolytes, vitamins, water, and minerals across our digestive epithelium into the interstitial fluid of our digestive tract. So we're going to gain these nutrients. Defecation is the elimination of the waste from our body. So anything we can't use. from that broken down food. We get all we can out of it and then whatever's left will be compacted. dehydrated because we want to keep as much water as we can as well and then what's left over is called feces which is a waste that we do want to get rid of the lining of our digestive tract safeguards the surrounding tissues against the corrosive effects of digestive acid and enzymes which can be very strong mechanical stress such as abrasion bacteria that we ingest with our food or that already live in the digestive tract. So next let's go through the serous membranes of the digestive area before we move on into more specifics. So the peritoneum is a serous membrane that lines the peritoneal cavity and there are two types of this peritoneum. We have the visceral Peritoneum, which is also known as the serosa, it's going to cover the organs inside of the cavity and the parietal peritoneum is going to line the inner surface of your body wall. So this is similar to the visceral and parietal pleura that we talked about and similar to the visceral and parietal pericardium that we also talked about. Visceral covering organs. parietal lining cavities and the two of these are going to produce peritoneal fluid and peritoneal fluid is going to allow sliding of the parietal and visceral surfaces without any friction or irritation so this is kind of like a lubricant and about seven liters of it are produced and absorbed every day but very little is in the peritoneal cavity at one time There is a condition called ascites and this is when we have abdominal swelling due to the buildup of peritoneal fluid. So looking a little further at some other membranes and we're going to take a look at these in pictures as well so we can get a visual but mesenteries are double sheets of peritoneal membrane and their job is to support portions of the digestive tract inside your peritoneal cavity. They're going to help to support and suspend. They connect the parietal peritoneum with the visceral peritoneum and are also going to allow for a route to and from the digestive tract for blood nerves or blood vessels, specifically nerves and also lymphatic vessels. They'll stabilize the position of the organs. and prevent the intestines from becoming tangled up, which is obviously a big deal. So let's focus on two of the bigger, well, we're actually going to look at more than two, but there are two kind of big ones that we're going to see, and then we're going to add on some other ones as well. So let's begin with the lesser omentum. So the lesser omentum is, to me, it looks a little bit like a stomach hammock. It's going to stabilize the position of the stomach. The stomach is kind of cradled in the lesser omentum. And it's going to provide an access route for blood vessels and other structures entering or leaving the liver. So it kind of has a double purpose. But in this drawing, you can see what looks a little bit like a hammock around the bottom of the stomach. A fun little illustration of how that works over here as well. But that's the lesser omentum, a stomach hammock. Next, we have the dorsal mesentery. This is going to enlarge to form an enormous pouch. So this was the second large one that I was referring to called the greater omentum. And the greater omentum is going to extend inferiorly or down between your body wall and your anterior surface of the small intestine. Hangs like an apron from the lateral and inferior borders of the stomach So if that totally confused you all those directions what we're basically saying here is that it attaches and then hangs down in front of your intestines and between your skin and your Muscle a bit like an apron would if you were wearing an apron. So let's take a look at where exactly that is to make it a little bit more approachable. So this is a side view. through the abdominal area. And so this would be our vertebral column. Here's the liver. Here's the stomach. There's a piece of the colon. And most of the other intestines have been removed. But this little loop here that hangs down, loops back up, and then connects to the colon, that is the greater omentum, or the apron. So it hangs down like an apron. And we're going to look at this in an official book illustration as well. But the greater omentum is awesome because it does so many important things. It's going to actually conform to the shape of your surrounding organs. It pads and protects the surface of your abdomen because you don't really have any bone right there. It provides insulation to reduce heat loss and stores lipid energy reserves. So we store fat in our greater omentum. And this is good because it allows us to store energy reserves, but it can be bad. So if we gain too much weight and we store that fat in our greater momentum, it contributes to the beer belly. You know the kind of belly that gets so big that it hangs over the pants? So sort of like this over here. So that hanging belly, that's the beer belly. And that's fat stored in this shape. of the greater omentum. The mesentery proper is a thick mesentery sheet. It's going to give more stability but permits some movement and it will suspend all but the first 25 centimeters of the small intestine. The mesocolon is a mesentery associated with part of the large intestine. So whenever we see large intestine, we should think colon because it's the same. So during development, the mesocolon of your ascending colon, descending colon, and rectum will fuse to the posterior or back body wall to lock the regions in place. So let's get a view of all of this because reading about it is not so clear as actually seeing it. So here's a front view and let's just review. We've got the lesser omentum. That's our stomach hammock. The greater omentum is not pictured in this. That's the apron, because if it were here, it would be covering all of this. We wouldn't be able to see the other things that we were talking about. So the next one was the mesentery proper. So the mesentery proper is this one here, the mesentery sheet. or mesentery proper and you can see how it's helping to stabilize the organs in here and then we had the mesocolon and the mesocolon we've got some here and here and also here so that's all mesocolon remember that's going to help to stabilize the intestines and organs in this area so that they can't just kind of squirm around wherever they want to because our intestines do move. So they need to be able to move, but we don't want them to get all tangled up. So this helps to keep them stable and in position. Here's a side view. So we've got our vertebral column just to orient you and we've got our liver, stomach, colon, and then these smaller tubes are our small intestines. This is a female, so we have a uterus there, we have a bladder, and then here's the rectum. A couple things I want to point out here, a few things. This is the peritoneal cavity, so the inside here is peritoneal cavity, and it's lined with the parietal peritoneum, which we already talked about. That's the lining. And then the organs are covered with, you can see this white coating on the liver. and the stomach, for example, that's the visceral peritoneum. We can also see in this picture, here's the greater omentum, the apron hanging down and looping back up to the colon, which is where we store fat. But it's also important to note that there are some things in the digestive tract that are not within the peritoneal cavity. So you can see the peritoneal cavity border goes around like so, goes around the uterus and then it goes up and down and back around here. To make a loop, you just follow the white membrane, but look what's outside of it. Pancreas, duodenum, which is part of the small intestine, and the rectum are not in the peritoneal cavity. They're behind it. So the word for that, when you're behind or outside of the peritoneal cavity, that's called retroperitoneal. And another thing that's retroperitoneal is... going to be the kidneys, which we'll talk about in the next chapter, but they too are outside of the peritoneal cavity. So you will see the word retroperitoneal again, but I just wanted to be sure you knew what it was. All right, so what we're going to do now is we're kind of transitioning into looking at the tissue a little bit more closely. And whenever we look at tissues of anything, we call that studying the histology. So histology is taking a look at the tissues. And what we're going to be doing is we're basically going to imagine that we took a scalpel and cut a plug out of a digestive organ wall. Okay, so that could be the colon, it could be the stomach, it could be the esophagus. We're going to look at all of them. But when we take a chunk out of one of these digestive organs, we can see that the wall of the digestive organ is going to have some layers in it. And those layers are the tissue layers, which we refer to as histology. Now, I do want to give you one other important word here that you're going to see again. The hollow inside of an organ is called the lumen, L-U-M-E-N, lumen. So the hollow inside of the stomach, lumen. The hollow inside of the transverse colon, lumen. The hollow inside of the small intestine, the uterus, the bladder, lumen. So you may see that word, and it's certainly going to be important in this chapter. So the layers of the digestive tract. Here's why lumen matters. We're starting closest to the lumen, so closest to the inside, and working our way out to the outside of the organ. So mucosa is the first layer beside the lumen, followed by submucosa, muscularis layer, and serosa. And we're going to look at those individually. But the lining of the digestive tract will vary depending on where you are in the digestive tract. So every organ is not going to look the same as far as its histological organization, which is why we're going to look at all the... main organs. But before we do that, still keeping it general, here is a digestive organ. And this, of course, would be the, hopefully you said lumen, hollow inside. And as we work our way through, we go lumen. And then this would be mucosa because it's closest to the lumen. submucosa, muscle layer, and then the outermost, which covers the organ, is serosa. So let's get even closer to this. We are going to zoom in right about in here. So again, we have the mucosa, we have submucosa, muscularis or muscle layer and then finally serosa now let's focus on mucosa first naturally so the mucosa is made up of three components we've got mucosal epithelium so this is the the outermost epithelial tissue and remember epithelial tissue is tissue that is avascular it lines or covers body surfaces And it typically has glands in it to produce secretions. So we've got the epithelium. We then have under the epithelium this stuff, which is called lamina propria. And lamina propria is areolar tissue, which we learned about in Chapter 4. And it's also got in it blood vessels, lymphatics, sensory nerves. And then under that, we have... the muscularis mucosae and the muscularis mucosae is of course made of muscle so those three things make up the mucosa in general. So we've got an overview of that in our notes here as well. So let's take a look at that. So mucosa inner lining of your digestive tract. The mucous membrane is going to consist of, as we said, epithelial tissue, which has secretions from glands, the lamina propria, and then the muscularus mucosae, which is going to be under it. Digestive epithelium. So mucosal epithelium can be simple or stratified depending on the location, function, and stress of the area. The oral cavity, pharynx, esophagus, anal canal will all be stratified, squamous epithelium. The stomach, small intestine, and large intestine, or most of it, will be simple columnar epithelium. We will also find enteroendocrine cells scattered around as well. These are going to secrete hormones that will help to coordinate the digestive tract activities and also the accessory glands. So let's move down now into the submucosa. So the submucosa is here. Well actually before we go into submucosa, let's talk a little bit more about the lamina propria. So we talked about the mucosal epithelium of the mucosa, but let's talk a little bit about the lamina propria before we go on to submucosa. So the lamina propria is a layer of areolar tissue that has in it blood vessels, as we mentioned before, sensory nerve endings, lymphatics, smooth muscle cells, and scattered lymphatic tissue as well. Under that, the final part of our mucosa is our muscularis mucosae. And the muscularis mucosae is in most areas of your digestive tract under the lamina propria. It's a very thin sheet of smooth muscle and elastic fibers. The smooth muscle is arranged into two concentric layers. The inner layer circles the lumen. Remember what that is, that's circular muscle going around and around the lumen in a circular way. And then the outer layer contains cells arranged parallel to the long axis of the tract. So these are going longitudinally around or parallel to the long axis of the tract. So this again is the mucosa. So when we move out of the mucosa, which had three layers. epithelium, lamina propria, and muscularis mucosae. We go into the submucosa, which is a layer of dense irregular connective tissue. Now because it is connective tissue, it is highly vascular. Lots of blood vessels, lots of lymphatic vessels, and there may be some glands in here as well, depending on where we are, that will secrete buffers and enzymes. into your digestive tract. So that would be of course this area, submucosa, here and here. So moving down we have next the muscular layer which is here. And the muscular layer of course is going to be dominated by smooth muscle cells because We don't really have conscious control over our digestive processes for the most part. So smooth muscle and there's going to be an inner circular layer and an outer longitudinal layer. And these layers are involved in mechanical digestion. Here we are mechanical digestion, which is when we. crush the food up into smaller bits, and they're also responsible for helping to move materials along your digestive tracts and moving everything forward. These movements are coordinated by the ENS, which is the enteric nervous system, so the nervous system that controls digestion. It's controlled primarily by parasympathetic division. So we talked about previously that Parasympathetic is often referred to as rest and digest. So we digest better when we are not under stress. So the parasympathetic division does stimulate digestion. So last we have the serosa, which is the serous membrane covering the muscle along most parts of the digestive tract enclosed by the peritoneal cavity. In areas where there is no serosa, we might have instead an adventitia. An adventitia is a dense network of collagen that firmly attaches the digestive tract to other structures. So let's go back. to serosa and see where that is again. So the serosa is the outermost and covers the muscle. So let's talk now about the motility of the digestive tract. So the actual movement process of things through the digestive tract. So visceral smooth muscle tissue is going to cause rhythmic cycles of activity. which is controlled by pace setter cells. And these cells are going to undergo spontaneous depolarization. Waves of contraction will spread throughout the entire muscular sheet. And one of these processes is called peristalsis. Peristalsis is the waves of muscular contractions that move a bolus along the length of the digestive tract. A bolus is a small oval mass of partially digested food. So once you chew up the food, mix it with a little bit of saliva, and push it back into the pharynx and esophagus, peristalsis will push that ball of food forward onto the next phase. So this is what peristalsis looks like. So here is the bolus. And you can see the muscle layer here. We've got the circular muscle closest to the bolus and the longitudinal furthest from the bolus. And when the muscle contracts, it pinches behind the bolus and that pinching motion will propel the bolus forward. So a wave of contraction in your circular muscle will move the bolus forward. And we have peristalsis. Segmentation are cycles of contraction that will churn and fragment the bolus, mixing the contents with secretions from your intestine. Regulation of digestive functions. So we have local factors, neural mechanisms, and hormonal mechanisms, which will all affect the regulation of digestive functions. So looking at local first, the pH, the volume, or the chemical composition of the contents in your intestines or stomach can have direct localized effects on your digestive activity. Stretching of the intestinal wall can stimulate localized contractions and local factors can stimulate the release of chemicals like prostaglandin, histamine, and other chemicals that can affect adjacent cells. And this can all stimulate digestion. Neural mechanisms, visceral motor neurons, control smooth muscle contraction and when the glands actually secrete. Neural mechanisms, those enteroendocrine cells we mentioned before. the digestive tract produce many peptide hormones that can affect every aspect of digestion and even some other systems they travel through the bloodstream to reach the target organs so we're going to see some examples of these three types of regulation as we go through the details of the chapter so let's begin where it all begins in the oral cavity So the oral cavity is going to function in sensory analysis, so taste, temperature of food before we swallow, mechanical digestion by using our teeth, tongue, and palate to help break the food down, lubrication by mixing the food with mucus and saliva, so getting it all slippery, to send it down, and limited chemical... digestion. So there will not be much chemical digestion in the mouth, but what we can begin Kind of preliminary digestion is of carbohydrates and lipids. The lining of the oral cavity, the oral mucosa, is made of stratified squamous or squamous epithelium. It's thin and non-keratinized on your cheeks, lips, and inferior surface of your tongue. Thin vascular mucosa inferior or below the tongue. can rapidly absorb lipid soluble drugs. The mucosi of the cheeks are supported by pads of fat and muscle. And you know that little hanging down thing in the back of your throat, the uvula, that little punching bag? The uvula is found hanging off of the soft palate. And it does have a job. It's going to prevent food from entering the pharynx too soon. So here are some of the components of the oral cavity. So there's our nasal cavity upstairs, which we looked at in Chapter 23. Here's our hard palate. Then we've got our soft palate. And then hanging down, we have the uvula. Here's our teeth and tongue. There's the epiglottis that we talked about in chapter 23. And then here's the pharynx, which is the back of our throat. We've got nasopharynx, oropharynx, and then laryngopharynx that were discussed in chapter 23 as well. So let's focus on the tongue for a minute. The tongue has four primary functions. Mechanical digestion by helping to compress. Provide abrasion and distortion to the food. Manipulation to assist in chewing and to prepare the food for swallowing. Sensory analysis by touch, temperature, and taste receptors. We also use the tongue for something else. So, speaking, for example. There are two main tongue muscles to focus on right now. The intrinsic tongue muscles, these are the ones that are smaller and they perform precise movements, like when our tongue changes in shape during speech. Extrinsic tongue movements perform most movements of the tongue. So the tongue is important in chewing and mechanical breakdown, but so are the teeth. The teeth assist the tongue while chewing. and are made up of a mineralized matrix similar to bone called dentin. It's very hard. The pulp cavity is the inside chamber of the tooth that receives blood vessels and nerves through the root canal. The apical foramen is the opening through which blood vessels and nerves enter the root canal. So we're going to take a look at all these parts in a picture in just a second. The root of the tooth sits in a bony socket, also known as the alveolus, not to be confused with the air sacs in the lungs. A layer of cement or cementum covers the dentin of the root to help anchor it down into the alveolus. We've got a periodontal ligament that extends from the dentin of the root to the alveolar bone to create the lock down of that tooth and this is called a Gomphosis which is a joint a type of immovable joint that we talked about in chapter 9 It's a very strong joint or articulation. The crown is the exposed portion of the tooth so what we can see when we look in someone's mouth and It projects beyond the soft tissue of the gingiva which are our gums and is separated from the root by the neck of the tooth. Enamel covers the dentin and forms the biting surface of the teeth which is where the food gets crushed. That's called the occlusal surface. And cusps are the little pointy elevations or projections on top of your teeth. So let's take a look at some of these anatomical features. in a picture. So this area that's exposed is known as the crown. of the tooth. The middle area where the tooth goes into the jaw right at the gums is the neck of the tooth. And then the part of the tooth that is anchored in the jaw is called the root. So here's the apical foramen where the blood vessels and nerves go into the dentin of the tooth. So this brown stuff here is all dentin. And then here is the periodontal ligament that holds the tooth into the jaw. It is coated by cement, also known as cementum. So not like cement, like, you know, that kind of cement, but a hard substance that will lock the tooth into the jaw. And then the crown has on it enamel, which is the hard coating. And. The cusps are the sharp projections, the little bumps that come off of the top of the tooth. So those are some of our main features of a typical adult tooth. Now we know we have different kinds of teeth. We have incisors, canines, premolars, and also molars. The different groups. Here's our incisors, canines, which are a little pointier. premolars, and then finally the molars. So let's talk about the difference between those. So incisors are blade shaped, located at the front of the mouth, and are used for clipping or cutting like when you bite into a carrot stick. They have a single root, and the canines, also known as cuspids, are more conical in shape, so a little pointier. one single pointed cusp used for tearing or slashing. That seems a little dramatic, but you know, we'll go with it. And they have a single root. So if you were to bite into a celery stick and then pull, that would be an example of using your canine teeth to kind of shear something. Maybe not so slashy, but more shearing. Premolars, also known as bicuspids have flattened crowns with two prominent rounded cusps and they are used to crush, mash, and grind. They have one or two roots. The molars in the back are the largest with flattened crowns, four to five prominent rounded cusps, and they are used for crushing and grinding too, but they excel at crushing and grinding really tough. things have two or three roots so during development there are two sets of teeth that will form first deciduous which are also called primary teeth milk teeth or what most people call baby teeth there will be 20 of those and then permanent teeth after so here are the baby teeth so they're about 20 baby teeth and then after your baby teeth fall out we have permanent teeth which are going to replace those baby teeth by erupting from up underneath the baby teeth so they'll actually push up on the baby tooth until the baby tooth falls out and there can be 32 permanent teeth so And here's a picture of the permanent teeth in the top jaw and the bottom jaw. But here's a really cool picture of a juvenile skull and you can see some of the mandible surface bone has been chipped away and the mandible. or the maxillary surface bone has been chipped away. These are baby teeth that you can see in the mouth and take a look under the baby teeth. You see these adult teeth, these permanent teeth that are forming in the jaw and what they'll do is they'll erupt or push upward and that will help to loosen the baby teeth so that they fall out. You can see the molars developing and and getting ready to come in later and erupt and then there is an adult tooth there you can see one there as well that will eventually push down okay on to the salivary glands there are three major pairs of salivary glands that secrete into the oral cavity we have the parotid glands the sublingual glands, and the submandibular glands. Each pair has a distinctive cellular organization and produce saliva with different properties. So let's start with the parotid. Parotids are below the zygomatic arch in your skull and they produce a serous secretion that contains salivary amylase. So amylase is an enzyme that's going to break down starches. Sublingual glands are found under the tongue. They're covered by mucous membrane of the floor of the mouth and produce mucus which acts as a buffer and lubricant for the mouth. Submandibular glands, which are found under the jaw, lie within the mandibular groove and secrete buffers, glycoproteins known as mucans, and salivary amylase 2. Mucins are going to help reduce infection and also form a gel-like like substance. So in this picture we can see right in front of the ear under the zygomatic arch is the parotid gland. Here's the submandibular, excuse me the sublingual, under the tongue. And then we've got the submandibular, which is under the jaw. We're only looking at one side. So you can see there's three on this side. There would be three on the other side for a total of six salivary glands. Saliva is produced a lot. And so there's about one to one and a half liters made every day. Ninety nine point four percent of it is water. And the remaining 0.6% is going to contain electrolytes, buffers, glycoproteins, antibodies, enzymes and wastes. The reason for saliva is to clean the oral surfaces, moisten and lubricate food, and to keep the pH of your mouth about 7, controls populations of bacteria, and limiting acid. that they produce, which could damage your teeth and mucosa, dissolving chemicals that can stimulate taste buds, and it initiates the digestion of complex carbohydrates with salivary amylase. Mastication or chewing is when food is forced from the oral cavity to the vestibule and back across the occlusal surfaces of your teeth. The muscles of chewing will close the jaws and slide the lower jaw from side to side. The tongue will compact this food into a bolus, which is a moist, rounded bowl that's easier to swallow. We then move to the pharynx or the throat, which is the common passageway for food, liquid, and air. Regions of the pharynx we already know, naso, oro, and laryngeal pharynx. The food passes through parts of the pharynx on its way to the esophagus. The esophagus is a hollow muscular tube that will bring food and liquid to the stomach. It's about 25 centimeters long and 2 centimeters wide. And it begins posterior to the cricoid cartilage and will enter the abdominal pelvic cavity. through the esophageal hiatus. So the esophageal hiatus is an opening in the diaphragm that the esophagus passes through on its way to the stomach. The resting muscle tone in the circular muscle layer of the esophagus will prevent air from entering the esophagus and backflow of materials from the stomach. The histology of the esophagus, there will be three layers. So remember we had mucosa, submucosa, muscularis, and serosa. Notice that serosa is not in this list. The mucosa contains non-keratinized stratified squamous epithelium. The mucosa and submucosa form large folds. The muscularis mucosae consist of smooth muscle. The submucosa contains esophageal glands that produce mucus. And the muscular layer has an inner circular and an outer longitudinal layer. Now something to add about the muscle layer of the esophagus. The muscle layer of the esophagus is unique because in it we are going to find smooth muscle, but also skeletal, which is not typical for most parts of the digestive tract. Now we know that we swallow many times during the day and night without thinking about it. So, swallowing can be something we're not focused on and it's happening involuntarily or it can happen voluntarily. For example, if you choose to take a sip of a drink or you choose to swallow some food, you are in control of that. So the skeletal muscle portion gives us some control, whereas the smooth muscle portion allows us to swallow involuntarily as well. Instead of a serosa, we have an adventitia. which will anchor the esophagus to the body wall. So here is a cross section through the esophagus. You can see it's taken about right here. The esophagus looks closed. There's not really a big hollow opening in it. You can see right here this white area. This is the opening and you can see there's not much room. And this is good because this folded interior allows for us to prevent air. coming down every time you open your mouth and you breathe. So this would be the lumen, which means that this dark purple area would be the mucosa. Okay, then we have the submucosa muscularis or muscle layer. And finally, adventitia. So going back to swallowing, we talked about it in our notes, but let's look at it visually. So deglutition is the fancy word for swallowing. It can be initiated voluntarily, but proceeds automatically after that. The swallowing reflex begins when tactile receptors on the palatal arches and uvula are stimulated by the bolus so when the ball of food moves closer to the back of the throat that will stimulate us to swallow there are three phases in deglutition buccal pharyngeal and esophageal so let's begin with buccal in the buccal phase we're going to compress the bolus against the hard palate we will then use the teeth and the tongue to shape that bolus and then advance the bolus towards the oropharynx. Okay, so once the bolus enters the oropharynx, we then begin the swallowing reflex. Next is pharyngeal. The bolus comes into contact with the arches of your palate. The larynx elevates and the epiglottis folds down. to block the airway because we don't want food to get into our airway. Then the bolus goes past the airway through the pharynx down into the esophagus, which is the esophageal phase. So in the esophageal phase, we are now in the esophagus, and peristalsis will squeeze and push that bolus down the esophagus towards the stomach. So as the bolus gets down to the bottom of the esophagus, there is an esophageal sphincter here that closes off the esophagus from the stomach. That sphincter will open and the bolus will drop into the stomach. And now we can really start breaking things down. But I want to point out as well the diaphragm here. You can see a piece of it there and you can see it here. Notice that the esophagus goes through the diaphragm. So that opening in the diaphragm is what we were talking about earlier is called the esophageal hiatus So now that the bolus is in the stomach, this is where we're going to temporarily store the ingested food as we break it down. Mechanical digestion will happen because the stomach is going to squeeze and churn and that's going to break the food down mechanically. And then we will have chemical digestion of food with acid and enzymes. Once the bolus enters the stomach, and we mix that bolus with acidic secretions of the stomach, we form what is called chyme. Chyme is the acidic, soupy mixture that we convert the bolus into once it enters the stomach. There are some main regions of stomach that you'll be looking at in your lab, most likely. Things like the cardia, fundus, body, and the pyloric part of the stomach. The cardia is the superior medial portion of the stomach where there's lots of mucus glands. The fundus is superior to where the stomach and esophagus join. The body is between the fundus and the curve of the J in the stomach. It's the largest region of the stomach and is the mixing tank for ingested food and secretions. The pyloric part is between the body and the duodenum or duodenum. The shape changes often during digestion. The pyloric antrum connects to the body and the pyloric canal empties into the duodenum or duodenum. The pylorus is muscular tissue surrounding the pyloric orifice, which is the stomach outlet, and the pyloric sphincter is a thick circular layer of muscle within the pylorus. So let's point out some of these things we just listed. So looking at this abdominal cavity here, we can see for one, there's the diaphragm. There's the esophagus, which is going through the diaphragm into the stomach. See all this yellow stuff? That's the greater omentum, which has got fat in it. And then we can see the liver here, gallbladder, which we will eventually talk about. So here's the stomach, and we can look at some of that anatomy now. So here's our esophagus. This area would be the cardia. This raised area, fundus. this middle area body and then down here we have the pylorus which is the bottom part of the stomach so here is the pylorus where it narrows is the pyloric antrum and then right before the pyloric sphincter we have the pyloric canal There's the pyloric orifice or sphincter which leads into the duodenum. Okay, the rugae, which are these little waves inside the empty stomach, are folds in the mucosa of the empty stomach. And they'll stretch out as your stomach fills with food or liquids. This allows for expansion. of your stomach lumen up to 50 times its empty size. The muscularis mucosae and the muscular layer of the stomach contain extra layers of smooth muscle cells. Specifically, we've got an oblique layer added in the stomach. This is in addition to the typical circular and longitudinal layers that we see elsewhere. So looking at the histology of the stomach, the stomach is lined with simple columnar epithelia. The epithelium is a secretory sheet that will make mucus to cover the interior surface of the mucosa. In the mucosa, we're also going to see gastric pits. Gastric pits are depressions that open out into the stomach lumen. There are mucus cells in these pits. that divide and are going to replace superficial cells. Gastric glands are found in the fundus and body. These extend deeply down into the underlying lamina propria. Each gastric pit communicates with several gastric glands and in those glands we have parietal cells and cheap cells. They secrete about 1500 milliliters of gastric or stomach juices each day. So here's the histology of the stomach. So we've got the mucosa where we can see the rugae. We can also see the simple columnar epithelia that line the mucosa. There's our submucosa, our muscle layer. But notice we've got three. layers in the muscle. We usually have two, longitudinal, circular, oblique, and there is a serosa. But notice up in the mucosa we have these gastric pits, which are openings to the gastric glands that go deep down in the mucosa. And we said that in those glands we're going to have parietal and chief cells. And we're also going to have mucus cells as well. And we will have some cells that release hormones, which we haven't mentioned yet. So let's take a zoomed in view of one of these gastric glands. OK, so here's the neck of the gastric pit, which connects to the gastric gland. And we can see parietal cells. We can see chief cells and we're going to talk about these G cells. We haven't talked about those yet. So let's talk about parietal and chief first. So the parietal and chief cells, the parietal are common in the gastric glands and they secrete what's called intrinsic factor. Intrinsic factor is a glycoprotein that will help us absorb vitamin B12. Vitamin B12 is very important for red blood cell production. We also secrete hydrochloric acid, and we need a strong acid to help us break down the food that enters the stomach. Chief cells are found near the base of the gastric glands, and they secrete pepsinogen. Pepsinogen is inactive, so once it's secreted, it has to be converted to... pepsin which is an active enzyme that breaks down protein but as soon as that pepsinogen makes it into the stomach lumen and comes in contact with hydrochloric acid it converts to pepsin and then it can start breaking down protein pyloric glands are located in the pyloric part of the stomach and produce mucus Enteroendocrine cells produce at least seven hormones. There are two we're focusing on now. G cells make gastrin and D cells make somatostatin. Gastrin is going to stimulate the parietal and chief cells to secrete more and also cause contractions in the gastric wall. So this is going to be very important once the food actually enters the stomach. We want this gastrin to be released because it will crank up the activity of the parietal and chief cells. D cells do the opposite. They're going to release somatostatin, which will inhibit the release of gastrin. So if we're nearing the end of stomach activity, then we would want to kind of calm everything down. Somatostatin can help us do that by inhibiting gastrin. So what is actually broken down in the stomach? We have some digestion of carbs by the salivary amylase that we added and lipids by lingual lipase. As stomach contents become more fluid, the pH will approach 2, which is pretty acidic. The preliminary digestion of proteins will begin by pepsin. But nutrients are not absorbed well in the stomach at all. And one of the main reasons for this is we still aren't quite. chemically broken down enough but also the stomach is coated in a very thick mucus that protects the stomach mucosa from the acid that is in the stomach and because of that mucus that also makes it very difficult for nutrients to get through the stomach wall. So gastric or stomach activity is regulated by the production of acid and enzymes you by the gastric mucosa, which can be controlled by your central nervous system, the enteric nervous system, which is your digestive nervous system, and we've seen it can also be affected by hormones of the digestive tract. There are three overlapping phases of gastric control that we're going to look at. We have the cephalic phase, the gastric phase, and the intestinal phase. Cephalic, gastric, and intestinal. So let's begin with cephalic phase. The cephalic phase of gastric secretion will begin when you see, smell, taste, or think about food. This phase is directed by your central nervous system because you're thinking about food. It will prepare the stomach to receive the food. It's going to cause gastric juice production to speed up. So the stomach kind of waters with your mouth getting ready for you to bring food in. So there's going to be production of acid and enzymes and everything's going to be all good and ready for when that food drops in. Next we have the gastric phase. The gastric phase is once food has finally arrived in the stomach. We kind of started stimulating the stomach in the cephalic phase, so we're going to build on that now. This phase is longer and can continue for three to four hours while the acid and enzymes process the ingested materials. It could be even longer than that. Some things that initiate this phase are stretching of the stomach wall because you added food to it, an increase in the pH of the gastric contents, again because you added food to it, and the presence of undigested materials in the stomach like proteins and peptides all stimulate the gastric phase. Now in the gastric phase there are three mini phases we should be aware of, starting first with the local response. If you're following along in the text that goes with this lecture you would find This information on page 906. The local response begins with stretching in the stomach wall. This stretching will cause the release of histamine, which will stimulate acid secretion. So stretching will cause more acid secretion. The stimulation of stretch receptors and chemoreceptors in the wall of the stomach will activate the stomach's secretory cells, producing powerful contractions called mixing waves. This is the neural response. So again, the neural response, the stretching of the stomach wall and chemoreceptors trigger the activation of the stomach's secretory cells. So we'll have more secretions and we'll also have stronger mixing waves in the stomach wall. This is the neural response. Finally, we've got the hormonal response. Neural stimulation and the presence of peptides and amino acids in the kind will stimulate secretion of gastrin by the G cells. Remember, gastrin is a hormone that stimulates secretion of those. Parietal and chief cells. Gastrin will travel in the bloodstream to the parietal and chief cells who increase their secretions reducing the pH of the gastric juice. We'll also have more stomach churning. So all of this secretion and all of this stomach churning is going to cause that food to get mixed up really well and dissolve down really well. So once the chyme has been broken down as much as we can break it down, we are then going to enter the intestinal phase. And the intestinal phase is the emptying. the stomach. So this is when chyme drips down into the small intestine. So when the chyme leaves the stomach and we're moving it down into the small intestine, the chief and parietal cells will stop secreting and peristalsis and segmentation will also stop. So everything has just gotten quiet and we're allowing this chyme to exit the stomach. So the pancreas and liver and gallbladder are going to help out a bit too. These are our accessory organs. The pancreas is behind the stomach and extends from the duodenum towards or duodenum toward the spleen. It is retroperitoneal. We talked about that at the beginning of the chapter and bound to the posterior wall of the abdominal cavity. It's wrapped in a very thin connective tissue capsule. So here's a picture of the pancreas. You can see that it has a duct running through it and that duct is going to enter the duodenum which is your small intestine. So we talked about the pancreas before. in the endocrine system, but we're not really focusing on the endocrine part of the pancreas in this chapter. Remember when we talked about insulin and glucagon? We're going to focus on the exocrine function, so what the pancreas releases through the duct. So the acinar cells and the epithelial cells of the duct system are found in the pancreas, and they will help secrete an alkaline pancreatic juice. into the small intestine. About a thousand milliliters per day and this is going to contain digestive enzymes, water, and ions. It's controlled by hormones from the duodenum. Pancreatic enzymes include pancreatic alpha amylase, pancreatic lipase, nuclease, and proteolytic enzymes. So looking first at the pancreatic alpha amylase. This is a carbohydrate. And remember that anything that ends with ace is an enzyme. So a carbohydrate is going to break down carbohydrates like starches. It's very similar to salivary amylase. Pancreatic lipase will break down certain complex lipids. Nuclease breaks down RNA or DNA. and proteolytic enzymes will break apart proteins. Protease is one type which breaks apart large protein complexes and peptidases break down small peptide chains into individual amino acids which we know is the component of protein. Liver is the largest visceral organ and performs essential metabolic and synthetic functions. The liver is made up of hepatocytes, which are liver cells, and they will help to adjust the circulating level of nutrients through selective absorption and secretion. Here is the liver. We have a right lobe of the liver and we have a left lobe of the liver. And underneath we have the gallbladder, the histology of the liver. So each lobe is divided by connective tissue into approximately 100,000 lobules. Lobules are the basic functional units of liver and are only about one millimeter in diameter. The hepatocytes form a very organized pattern, a series of irregular plates arranged like wheel spokes. So here's a picture of that. You can see the individual liver cells, which are these, and they are in fact arranged in a wagon wheel pattern. If we zoom in on this a little bit, can look a little bit closer, here are the individual liver cells, hepatocytes. So the liver will secrete bile and bile breaks down fat. secretes the bile into a network of little chambers or channels, excuse me, called bile canaliculi. So the cells are making bile and they put the bile into the bile canaliculi. this is going to train down toward the bile duct. So the right and left hepatic ducts collect bile from all the bile ducts of the liver lobes and unite to form a common hepatic duct. From the common hepatic duct the bile enters either the bile duct which empties into the duodenal or duodenal ampulla and the cystic duct which leads to the gallbladder. So I'm going to show you this network of ducts so that you can actually see what the heck's going on because it's a little bit confusing when you just read it. The bile duct is formed by the union of the cystic duct, which is coming from the gallbladder, and the common hepatic duct, which is coming from the liver. This will penetrate the wall of the duodenum and meet the pancreatic duct at the duodental ampulla. So let's work all of that mess out. So here we are underneath the liver. So there's our liver. Here's our gallbladder. Here's the stomach, duodenum, curving around. And there's pancreas, just a little piece of it. So this is a hepatic duct coming from the left lobe of the liver. And this is the hepatic duct coming from the right lobe of the liver. So each of these is draining bile. The left and right hepatic duct combine to form the common hepatic duct. The common hepatic duct moves down and connects with the cystic duct of the gallbladder. So here's the cystic duct. The cystic duct joins with the common hepatic duct to form the big duct. which is the bile duct. The bile duct moves down, goes behind the stomach, and then enters the duodenum here. So let's zoom in right here at this little opening. Here's our bile duct that we followed right to the opening. And here is our pancreatic duct. And both of them connect to the duodenal. ampulla. So we've got the bile coming out here and we've got the pancreatic juices coming out here right into the duodenum. And the duodenum is sometimes called the mixing bowl of the small intestine because it receives chyme and it also receives the juices from the liver and the pancreas. So all blood leaving the absorptive surfaces of the digestive tract will enter the hepatic portal system. flow into the liver, and then the liver cells can extract nutrients or toxins from that blood before the blood reaches the systemic circulation through hepatic veins. The liver will remove and store excess nutrients for later. It can correct nutrient deficiencies by sending out stored reserves if we need them or performing synthetic activities, which means building new things that we're deficient in so that we can correct those deficiencies. The liver produces bile. Bile salt in bile breaks lipid droplets apart, which is called emulsification in the duodenum. This creates tiny emulsion droplets that are coated with bile salt. This helps to increase the surface area exposed to enzymes, which is really necessary because mechanical digestion in the stomach creates large droplets of lipids, so we need to get smaller. Pancreatic lipase can interact only at the surface. The gallbladder is a hollow pear-shaped muscular sac that stores and concentrates bile prior to secretion into the small intestine. Moving to the small intestine. The small intestine is a long muscular tube where we are going to have the completion of chemical digestion and 90% of nutrients will be absorbed here. So this is really where the nutrient payoff happens. They consist of three segments, the duodenum, the pancreas, Jejunum and the ileum. Looking at the duodenum first. The duodenum is the first part of the small intestine that connects to the stomach. It's 25 centimeters long and is known as the mixing bowl because it's going to receive chyme from the stomach and also digestive secretions from the pancreas and liver as we just saw. The middle segment of the small intestine called the jejunum is 2.5 meters long and the site of most chemical digestion. and nutrient absorption. The ileum is the final segment of small intestine, three and a half meters long, and ends at the ileocecal valve. There's a sphincter here, which is a sphincter, that controls the flow of material from the ileum into the cecum of the large intestine, which is our last stop. So in this picture we have the duodenum at the top. We then have the jejunum, which is in purple, and the ileum, which is in pink. The ileum comes around and enters the large intestine right about here at the ileocecal valve or sphincter. The histology of the small intestine. We have circular folds, transverse folds in the intestinal lining. These are not like rugae, they're permanent. They don't disappear when the intestine fills. We have intestinal villi, which are finger-like projections in the mucosa of the small intestine, covered by simple columnar epithelia, and they have on them microvilli, which form a brush border. So this is a lot of folding. The circular folds are also called pleca. So this is the intestine cut open and these are the folds that I was talking about called circular folds or pleca and they are not for expansion of the organ. They're actually for increasing the surface area so we can absorb more nutrients. So they are permanent folds. Pleca or circular folds. Now going ahead, we're going to come back to these notes, but going ahead on the pleca or circular folds, we have villi, which are finger-like projections we've talked about a lot back in the anatomy one content. Villi we know are for absorption. So the pleca are covered in villi. And so we've got the pleca to absorb nutrients across. We have all the villi to absorb nutrients across. And then the villi have on them microvilli. So we're going to get even smaller in the next picture that we look at. But here's our histology. So we have the mucosa, which is covered in villi, submucosa, muscularis or muscular layer, and So what I want to do now is zoom in on one villus, one villus. So if we zoom in on one villus, that is what it would look like. So you can see a little peach fuzz of microvilli on this villus, which further increases surface area for absorption. So now we can absorb tons of nutrients across the intestinal epithelia. Inside of the villus, we can see a capillary network, and we can also see this green thing, which is called a lacteal. And the lacteals are connected to the lymphatic system. So let's talk about what they do. So going back, the lacteal is a lymphatic vessel in each villus. It's going to transport chylomicrons, which are fat globules. that are too large to enter the blood capillaries. Intestinal glands are going to extend deep into the lamina propria, and they contain stem cells near their base that produce new epithelial cells to repair any damage that we might have. 1.8 liters of intestinal juice enters the intestinal lumen every day. The intestinal juice will help keep the chyme moist and assist in buffering acid. Keeps the digestive enzymes and products of digestion in solution nice and liquidy as well. Intestinal motility. So after the chyme arrives in the duodenum or duodenum, weak peristaltic contractions will move it slowly toward the jejunum. Parasympathetic stimulation will accelerate local peristalsis and segmentation. The movements of the mucosa will increase the absorptive effectiveness because we're going to stir and mix the intestinal components, which will eliminate local differences in nutrient concentration. So once we're done in a small intestine, we move to the last stop, which is the large intestine or the colon. It is horseshoe shaped and extends from the end of the ileum to the anus. It lies inferior to the stomach and liver and frames the small intestine. It's about 1.5 meters long and 7.5 centimeters wide. Some of the main components. We have the cecum, which is the pouch-like first portion, the colon, which is the largest portion, and the rectum, which is the last 15 centimeters and the end of the digestive tract. So the cecum, or the expanded pouch, is going to receive and store materials from the ileum and begin the compaction process. The appendix is a skinny hollow structure about nine centimeters long. It's attached to the surface of the cecum and dominated by lymphoid nodules. There's also a mesoappendix mesentery that will connect the appendix to the ileum. the cecum. So let's go forward and take a look at that. So here's where the ileum, they've removed the small intestines from our picture, but here's the ileum coming in. There's the ileocecal valve or sphincter, and this is the first part of the large intestine known as the cecum, which is this pouch. So the cecum has on it the appendix. There that is. Okay, so next we move to the colon, and the colon has a larger diameter and thinner wall than the small intestine. It also has a feature called haustra. Haustra are pouches in the wall of the colon that permit expansion and elongation of the colon. We're also going to see tinea coli, and tinea coli are three longitudinal bands of smooth muscle that run along the outer surface of the colon. They're going to be responsible for creating the hostra because of their muscle tone. So let's take a look at that as well. Four regions of the colon, the ascending colon, transverse colon, descending colon, and then the sigmoid colon. So let's look at all of those. So first of all, These little pouches or little segments that look like the segments of a caterpillar body, those are the hostra labeled here. Then we've got this ribbon of muscle, the tinea coli, that goes around and around. And it's responsible for helping to create these hostra. for expansion and then we've got the region so we talked about the cecum but here going up is the ascending colon and it turns right here at the right colic flexure we then have the transverse colon going this way we have the left colic flexure so another twist descending colon and then sigmoid colon. And then we'll eventually talk about the rectum, which I believe will happen actually now. So the rectum forms the last 15 centimeters of the digestive tract, and it is an expandable organ for temporary storage of feces. The anal canal is the last portion of the rectum that contains small longitudinal folds. And the anus is the exit of the anal canal with keratinized epidermis like the skin. The internal anal sphincter has circular muscle, smooth muscle cells, and is not under voluntary control. The external anal sphincter encircles the distal portion of the anal canal, has skeletal muscle fibers, and is. under voluntary control. The histology of the large intestine. It's going to lack villi because we're really not trying to absorb tons and tons of nutrients anymore. We've already done about 90% of that. We have an abundance of goblet cells which make mucus and intestinal glands that are deeper than the glands of the small intestine with goblet cells in them. to provide mucus for lubrication for fecal material. So here's the histology of the colon or large intestine. And we have the mucosa with the deep mucus glands. We have the submucosa, muscularis or muscular layer, and then the serosa. The functions of the large intestine are mainly for absorption or reabsorption of water, less than 10% of nutrients. We're going to get rid of bile salt, organic waste, and we're going to absorb vitamins and toxins produced by bacteria. Compaction of intestinal contents into feces and the storage of fecal material. prior to release or defecation. The microbiome or microbes in the gut or that live in the human body, these are things like bacteria, fungi, and viruses, and we have quite a few of them in the large intestine. They are very important in helping us to absorb some of the vitamins that we would not otherwise be able to. get from our food without their help. So vitamins are organic molecules that are important as cofactors or coenzymes in your metabolism. Normal bacteria in your colon make three vitamins that supplement your diet. So we need this bacteria. Vitamin K which is produced by the bacteria is required by your liver for synthesizing four different clotting factors including prothrombin. Biotin is important in glucose breakdown. Vitamin B5 is required in the manufacture of steroid hormones and some neurotransmitters. Organic waste we want to get rid of. The bacteria convert bilirubin to urobilinogen and stercobilinogen. So these are all products of the breakdown of hemoglobin. from recycling of blood. Some urobilinogen are absorbed into the bloodstream and excreted in your urine. Urobilinogen and stercobilinogen remaining in the colon are converted to urobilins and stercobilins by exposure to oxygen. We're going to get rid of those. Organic waste continued. Bacteria break down peptides in feces and generate ammonia soluble ammonium ions. The large intestine motility, gastro ileal and gastroenteric reflexes will move materials toward the cecum or into the cecum while you eat. The movement from the cecum to the transverse colon is very slow allowing hours for water absorption. Peristaltic waves will move the material along the length of the colon. Segmentation movements will mix the contents of the adjacent hostra. Movement from the transverse colon through the rest of the large intestine results from powerful peristaltic contractions or mass movements. The stimulus is distension of the stomach and duodenum. which is relayed over the intestinal nerves. The distension of the rectal walls initiates the defecation reflex. Both of these are triggered by stretch receptors in the rectum. So a balanced diet is going to contain nutrients like carbohydrates, lipids, proteins, vitamins, minerals, and water. The processing and absorption of nutrients. The digestive system is going to break down the physical structure of food and then disassemble that into molecules. The molecules released into the bloodstream will be absorbed by cells and either broken down to give us ATP or energy or used to make new things like carbohydrates, proteins, and lipids. Digestive enzymes will break molecular bonds in large organic molecules, carbohydrates, proteins, lipids, and nucleic acids. The process of this breakdown is called hydrolysis, which we talked about in chapter 3. It's divided into classes by their specific substrates. Carbohydrates break bonds between simple sugars. Proteases break bonds between amino acids. And lipases separate fatty acids from glyceride. The digestive enzymes we've seen. are secreted by the salivary glands, tongue, stomach, and pancreas. Some age-related changes that we see as well. The division rate of epithelial stem cells will decline. Smooth muscle tone will decrease. The effects of cumulative damage over time become more apparent, and cancer rates increase. Dehydration is common among the elderly and changes in other systems have direct or indirect effects on the digestive system. The digestive system has an extensive anatomical and physiological connection to several other systems, for example, the nervous, cardiovascular, endocrine, and lymphatic. The digestive tract is an endocrine organ because it produces a variety of hormones and it also will produce neurotransmitters. This concludes Chapter 24, The Digestive System. Next, The Urinary System.

The digestive tract, also known as the gastrointestinal tract, GI tract, or alimentary canal, this is a muscular tube. that extends from your oral cavity or mouth area all the way down to the anus. There will also be involved accessory organs that are helping with the process of digestion like the teeth, tongue, and various glands that will help to secrete products that will assist in digestion.

So this is an overview of the main components of the digestive tract that we will be going through individually, beginning at the oral cavity, which is where we ingest, where food comes in. We then move the food through the pharynx, to the esophagus, down to the stomach, small intestine, and then large intestine. In assisting along the way, we're going to use the liver, the pancreas, and also the gallbladder to help us out of it. The main processes of the digestive system include ingestion, mechanical digestion and propulsion, chemical digestion, secretion, absorption, and finally defecation. So let's go through each of those first.

Ingestion occurs when food enters your oral cavity. This is taking food into the mouth. We will then begin to mechanically digest. So mechanical digestion is different from chemical digestion in that with mechanical digestion, we are just breaking the food up into smaller, more manageable pieces so that we can actually swallow that food.

We're not really doing much in the way of actual chemical breakdown at this phase. We're more just changing the size of the food. So we're going to crush it and shear it in our oral cavity.

And then we will propel that food or move it down through the pharynx into the digestive tract. And once we swallow that food, it is no longer under our conscious control. Next is chemical digestion, which is when we chemically break down the food itself into small organic fragments.

So, for example, if we eat protein, we would like to get that protein down to amino acids. If we had some carbs, we want to get those carbs down to individual monosaccharides. We want to get down very small because at that point we can then absorb that material across.

the digestive epithelia and get benefit from it. Secretion is when the digestive tract releases water, acids, enzymes, buffers, and salts from the epithelium of the digestive tract or glands and gallbladder. And all these secretions are going to assist us in the breakdown of this food so that we can actually get nutrients from it.

Absorption is probably the best payoff of this whole process. We're going to move organic molecules, electrolytes, vitamins, water, and minerals across our digestive epithelium into the interstitial fluid of our digestive tract. So we're going to gain these nutrients. Defecation is the elimination of the waste from our body.

So anything we can't use. from that broken down food. We get all we can out of it and then whatever's left will be compacted.

dehydrated because we want to keep as much water as we can as well and then what's left over is called feces which is a waste that we do want to get rid of the lining of our digestive tract safeguards the surrounding tissues against the corrosive effects of digestive acid and enzymes which can be very strong mechanical stress such as abrasion bacteria that we ingest with our food or that already live in the digestive tract. So next let's go through the serous membranes of the digestive area before we move on into more specifics. So the peritoneum is a serous membrane that lines the peritoneal cavity and there are two types of this peritoneum. We have the visceral Peritoneum, which is also known as the serosa, it's going to cover the organs inside of the cavity and the parietal peritoneum is going to line the inner surface of your body wall.

So this is similar to the visceral and parietal pleura that we talked about and similar to the visceral and parietal pericardium that we also talked about. Visceral covering organs. parietal lining cavities and the two of these are going to produce peritoneal fluid and peritoneal fluid is going to allow sliding of the parietal and visceral surfaces without any friction or irritation so this is kind of like a lubricant and about seven liters of it are produced and absorbed every day but very little is in the peritoneal cavity at one time There is a condition called ascites and this is when we have abdominal swelling due to the buildup of peritoneal fluid. So looking a little further at some other membranes and we're going to take a look at these in pictures as well so we can get a visual but mesenteries are double sheets of peritoneal membrane and their job is to support portions of the digestive tract inside your peritoneal cavity. They're going to help to support and suspend.

They connect the parietal peritoneum with the visceral peritoneum and are also going to allow for a route to and from the digestive tract for blood nerves or blood vessels, specifically nerves and also lymphatic vessels. They'll stabilize the position of the organs. and prevent the intestines from becoming tangled up, which is obviously a big deal.

So let's focus on two of the bigger, well, we're actually going to look at more than two, but there are two kind of big ones that we're going to see, and then we're going to add on some other ones as well. So let's begin with the lesser omentum. So the lesser omentum is, to me, it looks a little bit like a stomach hammock. It's going to stabilize the position of the stomach. The stomach is kind of cradled in the lesser omentum.

And it's going to provide an access route for blood vessels and other structures entering or leaving the liver. So it kind of has a double purpose. But in this drawing, you can see what looks a little bit like a hammock around the bottom of the stomach.

A fun little illustration of how that works over here as well. But that's the lesser omentum, a stomach hammock. Next, we have the dorsal mesentery. This is going to enlarge to form an enormous pouch.

So this was the second large one that I was referring to called the greater omentum. And the greater omentum is going to extend inferiorly or down between your body wall and your anterior surface of the small intestine. Hangs like an apron from the lateral and inferior borders of the stomach So if that totally confused you all those directions what we're basically saying here is that it attaches and then hangs down in front of your intestines and between your skin and your Muscle a bit like an apron would if you were wearing an apron. So let's take a look at where exactly that is to make it a little bit more approachable.

So this is a side view. through the abdominal area. And so this would be our vertebral column.

Here's the liver. Here's the stomach. There's a piece of the colon. And most of the other intestines have been removed.

But this little loop here that hangs down, loops back up, and then connects to the colon, that is the greater omentum, or the apron. So it hangs down like an apron. And we're going to look at this in an official book illustration as well.

But the greater omentum is awesome because it does so many important things. It's going to actually conform to the shape of your surrounding organs. It pads and protects the surface of your abdomen because you don't really have any bone right there. It provides insulation to reduce heat loss and stores lipid energy reserves.

So we store fat in our greater omentum. And this is good because it allows us to store energy reserves, but it can be bad. So if we gain too much weight and we store that fat in our greater momentum, it contributes to the beer belly.

You know the kind of belly that gets so big that it hangs over the pants? So sort of like this over here. So that hanging belly, that's the beer belly.

And that's fat stored in this shape. of the greater omentum. The mesentery proper is a thick mesentery sheet. It's going to give more stability but permits some movement and it will suspend all but the first 25 centimeters of the small intestine.

The mesocolon is a mesentery associated with part of the large intestine. So whenever we see large intestine, we should think colon because it's the same. So during development, the mesocolon of your ascending colon, descending colon, and rectum will fuse to the posterior or back body wall to lock the regions in place. So let's get a view of all of this because reading about it is not so clear as actually seeing it. So here's a front view and let's just review.

We've got the lesser omentum. That's our stomach hammock. The greater omentum is not pictured in this. That's the apron, because if it were here, it would be covering all of this. We wouldn't be able to see the other things that we were talking about.

So the next one was the mesentery proper. So the mesentery proper is this one here, the mesentery sheet. or mesentery proper and you can see how it's helping to stabilize the organs in here and then we had the mesocolon and the mesocolon we've got some here and here and also here so that's all mesocolon remember that's going to help to stabilize the intestines and organs in this area so that they can't just kind of squirm around wherever they want to because our intestines do move.

So they need to be able to move, but we don't want them to get all tangled up. So this helps to keep them stable and in position. Here's a side view.

So we've got our vertebral column just to orient you and we've got our liver, stomach, colon, and then these smaller tubes are our small intestines. This is a female, so we have a uterus there, we have a bladder, and then here's the rectum. A couple things I want to point out here, a few things.

This is the peritoneal cavity, so the inside here is peritoneal cavity, and it's lined with the parietal peritoneum, which we already talked about. That's the lining. And then the organs are covered with, you can see this white coating on the liver.

and the stomach, for example, that's the visceral peritoneum. We can also see in this picture, here's the greater omentum, the apron hanging down and looping back up to the colon, which is where we store fat. But it's also important to note that there are some things in the digestive tract that are not within the peritoneal cavity. So you can see the peritoneal cavity border goes around like so, goes around the uterus and then it goes up and down and back around here.

To make a loop, you just follow the white membrane, but look what's outside of it. Pancreas, duodenum, which is part of the small intestine, and the rectum are not in the peritoneal cavity. They're behind it. So the word for that, when you're behind or outside of the peritoneal cavity, that's called retroperitoneal.

And another thing that's retroperitoneal is... going to be the kidneys, which we'll talk about in the next chapter, but they too are outside of the peritoneal cavity. So you will see the word retroperitoneal again, but I just wanted to be sure you knew what it was. All right, so what we're going to do now is we're kind of transitioning into looking at the tissue a little bit more closely.

And whenever we look at tissues of anything, we call that studying the histology. So histology is taking a look at the tissues. And what we're going to be doing is we're basically going to imagine that we took a scalpel and cut a plug out of a digestive organ wall.

Okay, so that could be the colon, it could be the stomach, it could be the esophagus. We're going to look at all of them. But when we take a chunk out of one of these digestive organs, we can see that the wall of the digestive organ is going to have some layers in it.

And those layers are the tissue layers, which we refer to as histology. Now, I do want to give you one other important word here that you're going to see again. The hollow inside of an organ is called the lumen, L-U-M-E-N, lumen.

So the hollow inside of the stomach, lumen. The hollow inside of the transverse colon, lumen. The hollow inside of the small intestine, the uterus, the bladder, lumen. So you may see that word, and it's certainly going to be important in this chapter.

So the layers of the digestive tract. Here's why lumen matters. We're starting closest to the lumen, so closest to the inside, and working our way out to the outside of the organ.

So mucosa is the first layer beside the lumen, followed by submucosa, muscularis layer, and serosa. And we're going to look at those individually. But the lining of the digestive tract will vary depending on where you are in the digestive tract.

So every organ is not going to look the same as far as its histological organization, which is why we're going to look at all the... main organs. But before we do that, still keeping it general, here is a digestive organ. And this, of course, would be the, hopefully you said lumen, hollow inside. And as we work our way through, we go lumen.

And then this would be mucosa because it's closest to the lumen. submucosa, muscle layer, and then the outermost, which covers the organ, is serosa. So let's get even closer to this.

We are going to zoom in right about in here. So again, we have the mucosa, we have submucosa, muscularis or muscle layer and then finally serosa now let's focus on mucosa first naturally so the mucosa is made up of three components we've got mucosal epithelium so this is the the outermost epithelial tissue and remember epithelial tissue is tissue that is avascular it lines or covers body surfaces And it typically has glands in it to produce secretions. So we've got the epithelium.

We then have under the epithelium this stuff, which is called lamina propria. And lamina propria is areolar tissue, which we learned about in Chapter 4. And it's also got in it blood vessels, lymphatics, sensory nerves. And then under that, we have...

the muscularis mucosae and the muscularis mucosae is of course made of muscle so those three things make up the mucosa in general. So we've got an overview of that in our notes here as well. So let's take a look at that.

So mucosa inner lining of your digestive tract. The mucous membrane is going to consist of, as we said, epithelial tissue, which has secretions from glands, the lamina propria, and then the muscularus mucosae, which is going to be under it. Digestive epithelium.

So mucosal epithelium can be simple or stratified depending on the location, function, and stress of the area. The oral cavity, pharynx, esophagus, anal canal will all be stratified, squamous epithelium. The stomach, small intestine, and large intestine, or most of it, will be simple columnar epithelium. We will also find enteroendocrine cells scattered around as well.

These are going to secrete hormones that will help to coordinate the digestive tract activities and also the accessory glands. So let's move down now into the submucosa. So the submucosa is here. Well actually before we go into submucosa, let's talk a little bit more about the lamina propria. So we talked about the mucosal epithelium of the mucosa, but let's talk a little bit about the lamina propria before we go on to submucosa.

So the lamina propria is a layer of areolar tissue that has in it blood vessels, as we mentioned before, sensory nerve endings, lymphatics, smooth muscle cells, and scattered lymphatic tissue as well. Under that, the final part of our mucosa is our muscularis mucosae. And the muscularis mucosae is in most areas of your digestive tract under the lamina propria. It's a very thin sheet of smooth muscle and elastic fibers.

The smooth muscle is arranged into two concentric layers. The inner layer circles the lumen. Remember what that is, that's circular muscle going around and around the lumen in a circular way. And then the outer layer contains cells arranged parallel to the long axis of the tract. So these are going longitudinally around or parallel to the long axis of the tract.

So this again is the mucosa. So when we move out of the mucosa, which had three layers. epithelium, lamina propria, and muscularis mucosae.

We go into the submucosa, which is a layer of dense irregular connective tissue. Now because it is connective tissue, it is highly vascular. Lots of blood vessels, lots of lymphatic vessels, and there may be some glands in here as well, depending on where we are, that will secrete buffers and enzymes.

into your digestive tract. So that would be of course this area, submucosa, here and here. So moving down we have next the muscular layer which is here. And the muscular layer of course is going to be dominated by smooth muscle cells because We don't really have conscious control over our digestive processes for the most part.

So smooth muscle and there's going to be an inner circular layer and an outer longitudinal layer. And these layers are involved in mechanical digestion. Here we are mechanical digestion, which is when we. crush the food up into smaller bits, and they're also responsible for helping to move materials along your digestive tracts and moving everything forward.

These movements are coordinated by the ENS, which is the enteric nervous system, so the nervous system that controls digestion. It's controlled primarily by parasympathetic division. So we talked about previously that Parasympathetic is often referred to as rest and digest.

So we digest better when we are not under stress. So the parasympathetic division does stimulate digestion. So last we have the serosa, which is the serous membrane covering the muscle along most parts of the digestive tract enclosed by the peritoneal cavity.

In areas where there is no serosa, we might have instead an adventitia. An adventitia is a dense network of collagen that firmly attaches the digestive tract to other structures. So let's go back.

to serosa and see where that is again. So the serosa is the outermost and covers the muscle. So let's talk now about the motility of the digestive tract. So the actual movement process of things through the digestive tract. So visceral smooth muscle tissue is going to cause rhythmic cycles of activity.

which is controlled by pace setter cells. And these cells are going to undergo spontaneous depolarization. Waves of contraction will spread throughout the entire muscular sheet. And one of these processes is called peristalsis.

Peristalsis is the waves of muscular contractions that move a bolus along the length of the digestive tract. A bolus is a small oval mass of partially digested food. So once you chew up the food, mix it with a little bit of saliva, and push it back into the pharynx and esophagus, peristalsis will push that ball of food forward onto the next phase.

So this is what peristalsis looks like. So here is the bolus. And you can see the muscle layer here. We've got the circular muscle closest to the bolus and the longitudinal furthest from the bolus. And when the muscle contracts, it pinches behind the bolus and that pinching motion will propel the bolus forward.

So a wave of contraction in your circular muscle will move the bolus forward. And we have peristalsis. Segmentation are cycles of contraction that will churn and fragment the bolus, mixing the contents with secretions from your intestine. Regulation of digestive functions. So we have local factors, neural mechanisms, and hormonal mechanisms, which will all affect the regulation of digestive functions.

So looking at local first, the pH, the volume, or the chemical composition of the contents in your intestines or stomach can have direct localized effects on your digestive activity. Stretching of the intestinal wall can stimulate localized contractions and local factors can stimulate the release of chemicals like prostaglandin, histamine, and other chemicals that can affect adjacent cells. And this can all stimulate digestion. Neural mechanisms, visceral motor neurons, control smooth muscle contraction and when the glands actually secrete. Neural mechanisms, those enteroendocrine cells we mentioned before.

the digestive tract produce many peptide hormones that can affect every aspect of digestion and even some other systems they travel through the bloodstream to reach the target organs so we're going to see some examples of these three types of regulation as we go through the details of the chapter so let's begin where it all begins in the oral cavity So the oral cavity is going to function in sensory analysis, so taste, temperature of food before we swallow, mechanical digestion by using our teeth, tongue, and palate to help break the food down, lubrication by mixing the food with mucus and saliva, so getting it all slippery, to send it down, and limited chemical... digestion. So there will not be much chemical digestion in the mouth, but what we can begin Kind of preliminary digestion is of carbohydrates and lipids. The lining of the oral cavity, the oral mucosa, is made of stratified squamous or squamous epithelium.

It's thin and non-keratinized on your cheeks, lips, and inferior surface of your tongue. Thin vascular mucosa inferior or below the tongue. can rapidly absorb lipid soluble drugs. The mucosi of the cheeks are supported by pads of fat and muscle. And you know that little hanging down thing in the back of your throat, the uvula, that little punching bag?

The uvula is found hanging off of the soft palate. And it does have a job. It's going to prevent food from entering the pharynx too soon. So here are some of the components of the oral cavity. So there's our nasal cavity upstairs, which we looked at in Chapter 23. Here's our hard palate.

Then we've got our soft palate. And then hanging down, we have the uvula. Here's our teeth and tongue.

There's the epiglottis that we talked about in chapter 23. And then here's the pharynx, which is the back of our throat. We've got nasopharynx, oropharynx, and then laryngopharynx that were discussed in chapter 23 as well. So let's focus on the tongue for a minute. The tongue has four primary functions.

Mechanical digestion by helping to compress. Provide abrasion and distortion to the food. Manipulation to assist in chewing and to prepare the food for swallowing. Sensory analysis by touch, temperature, and taste receptors. We also use the tongue for something else.

So, speaking, for example. There are two main tongue muscles to focus on right now. The intrinsic tongue muscles, these are the ones that are smaller and they perform precise movements, like when our tongue changes in shape during speech. Extrinsic tongue movements perform most movements of the tongue.

So the tongue is important in chewing and mechanical breakdown, but so are the teeth. The teeth assist the tongue while chewing. and are made up of a mineralized matrix similar to bone called dentin.

It's very hard. The pulp cavity is the inside chamber of the tooth that receives blood vessels and nerves through the root canal. The apical foramen is the opening through which blood vessels and nerves enter the root canal. So we're going to take a look at all these parts in a picture in just a second.

The root of the tooth sits in a bony socket, also known as the alveolus, not to be confused with the air sacs in the lungs. A layer of cement or cementum covers the dentin of the root to help anchor it down into the alveolus. We've got a periodontal ligament that extends from the dentin of the root to the alveolar bone to create the lock down of that tooth and this is called a Gomphosis which is a joint a type of immovable joint that we talked about in chapter 9 It's a very strong joint or articulation.

The crown is the exposed portion of the tooth so what we can see when we look in someone's mouth and It projects beyond the soft tissue of the gingiva which are our gums and is separated from the root by the neck of the tooth. Enamel covers the dentin and forms the biting surface of the teeth which is where the food gets crushed. That's called the occlusal surface.

And cusps are the little pointy elevations or projections on top of your teeth. So let's take a look at some of these anatomical features. in a picture. So this area that's exposed is known as the crown.

of the tooth. The middle area where the tooth goes into the jaw right at the gums is the neck of the tooth. And then the part of the tooth that is anchored in the jaw is called the root. So here's the apical foramen where the blood vessels and nerves go into the dentin of the tooth. So this brown stuff here is all dentin.

And then here is the periodontal ligament that holds the tooth into the jaw. It is coated by cement, also known as cementum. So not like cement, like, you know, that kind of cement, but a hard substance that will lock the tooth into the jaw. And then the crown has on it enamel, which is the hard coating. And.

The cusps are the sharp projections, the little bumps that come off of the top of the tooth. So those are some of our main features of a typical adult tooth. Now we know we have different kinds of teeth.

We have incisors, canines, premolars, and also molars. The different groups. Here's our incisors, canines, which are a little pointier.

premolars, and then finally the molars. So let's talk about the difference between those. So incisors are blade shaped, located at the front of the mouth, and are used for clipping or cutting like when you bite into a carrot stick.

They have a single root, and the canines, also known as cuspids, are more conical in shape, so a little pointier. one single pointed cusp used for tearing or slashing. That seems a little dramatic, but you know, we'll go with it.

And they have a single root. So if you were to bite into a celery stick and then pull, that would be an example of using your canine teeth to kind of shear something. Maybe not so slashy, but more shearing.

Premolars, also known as bicuspids have flattened crowns with two prominent rounded cusps and they are used to crush, mash, and grind. They have one or two roots. The molars in the back are the largest with flattened crowns, four to five prominent rounded cusps, and they are used for crushing and grinding too, but they excel at crushing and grinding really tough. things have two or three roots so during development there are two sets of teeth that will form first deciduous which are also called primary teeth milk teeth or what most people call baby teeth there will be 20 of those and then permanent teeth after so here are the baby teeth so they're about 20 baby teeth and then after your baby teeth fall out we have permanent teeth which are going to replace those baby teeth by erupting from up underneath the baby teeth so they'll actually push up on the baby tooth until the baby tooth falls out and there can be 32 permanent teeth so And here's a picture of the permanent teeth in the top jaw and the bottom jaw.

But here's a really cool picture of a juvenile skull and you can see some of the mandible surface bone has been chipped away and the mandible. or the maxillary surface bone has been chipped away. These are baby teeth that you can see in the mouth and take a look under the baby teeth.

You see these adult teeth, these permanent teeth that are forming in the jaw and what they'll do is they'll erupt or push upward and that will help to loosen the baby teeth so that they fall out. You can see the molars developing and and getting ready to come in later and erupt and then there is an adult tooth there you can see one there as well that will eventually push down okay on to the salivary glands there are three major pairs of salivary glands that secrete into the oral cavity we have the parotid glands the sublingual glands, and the submandibular glands. Each pair has a distinctive cellular organization and produce saliva with different properties.

So let's start with the parotid. Parotids are below the zygomatic arch in your skull and they produce a serous secretion that contains salivary amylase. So amylase is an enzyme that's going to break down starches.

Sublingual glands are found under the tongue. They're covered by mucous membrane of the floor of the mouth and produce mucus which acts as a buffer and lubricant for the mouth. Submandibular glands, which are found under the jaw, lie within the mandibular groove and secrete buffers, glycoproteins known as mucans, and salivary amylase 2. Mucins are going to help reduce infection and also form a gel-like like substance.

So in this picture we can see right in front of the ear under the zygomatic arch is the parotid gland. Here's the submandibular, excuse me the sublingual, under the tongue. And then we've got the submandibular, which is under the jaw.

We're only looking at one side. So you can see there's three on this side. There would be three on the other side for a total of six salivary glands.

Saliva is produced a lot. And so there's about one to one and a half liters made every day. Ninety nine point four percent of it is water.

And the remaining 0.6% is going to contain electrolytes, buffers, glycoproteins, antibodies, enzymes and wastes. The reason for saliva is to clean the oral surfaces, moisten and lubricate food, and to keep the pH of your mouth about 7, controls populations of bacteria, and limiting acid. that they produce, which could damage your teeth and mucosa, dissolving chemicals that can stimulate taste buds, and it initiates the digestion of complex carbohydrates with salivary amylase. Mastication or chewing is when food is forced from the oral cavity to the vestibule and back across the occlusal surfaces of your teeth. The muscles of chewing will close the jaws and slide the lower jaw from side to side.

The tongue will compact this food into a bolus, which is a moist, rounded bowl that's easier to swallow. We then move to the pharynx or the throat, which is the common passageway for food, liquid, and air. Regions of the pharynx we already know, naso, oro, and laryngeal pharynx. The food passes through parts of the pharynx on its way to the esophagus.

The esophagus is a hollow muscular tube that will bring food and liquid to the stomach. It's about 25 centimeters long and 2 centimeters wide. And it begins posterior to the cricoid cartilage and will enter the abdominal pelvic cavity.

through the esophageal hiatus. So the esophageal hiatus is an opening in the diaphragm that the esophagus passes through on its way to the stomach. The resting muscle tone in the circular muscle layer of the esophagus will prevent air from entering the esophagus and backflow of materials from the stomach. The histology of the esophagus, there will be three layers.

So remember we had mucosa, submucosa, muscularis, and serosa. Notice that serosa is not in this list. The mucosa contains non-keratinized stratified squamous epithelium. The mucosa and submucosa form large folds.

The muscularis mucosae consist of smooth muscle. The submucosa contains esophageal glands that produce mucus. And the muscular layer has an inner circular and an outer longitudinal layer.

Now something to add about the muscle layer of the esophagus. The muscle layer of the esophagus is unique because in it we are going to find smooth muscle, but also skeletal, which is not typical for most parts of the digestive tract. Now we know that we swallow many times during the day and night without thinking about it.

So, swallowing can be something we're not focused on and it's happening involuntarily or it can happen voluntarily. For example, if you choose to take a sip of a drink or you choose to swallow some food, you are in control of that. So the skeletal muscle portion gives us some control, whereas the smooth muscle portion allows us to swallow involuntarily as well.

Instead of a serosa, we have an adventitia. which will anchor the esophagus to the body wall. So here is a cross section through the esophagus.

You can see it's taken about right here. The esophagus looks closed. There's not really a big hollow opening in it.

You can see right here this white area. This is the opening and you can see there's not much room. And this is good because this folded interior allows for us to prevent air. coming down every time you open your mouth and you breathe.

So this would be the lumen, which means that this dark purple area would be the mucosa. Okay, then we have the submucosa muscularis or muscle layer. And finally, adventitia. So going back to swallowing, we talked about it in our notes, but let's look at it visually. So deglutition is the fancy word for swallowing.

It can be initiated voluntarily, but proceeds automatically after that. The swallowing reflex begins when tactile receptors on the palatal arches and uvula are stimulated by the bolus so when the ball of food moves closer to the back of the throat that will stimulate us to swallow there are three phases in deglutition buccal pharyngeal and esophageal so let's begin with buccal in the buccal phase we're going to compress the bolus against the hard palate we will then use the teeth and the tongue to shape that bolus and then advance the bolus towards the oropharynx. Okay, so once the bolus enters the oropharynx, we then begin the swallowing reflex. Next is pharyngeal.

The bolus comes into contact with the arches of your palate. The larynx elevates and the epiglottis folds down. to block the airway because we don't want food to get into our airway.

Then the bolus goes past the airway through the pharynx down into the esophagus, which is the esophageal phase. So in the esophageal phase, we are now in the esophagus, and peristalsis will squeeze and push that bolus down the esophagus towards the stomach. So as the bolus gets down to the bottom of the esophagus, there is an esophageal sphincter here that closes off the esophagus from the stomach. That sphincter will open and the bolus will drop into the stomach.

And now we can really start breaking things down. But I want to point out as well the diaphragm here. You can see a piece of it there and you can see it here.

Notice that the esophagus goes through the diaphragm. So that opening in the diaphragm is what we were talking about earlier is called the esophageal hiatus So now that the bolus is in the stomach, this is where we're going to temporarily store the ingested food as we break it down. Mechanical digestion will happen because the stomach is going to squeeze and churn and that's going to break the food down mechanically. And then we will have chemical digestion of food with acid and enzymes.

Once the bolus enters the stomach, and we mix that bolus with acidic secretions of the stomach, we form what is called chyme. Chyme is the acidic, soupy mixture that we convert the bolus into once it enters the stomach. There are some main regions of stomach that you'll be looking at in your lab, most likely.

Things like the cardia, fundus, body, and the pyloric part of the stomach. The cardia is the superior medial portion of the stomach where there's lots of mucus glands. The fundus is superior to where the stomach and esophagus join.

The body is between the fundus and the curve of the J in the stomach. It's the largest region of the stomach and is the mixing tank for ingested food and secretions. The pyloric part is between the body and the duodenum or duodenum. The shape changes often during digestion. The pyloric antrum connects to the body and the pyloric canal empties into the duodenum or duodenum.

The pylorus is muscular tissue surrounding the pyloric orifice, which is the stomach outlet, and the pyloric sphincter is a thick circular layer of muscle within the pylorus. So let's point out some of these things we just listed. So looking at this abdominal cavity here, we can see for one, there's the diaphragm. There's the esophagus, which is going through the diaphragm into the stomach.

See all this yellow stuff? That's the greater omentum, which has got fat in it. And then we can see the liver here, gallbladder, which we will eventually talk about.

So here's the stomach, and we can look at some of that anatomy now. So here's our esophagus. This area would be the cardia.

This raised area, fundus. this middle area body and then down here we have the pylorus which is the bottom part of the stomach so here is the pylorus where it narrows is the pyloric antrum and then right before the pyloric sphincter we have the pyloric canal There's the pyloric orifice or sphincter which leads into the duodenum. Okay, the rugae, which are these little waves inside the empty stomach, are folds in the mucosa of the empty stomach. And they'll stretch out as your stomach fills with food or liquids.

This allows for expansion. of your stomach lumen up to 50 times its empty size. The muscularis mucosae and the muscular layer of the stomach contain extra layers of smooth muscle cells. Specifically, we've got an oblique layer added in the stomach.

This is in addition to the typical circular and longitudinal layers that we see elsewhere. So looking at the histology of the stomach, the stomach is lined with simple columnar epithelia. The epithelium is a secretory sheet that will make mucus to cover the interior surface of the mucosa.

In the mucosa, we're also going to see gastric pits. Gastric pits are depressions that open out into the stomach lumen. There are mucus cells in these pits.

that divide and are going to replace superficial cells. Gastric glands are found in the fundus and body. These extend deeply down into the underlying lamina propria.

Each gastric pit communicates with several gastric glands and in those glands we have parietal cells and cheap cells. They secrete about 1500 milliliters of gastric or stomach juices each day. So here's the histology of the stomach. So we've got the mucosa where we can see the rugae.

We can also see the simple columnar epithelia that line the mucosa. There's our submucosa, our muscle layer. But notice we've got three. layers in the muscle. We usually have two, longitudinal, circular, oblique, and there is a serosa.

But notice up in the mucosa we have these gastric pits, which are openings to the gastric glands that go deep down in the mucosa. And we said that in those glands we're going to have parietal and chief cells. And we're also going to have mucus cells as well. And we will have some cells that release hormones, which we haven't mentioned yet.

So let's take a zoomed in view of one of these gastric glands. OK, so here's the neck of the gastric pit, which connects to the gastric gland. And we can see parietal cells. We can see chief cells and we're going to talk about these G cells. We haven't talked about those yet.

So let's talk about parietal and chief first. So the parietal and chief cells, the parietal are common in the gastric glands and they secrete what's called intrinsic factor. Intrinsic factor is a glycoprotein that will help us absorb vitamin B12.

Vitamin B12 is very important for red blood cell production. We also secrete hydrochloric acid, and we need a strong acid to help us break down the food that enters the stomach. Chief cells are found near the base of the gastric glands, and they secrete pepsinogen.

Pepsinogen is inactive, so once it's secreted, it has to be converted to... pepsin which is an active enzyme that breaks down protein but as soon as that pepsinogen makes it into the stomach lumen and comes in contact with hydrochloric acid it converts to pepsin and then it can start breaking down protein pyloric glands are located in the pyloric part of the stomach and produce mucus Enteroendocrine cells produce at least seven hormones. There are two we're focusing on now. G cells make gastrin and D cells make somatostatin. Gastrin is going to stimulate the parietal and chief cells to secrete more and also cause contractions in the gastric wall.

So this is going to be very important once the food actually enters the stomach. We want this gastrin to be released because it will crank up the activity of the parietal and chief cells. D cells do the opposite.

They're going to release somatostatin, which will inhibit the release of gastrin. So if we're nearing the end of stomach activity, then we would want to kind of calm everything down. Somatostatin can help us do that by inhibiting gastrin.

So what is actually broken down in the stomach? We have some digestion of carbs by the salivary amylase that we added and lipids by lingual lipase. As stomach contents become more fluid, the pH will approach 2, which is pretty acidic. The preliminary digestion of proteins will begin by pepsin.

But nutrients are not absorbed well in the stomach at all. And one of the main reasons for this is we still aren't quite. chemically broken down enough but also the stomach is coated in a very thick mucus that protects the stomach mucosa from the acid that is in the stomach and because of that mucus that also makes it very difficult for nutrients to get through the stomach wall.

So gastric or stomach activity is regulated by the production of acid and enzymes you by the gastric mucosa, which can be controlled by your central nervous system, the enteric nervous system, which is your digestive nervous system, and we've seen it can also be affected by hormones of the digestive tract. There are three overlapping phases of gastric control that we're going to look at. We have the cephalic phase, the gastric phase, and the intestinal phase.

Cephalic, gastric, and intestinal. So let's begin with cephalic phase. The cephalic phase of gastric secretion will begin when you see, smell, taste, or think about food.

This phase is directed by your central nervous system because you're thinking about food. It will prepare the stomach to receive the food. It's going to cause gastric juice production to speed up.

So the stomach kind of waters with your mouth getting ready for you to bring food in. So there's going to be production of acid and enzymes and everything's going to be all good and ready for when that food drops in. Next we have the gastric phase. The gastric phase is once food has finally arrived in the stomach.

We kind of started stimulating the stomach in the cephalic phase, so we're going to build on that now. This phase is longer and can continue for three to four hours while the acid and enzymes process the ingested materials. It could be even longer than that. Some things that initiate this phase are stretching of the stomach wall because you added food to it, an increase in the pH of the gastric contents, again because you added food to it, and the presence of undigested materials in the stomach like proteins and peptides all stimulate the gastric phase. Now in the gastric phase there are three mini phases we should be aware of, starting first with the local response.

If you're following along in the text that goes with this lecture you would find This information on page 906. The local response begins with stretching in the stomach wall. This stretching will cause the release of histamine, which will stimulate acid secretion. So stretching will cause more acid secretion.

The stimulation of stretch receptors and chemoreceptors in the wall of the stomach will activate the stomach's secretory cells, producing powerful contractions called mixing waves. This is the neural response. So again, the neural response, the stretching of the stomach wall and chemoreceptors trigger the activation of the stomach's secretory cells.

So we'll have more secretions and we'll also have stronger mixing waves in the stomach wall. This is the neural response. Finally, we've got the hormonal response.

Neural stimulation and the presence of peptides and amino acids in the kind will stimulate secretion of gastrin by the G cells. Remember, gastrin is a hormone that stimulates secretion of those. Parietal and chief cells. Gastrin will travel in the bloodstream to the parietal and chief cells who increase their secretions reducing the pH of the gastric juice. We'll also have more stomach churning.

So all of this secretion and all of this stomach churning is going to cause that food to get mixed up really well and dissolve down really well. So once the chyme has been broken down as much as we can break it down, we are then going to enter the intestinal phase. And the intestinal phase is the emptying. the stomach. So this is when chyme drips down into the small intestine.

So when the chyme leaves the stomach and we're moving it down into the small intestine, the chief and parietal cells will stop secreting and peristalsis and segmentation will also stop. So everything has just gotten quiet and we're allowing this chyme to exit the stomach. So the pancreas and liver and gallbladder are going to help out a bit too. These are our accessory organs. The pancreas is behind the stomach and extends from the duodenum towards or duodenum toward the spleen.

It is retroperitoneal. We talked about that at the beginning of the chapter and bound to the posterior wall of the abdominal cavity. It's wrapped in a very thin connective tissue capsule.

So here's a picture of the pancreas. You can see that it has a duct running through it and that duct is going to enter the duodenum which is your small intestine. So we talked about the pancreas before.

in the endocrine system, but we're not really focusing on the endocrine part of the pancreas in this chapter. Remember when we talked about insulin and glucagon? We're going to focus on the exocrine function, so what the pancreas releases through the duct. So the acinar cells and the epithelial cells of the duct system are found in the pancreas, and they will help secrete an alkaline pancreatic juice.

into the small intestine. About a thousand milliliters per day and this is going to contain digestive enzymes, water, and ions. It's controlled by hormones from the duodenum.

Pancreatic enzymes include pancreatic alpha amylase, pancreatic lipase, nuclease, and proteolytic enzymes. So looking first at the pancreatic alpha amylase. This is a carbohydrate.

And remember that anything that ends with ace is an enzyme. So a carbohydrate is going to break down carbohydrates like starches. It's very similar to salivary amylase. Pancreatic lipase will break down certain complex lipids.

Nuclease breaks down RNA or DNA. and proteolytic enzymes will break apart proteins. Protease is one type which breaks apart large protein complexes and peptidases break down small peptide chains into individual amino acids which we know is the component of protein.

Liver is the largest visceral organ and performs essential metabolic and synthetic functions. The liver is made up of hepatocytes, which are liver cells, and they will help to adjust the circulating level of nutrients through selective absorption and secretion. Here is the liver. We have a right lobe of the liver and we have a left lobe of the liver.

And underneath we have the gallbladder, the histology of the liver. So each lobe is divided by connective tissue into approximately 100,000 lobules. Lobules are the basic functional units of liver and are only about one millimeter in diameter. The hepatocytes form a very organized pattern, a series of irregular plates arranged like wheel spokes. So here's a picture of that.

You can see the individual liver cells, which are these, and they are in fact arranged in a wagon wheel pattern. If we zoom in on this a little bit, can look a little bit closer, here are the individual liver cells, hepatocytes. So the liver will secrete bile and bile breaks down fat. secretes the bile into a network of little chambers or channels, excuse me, called bile canaliculi.

So the cells are making bile and they put the bile into the bile canaliculi. this is going to train down toward the bile duct. So the right and left hepatic ducts collect bile from all the bile ducts of the liver lobes and unite to form a common hepatic duct. From the common hepatic duct the bile enters either the bile duct which empties into the duodenal or duodenal ampulla and the cystic duct which leads to the gallbladder.

So I'm going to show you this network of ducts so that you can actually see what the heck's going on because it's a little bit confusing when you just read it. The bile duct is formed by the union of the cystic duct, which is coming from the gallbladder, and the common hepatic duct, which is coming from the liver. This will penetrate the wall of the duodenum and meet the pancreatic duct at the duodental ampulla.

So let's work all of that mess out. So here we are underneath the liver. So there's our liver.

Here's our gallbladder. Here's the stomach, duodenum, curving around. And there's pancreas, just a little piece of it.

So this is a hepatic duct coming from the left lobe of the liver. And this is the hepatic duct coming from the right lobe of the liver. So each of these is draining bile. The left and right hepatic duct combine to form the common hepatic duct. The common hepatic duct moves down and connects with the cystic duct of the gallbladder.

So here's the cystic duct. The cystic duct joins with the common hepatic duct to form the big duct. which is the bile duct.

The bile duct moves down, goes behind the stomach, and then enters the duodenum here. So let's zoom in right here at this little opening. Here's our bile duct that we followed right to the opening. And here is our pancreatic duct.

And both of them connect to the duodenal. ampulla. So we've got the bile coming out here and we've got the pancreatic juices coming out here right into the duodenum.

And the duodenum is sometimes called the mixing bowl of the small intestine because it receives chyme and it also receives the juices from the liver and the pancreas. So all blood leaving the absorptive surfaces of the digestive tract will enter the hepatic portal system. flow into the liver, and then the liver cells can extract nutrients or toxins from that blood before the blood reaches the systemic circulation through hepatic veins.

The liver will remove and store excess nutrients for later. It can correct nutrient deficiencies by sending out stored reserves if we need them or performing synthetic activities, which means building new things that we're deficient in so that we can correct those deficiencies. The liver produces bile.

Bile salt in bile breaks lipid droplets apart, which is called emulsification in the duodenum. This creates tiny emulsion droplets that are coated with bile salt. This helps to increase the surface area exposed to enzymes, which is really necessary because mechanical digestion in the stomach creates large droplets of lipids, so we need to get smaller.

Pancreatic lipase can interact only at the surface. The gallbladder is a hollow pear-shaped muscular sac that stores and concentrates bile prior to secretion into the small intestine. Moving to the small intestine. The small intestine is a long muscular tube where we are going to have the completion of chemical digestion and 90% of nutrients will be absorbed here.

So this is really where the nutrient payoff happens. They consist of three segments, the duodenum, the pancreas, Jejunum and the ileum. Looking at the duodenum first. The duodenum is the first part of the small intestine that connects to the stomach.

It's 25 centimeters long and is known as the mixing bowl because it's going to receive chyme from the stomach and also digestive secretions from the pancreas and liver as we just saw. The middle segment of the small intestine called the jejunum is 2.5 meters long and the site of most chemical digestion. and nutrient absorption.

The ileum is the final segment of small intestine, three and a half meters long, and ends at the ileocecal valve. There's a sphincter here, which is a sphincter, that controls the flow of material from the ileum into the cecum of the large intestine, which is our last stop. So in this picture we have the duodenum at the top.

We then have the jejunum, which is in purple, and the ileum, which is in pink. The ileum comes around and enters the large intestine right about here at the ileocecal valve or sphincter. The histology of the small intestine. We have circular folds, transverse folds in the intestinal lining.

These are not like rugae, they're permanent. They don't disappear when the intestine fills. We have intestinal villi, which are finger-like projections in the mucosa of the small intestine, covered by simple columnar epithelia, and they have on them microvilli, which form a brush border. So this is a lot of folding.

The circular folds are also called pleca. So this is the intestine cut open and these are the folds that I was talking about called circular folds or pleca and they are not for expansion of the organ. They're actually for increasing the surface area so we can absorb more nutrients.

So they are permanent folds. Pleca or circular folds. Now going ahead, we're going to come back to these notes, but going ahead on the pleca or circular folds, we have villi, which are finger-like projections we've talked about a lot back in the anatomy one content.

Villi we know are for absorption. So the pleca are covered in villi. And so we've got the pleca to absorb nutrients across.

We have all the villi to absorb nutrients across. And then the villi have on them microvilli. So we're going to get even smaller in the next picture that we look at. But here's our histology. So we have the mucosa, which is covered in villi, submucosa, muscularis or muscular layer, and So what I want to do now is zoom in on one villus, one villus.

So if we zoom in on one villus, that is what it would look like. So you can see a little peach fuzz of microvilli on this villus, which further increases surface area for absorption. So now we can absorb tons of nutrients across the intestinal epithelia. Inside of the villus, we can see a capillary network, and we can also see this green thing, which is called a lacteal.

And the lacteals are connected to the lymphatic system. So let's talk about what they do. So going back, the lacteal is a lymphatic vessel in each villus.

It's going to transport chylomicrons, which are fat globules. that are too large to enter the blood capillaries. Intestinal glands are going to extend deep into the lamina propria, and they contain stem cells near their base that produce new epithelial cells to repair any damage that we might have.

1.8 liters of intestinal juice enters the intestinal lumen every day. The intestinal juice will help keep the chyme moist and assist in buffering acid. Keeps the digestive enzymes and products of digestion in solution nice and liquidy as well.

Intestinal motility. So after the chyme arrives in the duodenum or duodenum, weak peristaltic contractions will move it slowly toward the jejunum. Parasympathetic stimulation will accelerate local peristalsis and segmentation.

The movements of the mucosa will increase the absorptive effectiveness because we're going to stir and mix the intestinal components, which will eliminate local differences in nutrient concentration. So once we're done in a small intestine, we move to the last stop, which is the large intestine or the colon. It is horseshoe shaped and extends from the end of the ileum to the anus. It lies inferior to the stomach and liver and frames the small intestine. It's about 1.5 meters long and 7.5 centimeters wide.

Some of the main components. We have the cecum, which is the pouch-like first portion, the colon, which is the largest portion, and the rectum, which is the last 15 centimeters and the end of the digestive tract. So the cecum, or the expanded pouch, is going to receive and store materials from the ileum and begin the compaction process. The appendix is a skinny hollow structure about nine centimeters long. It's attached to the surface of the cecum and dominated by lymphoid nodules.

There's also a mesoappendix mesentery that will connect the appendix to the ileum. the cecum. So let's go forward and take a look at that. So here's where the ileum, they've removed the small intestines from our picture, but here's the ileum coming in. There's the ileocecal valve or sphincter, and this is the first part of the large intestine known as the cecum, which is this pouch.

So the cecum has on it the appendix. There that is. Okay, so next we move to the colon, and the colon has a larger diameter and thinner wall than the small intestine.

It also has a feature called haustra. Haustra are pouches in the wall of the colon that permit expansion and elongation of the colon. We're also going to see tinea coli, and tinea coli are three longitudinal bands of smooth muscle that run along the outer surface of the colon. They're going to be responsible for creating the hostra because of their muscle tone. So let's take a look at that as well.

Four regions of the colon, the ascending colon, transverse colon, descending colon, and then the sigmoid colon. So let's look at all of those. So first of all, These little pouches or little segments that look like the segments of a caterpillar body, those are the hostra labeled here. Then we've got this ribbon of muscle, the tinea coli, that goes around and around. And it's responsible for helping to create these hostra.

for expansion and then we've got the region so we talked about the cecum but here going up is the ascending colon and it turns right here at the right colic flexure we then have the transverse colon going this way we have the left colic flexure so another twist descending colon and then sigmoid colon. And then we'll eventually talk about the rectum, which I believe will happen actually now. So the rectum forms the last 15 centimeters of the digestive tract, and it is an expandable organ for temporary storage of feces. The anal canal is the last portion of the rectum that contains small longitudinal folds.

And the anus is the exit of the anal canal with keratinized epidermis like the skin. The internal anal sphincter has circular muscle, smooth muscle cells, and is not under voluntary control. The external anal sphincter encircles the distal portion of the anal canal, has skeletal muscle fibers, and is. under voluntary control. The histology of the large intestine. It's going to lack villi because we're really not trying to absorb tons and tons of nutrients anymore.

We've already done about 90% of that. We have an abundance of goblet cells which make mucus and intestinal glands that are deeper than the glands of the small intestine with goblet cells in them. to provide mucus for lubrication for fecal material.

So here's the histology of the colon or large intestine. And we have the mucosa with the deep mucus glands. We have the submucosa, muscularis or muscular layer, and then the serosa.

The functions of the large intestine are mainly for absorption or reabsorption of water, less than 10% of nutrients. We're going to get rid of bile salt, organic waste, and we're going to absorb vitamins and toxins produced by bacteria. Compaction of intestinal contents into feces and the storage of fecal material.

prior to release or defecation. The microbiome or microbes in the gut or that live in the human body, these are things like bacteria, fungi, and viruses, and we have quite a few of them in the large intestine. They are very important in helping us to absorb some of the vitamins that we would not otherwise be able to. get from our food without their help. So vitamins are organic molecules that are important as cofactors or coenzymes in your metabolism.

Normal bacteria in your colon make three vitamins that supplement your diet. So we need this bacteria. Vitamin K which is produced by the bacteria is required by your liver for synthesizing four different clotting factors including prothrombin.

Biotin is important in glucose breakdown. Vitamin B5 is required in the manufacture of steroid hormones and some neurotransmitters. Organic waste we want to get rid of.

The bacteria convert bilirubin to urobilinogen and stercobilinogen. So these are all products of the breakdown of hemoglobin. from recycling of blood. Some urobilinogen are absorbed into the bloodstream and excreted in your urine.

Urobilinogen and stercobilinogen remaining in the colon are converted to urobilins and stercobilins by exposure to oxygen. We're going to get rid of those. Organic waste continued.

Bacteria break down peptides in feces and generate ammonia soluble ammonium ions. The large intestine motility, gastro ileal and gastroenteric reflexes will move materials toward the cecum or into the cecum while you eat. The movement from the cecum to the transverse colon is very slow allowing hours for water absorption.

Peristaltic waves will move the material along the length of the colon. Segmentation movements will mix the contents of the adjacent hostra. Movement from the transverse colon through the rest of the large intestine results from powerful peristaltic contractions or mass movements. The stimulus is distension of the stomach and duodenum.

which is relayed over the intestinal nerves. The distension of the rectal walls initiates the defecation reflex. Both of these are triggered by stretch receptors in the rectum.

So a balanced diet is going to contain nutrients like carbohydrates, lipids, proteins, vitamins, minerals, and water. The processing and absorption of nutrients. The digestive system is going to break down the physical structure of food and then disassemble that into molecules.

The molecules released into the bloodstream will be absorbed by cells and either broken down to give us ATP or energy or used to make new things like carbohydrates, proteins, and lipids. Digestive enzymes will break molecular bonds in large organic molecules, carbohydrates, proteins, lipids, and nucleic acids. The process of this breakdown is called hydrolysis, which we talked about in chapter 3. It's divided into classes by their specific substrates.

Carbohydrates break bonds between simple sugars. Proteases break bonds between amino acids. And lipases separate fatty acids from glyceride. The digestive enzymes we've seen. are secreted by the salivary glands, tongue, stomach, and pancreas.

Some age-related changes that we see as well. The division rate of epithelial stem cells will decline. Smooth muscle tone will decrease.

The effects of cumulative damage over time become more apparent, and cancer rates increase. Dehydration is common among the elderly and changes in other systems have direct or indirect effects on the digestive system. The digestive system has an extensive anatomical and physiological connection to several other systems, for example, the nervous, cardiovascular, endocrine, and lymphatic. The digestive tract is an endocrine organ because it produces a variety of hormones and it also will produce neurotransmitters.

This concludes Chapter 24, The Digestive System. Next, The Urinary System.

Transcript for:Understanding the Digestive System

Transcript for:
Understanding the Digestive System