Greetings, today, we're going to start our discussion of the gastrointestinal tract I'll call it the GI tract. This is the tract that enables us to bring nutrients, electrolytes, and water into the body. We are moving materials from our external environment to the inside of the body, then into the bloodstream for delivery to all the cells of the body. So, how does this thing work? So, the GI tract is a tube which is about five meters in length. This tube has an opening at the top which is your mouth. That's what's shown here. Here is the mouth. At the end of the tube we have the anus. The tube is going to allow food materials to enter the mouth from your diet. These are complex foods such as steak and potatoes. We need to convert this steak and potatoes into simple molecules which can be used by the cells of the body for fuel. We want to convert the steak and the potatoes into amino acids and into simple sugars. This will be done by the gastrointestinal tract. It acts like a processing plant. The first thing that happens is that these complex food materials enter through the mouth. As they enter into the mouth, then we chew on them and break them down into smaller and smaller particles. This increases the surface area of the particle and allows for attack by enzymes. So we increase the surface area. Then the material is very rapidly transmitted from the oral cavity, through the esophagus which is essentially a tube, down to the stomach. Once we reach the stomach, once in the stomach, then we start the process of digestion. Digestion converts the complex materials such as a steak into polypeptides, and then from polypeptides into amino acids. Digestion occurs within the stomach, but it's also going to occur within the next region, which is called the small intestine. In the small intestine we further break down these food particles to very simple molecules such as glucose and amino acids. Then we absorb these small molecules across the epithelial cells which line the GI tube to enter into the body. Material that's left over is released from the anus as a fecal material. This is going to be the waste products that cannot be used by the body. In a typical day you take in something like a liter to two liters of solids as well as fluids in a given meal. And as it passes through the GI tract you will add about seven liters of secretions which can be acids, buffers, and enzymes. And as the food bolus moves through the small intestines then, there is as much as nine liters of fluid. Most of this fluid will be absorbed as it moves through the small intestine. Such that we end up with fecal matter which is only about 200 to 500 millilitres. The rest of the material is absorbed as we're moving through the small intestine and into the large intestine. That means that along this GI tract, we're going to have regions that'll have specific functions. And we want to move our food material in a unidirectional manner. We start at the mouth and move towards the anus. This unidirectional movement is due to smooth muscle contraction which propels the bolus of food and food stuffs through the tract. In addition to having the GI tract itself or GI tract proper, there other organs which help in this process of processing food. The first of these is the salivary gland. The salivary gland secretes saliva which moistens the food particle as it's coming into the mouth and as we're chewing. This allows for the food particles to easily slide down the esophagus to the stomach. The second is that we have the liver and the pancreas. These two organs secrete materials which aid in the digestion of the food stuffs within the GI tract itself. That's going to be predominately within the small intestine. As we're moving down this tract, there are specific sphincters which gate the entry and exit from the tract. At the top, we have a voluntary sphincter governed by swallowing. At the bottom we have a voluntary sphincter, by which we control voiding of the feces. So there are skeletal muscle sphincters at the top and at the bottom of the tube. There are also sphincters within different regions of this tube. These are are smooth muscle sphincters. They are under involuntary control. The smooth muscle sphincter, for instance, resides here between the esophagus and the stomach. There is another between the stomach and the beginning of the small intestine, the duodenum. Then again we have an involuntary sphincter between one region of the colon and the anus. So the tract governs movement of food materials along the tube. As it moves along the GI tract, we're going to change the actual structure of these complex materials and then absorb the simple maolecules. At the end we have waste product, which is not useful to the body. This is what's expelled from the body. Let's look a little bit more at the anatomy of the GI tract itself. I told you that the tube is a muscular tube. On the outermost regions of the tube wall we have two layers of muscle. That is what's diagramed here. The inner layer is here. This is the inner layer of this muscle, called the muscularis externus. This inner layer of muscle is oriented conferentially around the lumen. When it contracts then the diameter of the tube gets smaller. Conversely, if it relaxes, it will make the diameter of the tube larger. It is governing, then, the diameter of the tube. The muscle, which is present in the outermost layer of the muscularis externis, is oriented along the long axis of the tube. When it shortens, it shortens the tube. This allows for peristaltic movement of the bolus of food as it moves down the tract. Sort of a milking kind of an action, as it moves down the tract. On the inner most aspect of the tube, is a lumen. The lumen is obviously where the food particles are first delivered. Surrounding that lumen we have an epithelium, this is a layer of cells. At the top of the tract there are multiple layers of cells which are very flat. These are very much like what you see on your skin. This is a wear and tear surface because protects to abrasion. We have food materials coming into the tract which can be a bit rough. In the simple conduit from the mouth to the esophagus and then to the stomach. Within the esophagus, we have this wear and tear epithelium. There is no absorption and no secretion. We have a similar type of epithelium in the anus. So wear and tear epithelium at the top of the tube and wear and tear epithelium at the bottom of the tube. In the stomach, the epithelium is a single layer of cells. These cells are secretory. We'll talk about what they're secreting in just a second. Then we move into the small intestine. In the small intestine the cells there are also a single layer of cells. But here we have an absorptive function. We will move small particles generated in the lumen of the tract across these cells and into the body. The first part of the tract that I want to talk about today is the stomach. We will consider what secretions the stomach makes and how it starts the process of digestion. The next time we meet, we'll talk about how this process is regulated. The stomach has essentially three types of cells. This is in the major portion of the stomach. The major portion of the stomach is called the fundic, the fundic region of the stomach. Here, the cells which are on the very surface, facing the lumen, are called surface cells. These extend down into the glands, into these invaginations. The invaginations are called pits. these are also surface cells. As we descend into the invagination, the gland, we will have neck cells. These epithelial cells lining these gland surfaces looks like this. The surface cells and the neck cells secrete mucus. This is a complex carbohydrate containing material, which acts as a protective coating. It prevents the acid, which is generated by the stomach, from eroding through this epithelial layer. The second type of gland cell is called the parietal cell. The parietal cells are located within the grands of the fundic region. The parietal cells have two major functions. One is they make HCL. They make this acid which starts the digestion of protein. Secondly, they make a factor, called intrinsic factor. 144
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This is a carrier protein. This carrier binds vitamin B12 which in the human must come from the diet. You cannot make this vitamin. The vitamin is absolutely necessary for making red blood cells. Vitamin B12 comes in from the diet. It binds to this intrinsic factor, this carrier, and then passes through the small intestine to the distal region of the small intestine, called the ileum. And in the ileum, that carrier bids to a receptor on the epithelium. The vitamin B12 move across those cells, across the ileum epithelial cells. The vitamin B12 has entered into the body. In the absence of intrinsic factor, the individual will have pernicious anemia. They have pernicious anemia. T ytpe of anemia simply means you can't make red blood cells. Consequently you will have a lower oxygen carrying capacity within the body. The third type of cell which is present within this region of the stomach is called the chief cell. The chief cells is down here in the very base of the glands. The chief cells secrete pepsinogen. Pepsinogen is an inactive enzyme. The active enzyme is pepsin. Anything that has an "ogen" on the end of its name is an inactive enzyme. These are classified as zymogens. They are inactive enzymes. We'll talk about inactive enzyme more when we talk about the pancreas. Pepsinogen is an inactive enzyme which is secreted into the lumen of the stomach. Pepsinogen in the presence of acid is converted to pepsin. That requires HCL. We need to have a very acidic condition for this enzyme to become activated. The pepsin starts to break down protein into polypeptides. Our digestion of protein starts with this enzyme. Now because the secretion of acid is so important, I want to spend some time talking about it. The secretion of gastric acid is done by the parietal cells. These are the parietal cells. We are still in the stomach in the fundic region of the stomach. I've drawn here a parietal cell. The lumen of the stomach is on this side, and the blood portion of the cells are here. This is the basal portion of the cell which faces the blood on this side. The apical portion faces the lumen and the basal side faces the blood. We have a capillary running right here. Now these cells are going to use a reaction which is dependent on carbonic anhydrase to generate protons. These protons will move into the lumen of the stomach. This generates concnetrated acid. The enzyme is carbonic anhydrase. It uses the carbon dioxide, which comes from the blood and diffuses across the plasma membrane of these cells. The CO2 reacts with water in the presence of carbonic anhydrase to produce bicarbonate and a proton. The proton is extruded from the cells into the lumen by a pump. This is an ATPase. This particular ATPase is a hydrogen-potassium ATPase. It is so important that it has another name. We call it the proton pump. The bicarbonate which came from our carbonic anhydrase reaction, shown here, is extruded from the cells and enters into the blood in exchange for chloride. An antiporter moves the bicarbonate out of the cells and chloride into the cells. The chloride diffuses across the cells and leaves the cells to enter into the lumen of the stomach. So we now have chloride and protons within the lumen of the stomach. We just generated hydrochloric acid. In order to keep the proton pump active, we need potassium ions. This potassium is being furnished by the sodium-potassium ATPase, which is sitting in on the basal surface of these cells. Potassium comes into the cells from the blood. Potassium is then extruded from the cell through the leak channel on the luminal side. Potassium is used by the pump to move potassium in and the proton out of the cell into the lumen. The pH of this region can get as low as pH 2.0. pH as you recall, is the logarithmic scale for free hydrogen ions. So, this is extremely concentrated, very strong acid. This is a very, very acidic condition. Acid is made by the stomach under two conditions. One is in the fed state, we increase the amount of acid that's going to be made. So if we have food coming into the body, acid secretion will increase rapidly. The cells sense the presence of food. They're going to be making a lot of HCL. By 19 minutes after eating, we have a peak of secretion of acid within the stomach, within this region of the stomach. Then, the peak acid output drops by four hours after a meal, When the peak acids secretion drops, the stomach makes a basal amount of acid within this region. The pH of the stomach is changing as well. pH is our free proton. Initially we have a move from a very acidic condition that has a pH of two up towards a pH of five. The luminal content goes towards a basic condition. This is due to the protons binding to the food materials which are entering the stomach lumen. Food buffers the free protons. Eventually, the amount of food that's within the stomach decreases. It leaves slowly to enter into the small intestine. As food leaves the stomach, then the amount of free proton increases. Luminal pH then falls. By two hours after feeding, to four hours after feeding, we again, have a very low pH within the lumen. So the next time we meet, we will to talk about how we're going to control the production of acid when food enters the body, and how we turn off the production of acid when the need for very high amounts of acid is gone because the stomach is empty. So the general concepts are the gastrointestinal tract is a muscular tube in which nutrients are move in an unidirectional manner from mouth to anus. Secondly, we have regional specializations along this tract, which enable fragmentation of the food particles to make them smaller, increase the surface areas. In digestion the complex proteins are broken down into polypeptides, and then into amino acids. In absorption of nutrients, the amino acids moves across the epithelium that's lining the tract and allow them to enter the body. Third, acids are secreted by the parietal cells of the stomach. This is in a specific region of the stomach called the fundic stomach. This process requires carbonic anhydrase activity to generate the proton and the bicarbonate. They are generated from water and CO2, which is delivered to these cells by the blood. The protons are secreted into the stomach lumen by a hydrogen-potassium ATPase. This is the proton pump. Bicarbonate is secreted in to the blood in exchange for chloride ions. As the chloride enters, it crosses the cells and then it's transferred by diffusion across the membrane, facilitated diffusion, into the lumen., This generates hydrochloric acid. And fourth, acid secretion can be increased in our fed states and is decreased in the fasted states. Regulation of moving from a high acid secretion to a lower acid secretion, requires several factors. We will talk about that next time you come in here. So see you at that time, bye bye.
