This lecture cover information related to the digestive system at a level consistent with an undergraduate setting.
Paul Buttenhoff in English
Hello, my name is Paul Buttenhoff. I’m an Assistant Professor here at the College of St. Catherine in Minneapolis. Today I’d like to talk to you a little bit about the digestive system, one of 11 major organ systems in your body. Although the digestive system is unique in it’s own right, we do have to remember that it does function in conjunction with all of the ten other organs systems to allow you to be alive and to maintain homeostasis. That’s what we are all about.
From an anatomical standpoint the digestive system is divided into two regions or two portions. There is a digestive tract or sometimes the alimentary canal or gastrointestinal tract that is primarily a 30 foot tube that starts at the mouth and ends at the anus. Different regions of the alimentary canal are going to be specialized and will be given different names, for example the mouth, the esophagus, the stomach, the small intestine or the large intestine. In any of these regions we are going to see there are specialized functions. The accessory organs of the digestive system will contribute to the process. Usually they are not found exactly in the digestive system but they exist outside, in the periphery. Accessory organs include things like teeth, salivary glands, the liver, the gallbladder and the pancreas and we will get to their functions in a moment.
If we talk about the digestive system as a whole we can see that it carries out three primary functions for us. First of all at the top end, the digestive system is designed to allow us to take food and beverages into our body, this process is known as ingestion. The second step, or the second function, will be known as true digestion. Once food and beverages enter our body we need to be able to break them down into their smaller usable components. Digestion will occur in several key areas and we will talk about those in a moment. After material enters your body and is broken down, the third very important step will be known as absorption. Without absorption material will simply pass through your body and will be unusable to you. Absorption will involve the movement of nutrients, sugars, proteins and fats, vitamins and minerals, from the alimentary canal or gastrointestinal tract actually into your bloodstream across several layers of tissues and then they will be available for transport to every other organ system in your body. The fourth function is the waste removal function. Not everything that you eat is digestible and or absorbable. Your large intestine, which we will get to shortly, will handle the removal of feces from the body.
What we are going to do today is pretend we are a chicken sandwich or a cheeseburger or curried lentils and we are going to start the journey through the digestive system and we’re going to see what happens along the way. We’re going to primarily keep track of three macromolecules. We are going to keep track of sugars, or carbohydrates. We are going to keep track of proteins and we are going to keep track of fats, or lipids along the way.
When food enters your mouth, digestion starts. You use your teeth to physically, or mechanically, break large pieces of food down into smaller pieces of food. This is going to be important because along the way we are going to mix food particles with chemicals and with these smaller particles the surface area for chemical digestion is increased. Additionally in your mouth, saliva will be used to break carbohydrates, or sugars, down into smaller structures known as disaccharides. A disaccharide is not the smallest form of a sugar, but it’s a good start. After you chew food and mix it with saliva you form a small round ball known as a bolus. Swallowing will be the process in which you move this bolus from your mouth down into your stomach through a long muscular pipe known as the esophagus.
Your stomach receives balls of food, or boli, and the primary function of the stomach will be to break down or to initiate the digestion of proteins that were found in those food particles. From an anatomical standpoint your stomach is a three-dimensional mixing bowl. It’s got several layers of muscle that are designed to allow your stomach to contract or shrink in a three-dimensional fashion. Additionally, the walls of the stomach produce a fluid known as gastric juice. Gastric juice primarily will be designed to help you break down proteins. In gastric juice we’re going to find general water, we’ll find hydrochloric acid, or stomach acid, and we’re going to find a very important enzyme that will act as a chemical scissors to start breaking proteins down into their smaller pieces. In the stomach, pepsin will break proteins into smaller chains known as peptides. That is the primary function of your stomach. If you eat foods that contain a lot of protein, pepsin will have a greater job and hence food will stay in your stomach for longer periods of time. If you eat foods that contain primarily water or sugar, materials that do not contain proteins, they will pass relatively quickly through your stomach down to the next organ in the system. After protein digestion has started and has proceeded to a sufficient degree, material, at this point known as chyme, will enter your small intestine through a ring of smooth muscle known as the pyloric sphincter.
The digestive system, along the way, will contain several little key regions. If we had to talk about a region that was of primary importance we would have to focus on the small intestine. Anatomically the small intestine consists of a duodenum, which is attached to the stomach and two other lengths of tube, essentially, known as the jejunum and the ileum, respectively. The small intestine will receive bile from the liver and the gallbladder and it will also receive a chemical called pancreatic juice through special ducts from the pancreas. The functions of the small intestine will be two-fold. First of all the small intestine will continue the digestion process. The process was started in your mouth and continued nicely in your stomach but now we need to finish the deal. In your small intestines fats will be broken down into smaller structures known as triglycerides. Peptides from the stomach will be broken down into smaller structures known as amino acids and the remaining sugars, that have not yet been broken down in your mouth, will be turned into disaccharides or monosaccharides. Those are the particles that we are really after. In the small intestine final digestion will occur. What started out as a cheeseburger will now be entirely different. You have spent a lot of time and energy producing useful particles, it’s an energetically costly process and it’s very carefully maintained. Now we must move these particles from the small intestine into the bloodstream so we can transport the products of sugar and fats, minerals and proteins, to every other tissue in your body. The small intestine, primarily the duodenum and the jejunum; the first two regions, contain many small finger-like projections known as villi. These villi are going to serve as avenues for the movement of material from your digestive system into the bloodstream, technically that’s absorption. Most of the nutrients, most of the components of fats, sugars and proteins are absorbed in the duodenum and the jejunum. The ileum is going to be connected to the final portion of your digestive system known as the large intestine or sometimes referred to as the colon. In the ileum we are not going to find a lot of villi but we will see that water absorption occurs to a great degree.
After you have taken food into your body, chewed it up, sent it to your stomach for mixing and sent it to your small intestine for final digestion and absorption what we will end up with, at this point in the ileum and in the first portion of the large intestine, will be a material known as feces. This will be undigested matter. This will be unabsorbable matter, and from your body’s standpoint this is waste material and you must get this out of your body in order to remain healthy. In order to remain at homeostasis. The ileum of the small intestine is connected to the first portion of the large intestine known as the cecum, which is a small round ball shaped receiving chamber. Additionally, attached to the cecum there is a small piece of lymphatic tissue known as the vermiform appendix or sometimes simply the appendix. In humans the function of the appendix really is not known and there are many people that have their appendixes removed and suffer no ill effects.
The large intestine consists of the cecum, the colon, the rectum and the anus, down at the distil end. What we are going to do now is start at the cecum and trace the path of feces as it finishes out the process. Keep in mind we are talking about undigested, unabsorbable material at this point. From the cecum, feces will move up the right side of the body through the ascending colon. The ascending colon will turn to the left and run across your body, through the transverse colon, and then turn down on the left side to become the descending colon. As soon as the descending colon travels down into an imaginary ring formed by the bones in your pelvis, the descending colon will form an S-shaped curve sometimes known as the sigmoid colon. And last but not least, the sigmoid colon will deliver feces to the rectum. The rectum will deliver, ultimately, feces out of your body through the anus. If we take a look at the walls of the ascending, transverse, descending or sigmoid colon we will see that they are constructed a little bit differently than the walls of any other tissue in your digestive system. They do consists of large, hollow cup-shaped pouches that will be crucial for allowing you to absorb water from undigested, unabsorbed material. The large intestine does function very heavily in allowing you to get water from the foods and beverages that you eat. Additionally in your large intestines we are going to find large amounts of bacteria, helpful bacteria by the way, and these bacteria will handle several functions. For example, your intestinal bacteria will produce vitamin K, a very important component in blood clotting. Your intestinal bacteria will also help you further metabolize proteins and sugars, functions that could not take place in your small intestines.
As material travels through the ascending, transverse, descending and sigmoid colon it collects at the rectum. Special components of your nervous system in the rectum can detect increased presence of feces or increased stretch in the walls. Information will be sent to your brain and you will perceive that as the urge to use the restroom. From the rectum material will move out of your body through a structure known as the anus. The anus actually consists of two rings of muscle, an internal anal sphincter and an external anal sphincter. Because the external anal sphincter contains muscle that you can voluntarily control, you should consciously have control over the movement of feces from the rectum into the external environment.