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(Students: Bolded, italicized, and underlined sections will be on the exam) The Digestive System: I. Function of the System: A. The digestive system is a physical and chemical process that breaks down food into basic nutrients (including water, carbohydrates, proteins, fats, vitamins, and minerals) for the body’s cells, and packages the unused portion for waste disposal. When we eat such things as bread, meat, and vegetables, they are not in a form that the body can use as nourishment. Our food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy. II. The Main Parts of the System: System Overview: The digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. This tube is called the alimentary canal. The alimentary canal includes the mouth (with salivary glands), esophagus, stomach, small intestine, large intestine, rectum, and anus. Food does not travel through the liver, appendix, nor pancreas. Inside this tube is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. Two solid organs, the liver and the pancreas, produce digestive juices that reach the intestine through small tubes. The gall bladder stores one of these juices from the liver called bile, which digests fats. The appendix may also play a role in storing toxins from the system. In addition, parts of other organ systems (for instance, nerves and blood) play a major role in the digestive system. Here are the parts, in more detail. A. Mouth: The mouth is the beginning of the digestive tract; and, in fact, digestion starts here when taking the first bite of food. The teeth and tongue begin by breaking down the food. Chewing, called mastication, breaks the food into pieces that are more easily digested, while the salivary glands secrete saliva which mixes with food to begin the process of breaking it down into a form your body can absorb and use. B. Salivary Glands: Three pairs, parotid, submandibular, sublingual. There are salivary glands in the cheek and others that open into the floor of the mouth. Saliva is a mixture of mucus and enzymatic fluids, each produced to various extents in various glands. It also contains the enzyme salivary amylase, (starts to break down starch) lysozyme (antibacterial) and IgA antibodies. In some mammals (and snakes!) saliva may be poisonous, ‘quieting down’ living prey. C. Esophagus: It is a food passage that connects the mouth to the stomach. It is located in your throat near your trachea (windpipe), the esophagus receives food from your mouth when you swallow. By means of a series of wavelike muscular contractions called peristalsis, the esophagus delivers food to your stomach. D. Stomach: The stomach is a hollow organ, or "container" that holds food while it is being mixed with enzymes that continue the process of breaking down food E. F. G. H. I. into a usable form called chyme (chime). Cells in the lining of the stomach secrete hydrochloric acid and powerful enzymes that are responsible for the breakdown process. When the contents of the stomach are sufficiently processed, they are released into the small intestine. Small Intestine: The small intestine is a 22-foot (approximately) long muscular tube that breaks down food using enzymes released by the pancreas and bile from the liver, and it then absorbs the digested nutrients into the bloodstream. It is made up of three segments — the duodenum, jejunum and ileum. Peristalsis also is at work in this organ, moving food through and mixing it with the digestive secretions from the pancreas and liver. The duodenum is largely responsible for the continuous breaking-down process, with the jejunum and ileum mainly are responsible for absorption of nutrients into the bloodstream. Contents of the small intestine start out semi-solid (called chyme), and end in a liquid form after passing through the organ. Water, bile, enzymes and mucous contribute to the change in consistency. Once the nutrients have been absorbed and the leftover-food residue liquid has passed through the small intestine, it then moves on to the large intestine, or colon. Pancreas: The pancreas secretes digestive enzymes into the duodenum, the first segment of the small intestine. These enzymes break down protein, fats and carbohydrates. As part of the endocrine system, the pancreas also makes insulin, secreting it directly into the bloodstream. Insulin is the chief hormone for metabolizing sugar. Liver: The liver is the largest organ (by mass) in the digestive system and it has multiple functions. Its main function within the digestive system is to process the nutrients absorbed from the small intestine. Bile, a green liquid from the liver, is secreted into the small intestine and plays an important role in digesting fat. In addition, the liver is the body’s chemical "factory." It takes the raw materials absorbed by the intestine and makes many of the various chemicals the body needs to function. The liver also de-toxifies and breaks down potentially harmful chemicals, including alcohol. Gallbladder: The gallbladder stores and concentrates bile, and then releases it through the bile duct into the small intestines (duodenum part) to help absorb and digest fats. Colon (Large Intestine): The colon is a 6-foot long muscular tube that connects the small intestine to the rectum, and is largely responsible for the absorption of water and minerals into the bloodstream, as well as collecting food residue for excretion The large intestine is wider than the small intestines, and is made up of the cecum, the ascending (right) colon, the transverse (across) colon, the descending (left) colon and the sigmoid colon, which then connects to the rectum. The appendix is a small tube attached to the cecum, and its function is not known, although many speculate that it stores some chemical and bacterial wastes. The large intestine is a highly specialized organ that is responsible for processing wastes so that emptying the bowels is easy and convenient. Stool, or waste left over from the digestive process, is passed through the colon by means of peristalsis, first in a liquid state and ultimately in a solid form called feces. As stool passes through the colon, water is removed. Stool is stored in the sigmoid (Sshaped) colon until a "mass movement" empties it into the rectum once or twice a day. It normally takes about 36 hours for stool to get through the colon. The stool itself is mostly food debris and bacteria. These ‘good’ bacteria perform several useful functions, such as synthesizing various vitamins, processing waste products and food particles, and protecting against harmful bacteria. When the descending colon becomes full of stool, or feces, it empties its contents into the rectum to begin the process of elimination. J. Rectum: The rectum (Latin for "straight") is an 8-inch chamber that connects the colon to the anus. It is the rectum's job to receive stool from the colon, to ‘let the person know’ that there is stool to be evacuated and to store the stool until evacuation happens. When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters relax and the rectum contracts, disposing its contents. If the contents cannot be disposed, the sphincter contracts and the rectum accommodates so that the sensation temporarily goes away. K. Anus: The anus is the last part of the digestive tract. It is a 2-inch long canal consisting of the pelvic floor muscles and the two anal sphincters (internal and external). The lining of the upper anus is specialized to detect rectal contents. It lets you know whether the contents are liquid, gas or solid. The anus is surrounded by sphincter muscles that are important in allowing control of stool. The pelvic floor muscle creates an angle between the rectum and the anus that stops stool from coming out when it is not supposed to. The internal sphincter is always tight, except when stool enters the rectum. It keeps us continent when we are asleep or otherwise unaware of the presence of stool. When we get an urge to go to the bathroom, we rely on our external sphincter to hold the stool until reaching a toilet, where it then relaxes to release the contents. III. Interactions With Other Systems: The digestive system interacts with every system, since it provides nutrients for every cell in every organ system. Some examples of systems that directly interact with the system include: • The Excretory System: The rectum and anus are part of the body’s excretory system, since they store and expel waste products from the body. • The Nervous System: The nervous system controls the movement of the food through the digestive tract, as well as the secretion of digestive enzymes in the system. • The Circulatory System: The nutrients from the digestive system are absorbed into the blood in the intestines, and are then pumped to all of the cells throughout the body. • The Endocrine System: The pancreas is a part of both the digestive and endocrine systems, since it secretes digestive enzymes into the digestive tract, and insulin into the bloodstream. IV. Some Disease and Disorders: • Constipation: A very common disorder where the rectum stores solid fecal matter, and the body fails to expel it ‘in a timely fashion. People who live an easy, sedate life in a city and consume rich food are more prone to it than others. Treatment includes an array of medications and eating foods (roughage including fruits and certain grains) that soften stools and ‘encourage’ it to pass. • Peptic Ulcer: Excess hydrochloric acid in the stomach begins to ‘eat away’ at the muscle lining inside of the stomach, causing bleeding and severe pain. Stress may contribute to increased acid production. Treatment includes various medications, and eating ‘softer’, ‘easier to digest’ (such as yogurt and fruits) foods. • Gallstones: Stones formed by excess cholesterol that hardens into a ‘stone’ that blocks the bile duct, creating pressure and pain that is felt below the right ribs. In extreme cases, the gall bladder may need to be removed, and the person may have to eat less fatty foods for the remainder of their lives (since the bile stored in the gall bladder assists with the breakdown of fat). • Acid Reflux: Excess acid from the stomach travels through the ‘weakened’ sphincter muscle that separates the stomach from the esophagus, and up into the esophagus, causing chest pain and discomfort. Antacids may alleviate the pain. • Dyspepsia (Indigestion): Dyspepsia or indigestion is the term applied to difficulty in digesting food. Dyspepsia may be associated with pain in the region of the stomach, flatulence, eructations, a foul taste in the mouth, heartburn, and a general feeling of discomfort.! There is also loss of appetite in some cases of dyspepsia.! Irregular food habits in the form of overeating or eating at the wrong time, or the wrong type of food leads to indigestion. The general term applied to the symptoms associated with dyspepsia in Ayurveda is Agitimandya, which is a decreased digesting capacity of agnis, particularly, jatharagni, which aids digestion.! The aggravation of the doshas vata, pitia, and kapha-leads to this disease.! Excess of vata leads to pain in the stomach; that of pitta causes a burning sensation; and if kaplia is vitiated, there is nausea and vomiting. V. Interesting Facts about the System: • Swallowing: In leaving the mouth a bolus of food must cross the respiratory tract (trachea is anterior to the esophagus) by a complicated mechanism known as swallowing, or deglutination. During one of the stops of swallowing, the epiglottis is a flap of muscle that closes the trachea during swallowing, preventing food from entering the windpipe, allowing it to continue down the esophagus and into the stomach. • More on Peristalsis and the Movement of Food Through the System: The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ. Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ. • The first major muscle movement occurs when food or liquid is swallowed. Although we are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves. • The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ring like valve closing the passage between the two organs. However, as the food approaches the closed ring, the surrounding muscles relax and allow the food to pass. • The food then enters the stomach, which has three mechanical tasks to do. First, the stomach must store the swallowed food and liquid. This requires the stomach to relax and accept large volumes of swallowed material. The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine. • Several factors affect emptying of the stomach, including the nature of the food (mainly its fat and protein content) and the degree of muscle action of the emptying stomach and the next organ to receive the contents (the small intestine). As the food is digested in the small intestine and dissolved into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow further digestion. • Finally, all of the digested nutrients are absorbed through the intestinal walls. The waste products of this process include undigested parts of the food, known as fiber, and older cells that have been shed from the mucosa. These mouth, soft palate closes off the nose and the larynx rises so that the epiglottis closes off the trachea. Food thus moves into the pharynx and onwards by peristalsis aided by gravity. If we try to talk whilst swallowing food may enter the respiratory passages and a cough reflex expels the bolus.