Digestive System. Student Learning Objectives: Structures to be studied: Introduction

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1 Digestive System Student Learning Objectives: Identify the primary structures of the digestive system. Describe the use of various important nutrient materials in the body. Structures to be studied: Oral cavity Teeth Gingiva Hard palate Soft palate Uvula Tongue Salivary glands Pharynx Esophagus Stomach Cardiac sphincter Pyloric sphincter Small intestine Duodenum Jejunum Ileum Large intestine Appendix Cecum Ileocecal valve Ascending colon Transverse colon Descending colon Sigmoid colon Rectum Anus Liver Gallbladder Pancreas Introduction The digestive system is involved in helping us process food materials from the environment to extract the nutrients that they contain. The digestive system processes food materials both mechanically and chemically in order to reduce the food to its basic components. Most materials must be in their most basic forms in order to be absorbed into the blood stream. The circulation will transport most of these nutrients to the liver via the hepatic-portal vein. This will allow the liver to store excess nutrients and make up for any nutrient deficits as well as extract any harmful materials from the blood before it is sent off to the rest of the body organs. This unusual blood flow is critical for maintaining stable levels of nutrients in the blood supply, despite dramatic variations in the nutrient levels in the blood leaving the digestive system. Once absorbed, the nutrients will be used for a wide range functions in the body. In this section, we will look first at the various digestive organs and then will examine the use of some of the nutrients in the body.

2 Digestive System Tubing The digestive system is composed of a large tube, beginning at the oral cavity and ending at the anus, and several accessory glands. The oral cavity acquires large food materials from the environment and breaks these large chunks up into particles small enough to swallow. The process of breaking up the food in the oral cavity is called mastication. Teeth play a key role in mastication process. There are four different types of teeth. The incisors are the chisel-shaped teeth at the front of the mouth. There are four incisors, both top and bottom. These teeth are the first to emerge in the infant and designed for cutting and biting. The canines (formerly referred to as the cuspids) lie lateral and posterior to the incisors. There two canines on the top and two on the bottom. These teeth can sink into the food (since they tend to be a bit longer and more pointed) and are involved in tearing a chuck of food away from the main mass. Once the chunk has been obtained with the aid of the incisors and the canines, the food is pushed back further in the oral cavity to be crushed by the large, flat premolars (formerly known as bicuspids) and molars. The tongue aids the mastication process by moving the food around in the mouth to contact the teeth so that materials will be evenly processed. The tongue presses the food against the hard palate which forms the roof of the mouth. This firm ceiling is formed by parts of the maxilla and palatine bones. The tongue also mixes the food materials with the saliva, a watery substance, to speed up the breaking up of the chunks. The pulverized food is then formed into a ball (or bolus) and is ready to be swallowed. At the posterior part of the hard palate is the soft palate. This fleshy structure seals off the nasal cavities during swallowing. The uvula is a dangling extension of the soft palate that hangs down into the throat region (resembling a punching bag ). Special nerve receptors in the uvula detect the presence of a food bolus to initiate the swallowing reflex.

3 The tongue pushes the food bolus to the back of the throat, bumping the bolus against the uvula. Signals are sent to the nervous system and the swallowing reflex begins. Food moves into the pharynx, or throat region (which you previously with the respiratory system), and then into the esophagus. Peristaltic muscle contractions push the food bolus down the esophagus and into the stomach. The esophagus begins at the inferior pharynx and passes through the thorax posterior to the trachea. This tube then penetrates through a small opening in the diaphragm and enters the abdominal cavity to join with the stomach. The stomach is a large, bag-like structure that stores the food consumed in a large meal until it can be further processed. Folds on the inside of the stomach, known as rugae, allow the stomach lining to stretch to accommodate a meal without tearing. The stomach has very acidic conditions due to the presence of large amounts of hydrochloric acid which are produced in the glands of the stomach wall. Food materials are pulverized into a mostly homogenous soup that is then passed on to the small intestine. The small intestine is composed of a long tube, generally feet long, with numerous folds and specialized structures (e.g. villi and microvilli) on the inside to increase the inner (or enteric) surface area making nutrient processing much more efficient. The small intestine is the main place where food materials are digested and absorbed. There are three main functional sections in the small intestine: duodenum, jejunum, and ileum. The duodenum is the first section of the small intestine and attaches to the stomach. This is also the section of small intestine that receives secretions from the liver, gallbladder, and pancreas. These secretions are extremely important in processing food materials. The duodenum is a fairly short tube, only about inches long. This tube is continuous with the middle section of the small intestine, the jejunum. The jejunum then connects to the final section, the ileum which attaches to the large intestine. As one progresses through the intestine, the number of folds and villi decreases. Much of the food material is processed and absorbed in the short duodenum, leaving only a small amount of material to be processed in the rest of the small intestinal tubing.

4 The large intestine (or colon) is another long tubular structure of the digestive system. The appearance of the large intestine is very different from that of the small intestine. The large intestine also requires increased surface area in order to function efficiently. The large intestine is involved in absorption of most of the water that remains in the materials being processed after the small intestine completes its job. Unlike the small intestine, which increases its surface area with internal structures, the large intestine increases its surface area on the outer surface. Three bands of muscle, called taenia coli (seen as silver stripes on the model), pucker up the large intestine when they contract forming pouch-like areas called haustra (seen as gray bumps on the model). This feature allows the large intestine to have a surface area nearly double the 4-6 feet that the structure occupies in the abdominal cavity. The large intestine is composed of several sections. The ileum of the small intestine joins into the large intestine at the cecum. Hanging inferiorly from the cecum, one will find the appendix. Scientists believe that the appendix was likely part of a formerly much larger cecum in our ancestors. The ascending colon travels up the right side of the abdomen. The large intestine crosses from the right to the left side of the abdomen via the transverse colon. The descending colon travels down the left side of the body. The large intestine moves back to the midline with the sigmoid colon and ends with the rectum, which exits the body at the anus.

5 There are a number of sphincters in the digestive tubing. Sphincters are rings of muscle that can open and close to control the movement of materials through the tubing. The cardiac sphincter is found at the entrance to the stomach. This sphincter helps ensure that food materials don t reflux (i.e. backflow) into the esophagus. The pyloric sphincter is located at the exit from the stomach and controls the movement of food materials into the duodenum. Only materials that have been reduced to a sufficiently small size are normally allowed to pass through this sphincter. The ileum joins with the cecum at the ileocecal valve. Unlike the previous structures, this valve has more of a flap design than a sphincter arrangement. When food materials pass through the ileocecal valve, the flaps snap shut to prevent materials from backflowing into the small intestine. At the end of the digestive tubing is the final sphincter, the anal sphincter or anus. This sphincter controls the movement of feces (i.e. indigestible materials) out of the body. Accessory Organs The accessory organs of the digestive system produce many of the materials that are used to process foods during digestion. The salivary glands are located in the head and neck region below the ear. Tubes, or ducts, from these glands enter into the oral cavity. These ducts transport the saliva, a glandular secretion from the salivary glands, into the oral cavity. Saliva helps to moisten and break up the food materials during mastication. In addition, there is an enzyme in saliva that begins the digestion of the carbohydrates. There are three sets of salivary glands. The parotid salivary glands are very large and lie in front of and under the ears. The submandibular and sublingual (which cannot be seen on the picture) salivary glands lie beneath the mandible on the outside of the face. In the disease known as mumps, the salivary glands become inflamed and enlarged.

6 The other accessory glands are located in the abdominal cavity. The largest of these accessory glands is the liver. This organ plays many vital roles in the body including controlling nutrient levels in the blood; removing harmful chemicals from the blood; and producing proteins necessary for blood clotting, nutrient/waste exchange between the blood and body cells, and immune system activity. The role of the liver in the digestion process involves the production of bile. Bile is used in the processing of fats from our food. The gallbladder stores bile that the liver has produced between meals. This pouch is usually green in color because bile stains the tissues that form the gallbladder. Bile is transported to the small intestine through the common bile duct. This tube enters into the duodenum, just a short distance from where the duodenum connects onto the stomach. The final accessory gland is the pancreas which lies in the upper abdomen, just inferior to the stomach. The pancreas makes numerous enzymes that are involved in the digestion of carbohydrates, fats, proteins, and nucleic acids. In addition, sodium bicarbonate is made here to help neutralize the hydrochloric acid coming into the small intestine from the stomach and to raise the ph in the small intestine. The pancreatic duct transports the pancreatic enzyme and bicarbonate secretions to the small intestine. Just before entering the duodenum, the pancreatic duct joins with the common bile duct. The pancreas is part of the endocrine system also, making hormones such as insulin and glucagon which are secreted into the blood. These hormones are made by groups of endocrine cells that lie in the Islets of Langerhans or pancreatic islets. Insulin and glucagons help the body control blood glucose levels.

7 Use of Nutrients in the Body Once nutrients have been digested and absorbed in the digestive system, they eventually enter the blood stream and are transported to various tissues around the body. Different nutrients play vital roles in the functioning of the body organ systems. The following gives a list of the functions of various nutrients in the body. [This material has been included in this anatomy exercise to assist you for lecture and will not be tested on the Digestive Quiz.] Organic Molecules: Carbohydrates: - Glucose is a major energy source for most body cells and is the exclusive energy source for the neurons - Stored for future use as glycogen - Converted to other molecules, such as fats or amino acid, which are needed by the body Lipids: - Synthesis of: a. Cellular components such as the plasma membrane b. Certain hormones (endocrine gland secretions) c. Bile salts used in fat digestion d. Myelin sheath covering of some neuronal fibers e. Cholesterol - Important energy source which results in the formation of twice as much ATP as a comparable molecule of carbohydrate Proteins: - Synthesis of: a. Cell components such as hemoglobin and muscle myofilaments (i.e. actin and myosin) b. Certain hormones c. Cellular secretions such as enzymes, clotting factors, blood proteins, connective tissue fibers, and antibodies - Used for energy after all other sources of energy have been exhausted Inorganic molecules: Macro minerals: minerals that are used in large amounts in the body - Sodium: a. Important electrolyte (ion) in the body b. Helps to regulate the body fluid levels c. Important in nerve impulse transmission - Calcium: a. Forms crystalline framework of bones and teeth b. Important in muscle contraction c. Involved in blood clotting

8 - Phosphorus: a. Forms crystalline framework of bones and teeth b. Component of nucleic acids and some enzymes c. Involved in metabolism of nutrients for energy - Chloride: a. Important electrolyte (ion) in the body b. Component of gastric acid c. Involved in acid/base regulation of the body - Potassium: a. Important electrolyte (ion) in the body b. Helps control fluid and electrolyte levels in the body by serving as an exchange ion c. Important in nerve impulse transmission - Magnesium: a. Forms crystalline framework of bones b. Aids functioning of muscle and nervous tissue c. Involved in the metabolic process Trace elements: minerals that are used in small amounts in the body - Iron: a. Component of hemoglobin, myoglobin, and certain enzymes b. Involved in the metabolism process as a component of the cytochromes c. Important in transport of gases around the body - Iodine: component of the thyroid hormones - Zinc: a. Component of many enzymes b. Important in protein synthesis c. Important in wound healing and preventing anemia - Fluoride: important in maintaining bone and tooth structure - Copper: a. Component of many enzymes b. Important in the formation of hemoglobin - Selenium: important antioxidant which protects the body from diseases such as cancers - Chromium: a. Important in cellular processes in which body cells use sugars b. Enhances the effects of insulin c. Increases HDL while decreasing LDL - Manganese: a. Component of many enzymes b. Important for maintaining bone structure c. Important for functioning of the reproductive structures and the central nervous system - Molybdenum: a. Component of many enzymes b. May play a role in preventing formation of dental caries (cavities)

9 Vitamins: molecules important in cellular metabolic activities Fat-soluble: - Vitamin A: a. Important in the formation of connective tissue b. Important for the formation and maintenance of skin, mucous membranes, and hair c. Involved in the production of pigments for vision - Vitamin D: a. Important for the formation and maintenance of bones and teeth b. Assists with the absorption of calcium from the dietary sources - Vitamin E: anti-oxidant which protects the plasma membrane from damage and prevents development of cancers - Vitamin K: a. Essential for the formation of certain clotting proteins b. Deficiencies will lead to increased bleeding time and delayed clotting of blood Water-soluble: - B-complex vitamins: group of vitamins which are primarily involved in the formation of various enzymes and coenzymes which catalyze a variety of chemical reactions in the body - Vitamin C: a. Promotes metabolic activities b. Some antioxidant effects c. Important in formation of connective tissue collagen d. Helps to maintain healthy capillaries, bones, and teeth e. Improves function of the immune system