Digestive System (Part 1) PDF

.. Chapter 24: The Digestive System PART 1 Substances are used as raw materials for synthesizing essential compounds (anabolism) or are decomposed to provide the energy that cells need to continue functioning (catabolism). The catabolic reactions require two essential ingredients: (1) o...

.. Chapter 24: The Digestive System PART 1 Substances are used as raw materials for synthesizing essential compounds (anabolism) or are decomposed to provide the energy that cells need to continue functioning (catabolism). The catabolic reactions require two essential ingredients: (1) oxygen and (2) organic molecules, such as carbohydrates, fats, or proteins, that can be broken down by intracellular enzymes. The digestive system provides both the fuel that keeps all the body's cells running and the building blocks needed for cell growth and repair. The digestive system consists of a muscular tube, the digestive tract , also called the gastrointestinal (GI) tract or alimentary canal , and various accessory organs. The oral cavity (mouth), pharynx , esophagus , stomach , small intestine , and large intestine make up the digestive tract. Accessory digestive organs include the teeth, tongue, and various glandular organs , such as the salivary glands, liver, and pancreas, which secrete into ducts emptying into the digestive tract. 1 Functions of the Digestive System Digestion refers to the chemical breakdown of food into small organic fragments suitable for absorption by the digestive epithelium. Secretion is the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by glandular organs. Absorption is the movement of organic substrates, electrolytes (inorganic ions), vitamins, and water across the digestive epithelium and into the interstitial fluid of the digestive tract. Excretion is the removal of waste products from body fluids. The digestive tract and glandular organs discharge waste products in secretions that enter the lumen of the tract. Most of these waste products, after mixing with the indigestible residue of the digestive process, will leave the body. The ejection of materials from the digestive tract, a process called defecation, or egestion , eliminates materials as feces. The lining of the digestive tract also plays a protective role by safeguarding surrounding tissues against (1) the corrosive effects of digestive acids and enzymes; (2) mechanical stresses, such as abrasion; and (3) bacteria that either are swallowed with food or reside in the digestive tract. When bacteria reach the underlying layer of areolar tissue, the lamina propria , they are attacked by macrophages and other cells of the immune system. The Digestive Organs and the Peritoneum The abdominopelvic cavity contains the peritoneal cavity. Because a thin layer of peritoneal fluid separates the parietal and visceral surfaces, relative movement can occur without friction and resulting irritation. About seven liters of fluid is secreted and reabsorbed each day, although the volume within the peritoneal cavity at any one time is very small. Liver disease, kidney disease, and heart failure can cause an increase in the rate at which fluids move into the peritoneal cavity. The accumulation of fluid creates a characteristic abdominal swelling called ascites. The distortion of internal organs by this fluid can result in symptoms such as heartburn, indigestion, and lower back pain. Mesenteries Portions of your digestive tract are suspended within the peritoneal cavity by sheets of serous membrane that connect the parietal peritoneum with the visceral peritoneum. These mesenteries are double sheets of peritoneal membrane. 2 Mesenteries also stabilize the positions of the attached organs and prevent your intestines from becoming entangled during digestive movements or sudden changes in body position. (b) A diagrammatic view of the organization of mesenteries in an adult. As the digestive tract enlarges, mesenteries associated with the proximal portion of the small intestine, the pancreas, and the ascending and descending portions of the colon fuse to the body wall. (c) An anterior view of the empty peritoneal cavity, showing attachment sites where fusion occurs. (d) A sagittal section of the mesenteries of an adult. Notice that the pancreas, duodenum, and rectum are retroperitoneal 3 4 The lesser omentum stabilizes the position of the stomach and provides an access route for blood vessels and other structures entering or leaving the liver. The falciform ligament helps stabilize the position of the liver relative to the diaphragm and abdominal wall. The greater omentum hangs like an apron from the lateral and inferior borders of the stomach. All but the first 25 cm of the small intestine is suspended by the mesentery proper , a thick mesenterial sheet that provides stability, but permits some independent movement. A mesocolon is a mesentery associated with a portion of the large intestine The transverse mesocolon , which supports the transverse colon, and the sigmoid mesocolon , which supports the sigmoid colon, are all that remains of the original embryonic mesocolon. 5 An inflammation of the peritoneal membrane produces symptoms of peritonitis a painful condition that interferes with the normal functioning of the affected organs. Physical damage, chemical irritation, and bacterial invasion of the peritoneum can lead to severe and even fatal cases of peritonitis. In untreated appendicitis, peritonitis may be caused by the rupturing of the appendix and the subsequent release of bacteria into the peritoneal cavity. Peritonitis is a potential complication of any surgery in which the peritoneal cavity is opened or of any disease that perforates the walls of the stomach or intestines. Histological Organization of the Digestive Tract The major layers of the digestive tract include (1) the mucosa , (2) the submucosa , (3) the muscularis externa , and (4) the serosa. The Mucosa The inner lining, or mucosa , of the digestive tract is a mucous membrane consisting of an epithelium, moistened by glandular secretions, and a lamina propria of areolar tissue. The Digestive Epithelium The oral cavity, pharynx, and esophagus (where mechanical stresses are most severe) are lined by a stratified squamous epithelium, whereas the stomach, the small intestine, and almost the entire length of the large intestine (where absorption occurs) have a simple columnar epithelium that contains goblet cells. Scattered among the columnar cells are enteroendocrine cells , which secrete hormones that coordinate the activities of the digestive tract and the accessory glands. The lining of the digestive tract is often thrown into longitudinal folds. The folding dramatically increases the surface area available for absorption. The life span of a typical epithelial cell varies from two to three days in the esophagus to up to six days in the large intestine. The lining of the entire digestive tract is continuously renewed 6 through the divisions of epithelial stem cells, keeping pace with the rate of cell destruction and loss at epithelial surfaces. This high rate of cell division explains why radiation and anticancer drugs that inhibit mitosis have drastic effects on the digestive tract. Lost epithelial cells are no longer replaced, and the cumulative damage to the epithelial lining quickly leads to problems in absorbing nutrients. In addition, the exposure of the lamina propria to digestive enzymes can cause internal bleeding and other serious problems This submucosal plexus , or plexus of Meissner , contains sensory neurons, parasympathetic ganglionic neurons, and sympathetic postganglionic fibers that innervate the mucosa and submucosa The Muscularis Externa is a region dominated by smooth muscle cells. Like the smooth muscle cells in the muscularis mucosae, those in the muscularis externa are arranged in an inner, circular layer and an outer, longitudinal layer. The Movement of Digestive Materials The muscular layers of the digestive tract consist of visceral smooth muscle tissue , a type of smooth muscle introduced in Chapter 10. The smooth muscle along the digestive tract shows rhythmic cycles of activity due to the presence of pacesetter cells. These smooth muscle cells undergo spontaneous depolarization, and their contraction triggers a wave of contraction that spreads through the entire muscular sheet. Pacesetter cells are located in the muscularis mucosae and muscularis externa, the layers of which surround the lumen of the digestive tract. The coordinated contractions of the muscularis externa play a vital role in the movement of materials along the tract, through peristalsis , and in mechanical processing, through segmentation. The muscularis externa propels materials from one portion of the digestive tract to another by contractions known as peristalsis. Peristalsis consists of waves of muscular contractions that move a bolus , or small oval mass of digestive contents, along the length of the digestive tract. 7 Peristalsis. Peristalsis propels materials along the length of the digestive tract. Segmentation Most areas of the small intestine and some portions of the large intestine undergo cycles of contraction that produce segmentation. These movements churn and fragment the bolus, mixing the contents with intestinal secretions. Because they do not follow a set pattern, segmentation movements do not push materials along the tract in any one direction. Control of the Digestive Function The activities of the digestive system are regulated by neural, hormonal, and local mechanisms 8