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Lec.12 Medical Physiology Z.H.Al-Zubaydi Digestive System The organs of digestive system can be separated into two main group; gastrointestinal tract (GIT) and the accessory digestive organs. Gastrointestinal Tract (GIT) Gastrointestinal tract is a continuous coiled hollow muscular tube through the ventral body cavity and is open at both ends. Its organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The large intestine leads to the terminal opening, or anus. The GIT is approximately less than 9 m long. Mouth Food enters the digestive tract through the mouth, or oral cavity, a mucous membrane-lined cavity. The lips (labia) protect its anterior opening, The muscular tongue occupies the floor of the mouth. As food enters the mouth, it is mixed with saliva and masticated (chewed). The cheeks and closed lips hold the food between the teeth during chewing. The nimble tongue continually mixes food with saliva during chewing and initiates swallowing. Thus, the breakdown of food begins before the food has even left the mouth. Papillae containing taste buds, or taste receptors, are found on the tongue surface. Pharynx From the mouth, food passes posteriorly into the pharynx is subdivided into the nasopharynx, part of the respiratoiy passageway; the oropharynx, posterior to the oral cavity; and the laryngopharynx, which is continuous with the esophagus below. The walls of the pharynx contain two skeletal muscle layers. The cells of the inner layer run longitudinally; those of the outer layer run around the wall in a circular fashion. Alternating contractions of these two muscle layers propel food through the pharynx into the esophagus below. Esophagus The esophagus or gullet, runs from the pharynx through the diaphragm to the stomach, it is essentially a passageway that conducts food (by peristalsis) to the stomach. 1 The walls of GIT organs from the esophagus to the large intestine are made up of the same four basic tissue layers, or tunics: 1. The mucosa is the innermost layer, a moist membrane that lines the cavity, or lumen, of the organ. It consists primarily of a surface epithelium, plus a small amount of connective tissue (lamina propria) and a scanty smooth muscle layer. 2. The submucosa is found just beneath the mucosa. It is a soft connective tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphatic vessels. 3. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells. 4. The serosa is the outermost layer of the wall. It consists of a single layer of flat serous fluid-producing cells Stomach The C-shaped stomach is on the left side of the abdominal cavity. Different regions of the stomach have been named. The cardiac region (named for its position near the heart), through which food enters the stomach from the esophagus. The fundus is the expanded part of the stomach lateral to the cardiac region. The body is the midportion, and as it narrows inferiorly, it becomes the pyloric antrum, and then the funnel-shaped pylorus, the terminal part of the stomach. The stomach is approximately 25 cm long, but its diameter depends on how much food it contains. When it is full, it can hold about 4 liters of food. When it is empty, it collapses inward on itself. The stomach acts as a temporary "storage tank" for food as well as a site for food breakdown. Besides the usual longitudinal and circular muscle layers, its wall contains a third obliquely arranged layer in the muscularis externa. This arrangement allows the stomach not only to move food along the tract, but also to churn, mix, and pummel the food, physically breaking it down to smaller fragments. In addition, chemical breakdown of proteins begins in the stomach. The mucosa of the stomach is a simple columnar epithelium that produces large amounts of mucus. This otherwise smooth lining is dotted with millions of deep gastric pits, which lead into gastric glands that secrete the gastric juice. For example, some stomach cells produce intrinsic factor, a substance needed for the absorption of vitamin B12 from the small intestine. The chief cells produce proteindigesting enzymes, mostly pepsinogens, and the parietal cells produce corrosive hydrochloric acid, which makes the stomach contents acidic and activates the enzymes. The mucous neck cells 2 produce a sticky alkaline mucus, which clings to the stomach mucosa and protects the stomach wall itself from being damaged by the acid and digested by the enzymes. Still other cells, the enteroendocrine cells produce local hormones, such as gastrin, that are important to the digestive activities of the stomach. Most digestive activity occurs in the pyloric region of the stomach. After food has been processed in the stomach, it resembles heavy cream and is called chyme. The chyme enters the small intestine through the pyloric sphincter. Small Intestine The small intestine is the body's major digestive organ. Within its twisted passageways, usable food is finally prepared for its journey into the cells of the body. It is the longest section of the GIT, with an average length of 2.5 to 7 m in a living person. The small intestine has three subdivisions: the duodenum, the jejunum, and the ileum, which contribute 5 percent, nearly 40 percent, and almost 60 percent of the length of the small intestine respectively. Chemical digestion of foods begins in earnest in the small intestine. The small intestine is able to process only a small amount of food at one time. The pyloric sphincter controls food movement into the small intestine from the stomach and prevents the small intestine from being overwhelmed. Though the C-shaped duodenum is the shortest subdivision of the small intestine, it has the most interesting features. Some enzymes are produced by the intestinal cells. More important are enzymes produced by the pancreas through the pancreatic ducts, where they complete the chemical breakdown of foods in the small intestine. Bile (formed by the liver) also enters the duodenum through the bile duct in the same area. Nearly all food absorption occurs in the small intestine. The small intestine wall has three structures that increase the absorptive surface tremendously; microvilli, villi, and circular folds. · Microvilli are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy appearance, sometimes referred to as the brush border. · Villi are fingerlike projections of the mucosa. Within each villus is a rich capillary bed and a modified lymphatic capillary called a lacteal. The digested foodstuffs are absorbed through the mucosal cells into both the capillaries and the lacteal. · Circular folds are deep folds of both mucosa and submucosa layers. 3 Large Intestine The large intestine is much larger in diameter than the small intestine but shorter in length, about 1.5 m long. It extends from the ileocecal valve to the anus. Its major functions are to dry out the indigestible food residue by absorbing water and to eliminate these residues from the body as feces. It has the following subdivisions: cecum, appendix, colon, rectum, and anal canal. The saclike cecum is the first part of the large intestine. Hanging from the cecum is the wormlike appendix. Since it is usually twisted, it is an ideal location for bacteria to accumulate and multiply. Inflammation of the appendix, appendicitis, is the usual result. The colon is divided into several distinct regions. The ascending colon travels up the right side of the abdominal cavity and makes a turn to travel across the abdominal cavity as the transverse colon. It then turns again and continues down the left side as the descending colon to enter the pelvis. where it becomes the S-shaped sigmoid colon. The sigmoid colon, rectum, and anal canal lie in the pelvis. The anal canal ends at the anus, which opens to the exterior. Because most nutrient absorption has occurred before the large intestine is reached, no villi are seen in the large intestine, but there are tremedous numbers of goblet cells in its mucosa that produce an alkaline (HCO3~-rich) mucus. The mucus acts as a lubricant to ease the passage of feces to the end of the digestive tract. Accessory Digestive Organs Salivary Glands Three pairs of salivary glands empty their secretions into the mouth. The large parotid glands lie anterior to the ears, the submandibular glands and the small sublingual glands empty their secretions into 4 the floor of the mouth through tiny ducts. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids. The mucus moistens and helps to bind food together into a mass called a bolus, which makes chewing and swallowing easier. The clear serous portion contains an enzyme, salivary amylase, that begins the process of starch digestion in the mouth. Saliva also contains substances such as lysozyme and antibodies (IgA) that inhibit bacteria; therefore, it has a protective function as well. Also saliva dissolves food chemicals so they can be tasted. Pancreas The pancreas is a soft, pink, triangular gland that extends across the abdomen from the spleen to the duodenum. The pancreas produces enzymes that break down all categories of digestible foods. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid, which neutralizes the acidic chyme coming in from the stomach. The pancreas also has an endocrine function; it produces the hormones insulin and glucagon. Liver and Gallbladder The liver is the largest gland in the body. It is located under the diaphragm, more to the right side of the body. There is no question that the liver is one of the body's most important organs. It has many metabolic and regulatory roles; however, its digestive function is to produce bile that enters the duodenum through the bile duct. Bile is a yellow-to-green, watery solution containing bile salts, bile pigments (chiefly bilirubin, a breakdown product of hemoglobin), cholesterol, phospholipids, and a variety of electrolytes. Bile does not contain enzymes, but its bile salts emulsify fats by physically breaking large fat globules into smaller ones, thus providing more surface area for the fat-digesting enzymes to work on. The gallbladder is a small, thin-walled green sac in the inferior surface of the liver. When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored. While being stored in the gallbladder, bile is concentrated by the removal of water. Later, when fatty food enters the duodenum, a hormonal stimulus prompts the gallbladder to contract and spurt out stored bile, making it available to the duodenum. 5 6