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8/26/2016 Chapter 24: The Digestive System Digestive System Digestive Tract: mouth pharynx esophagus stomach small intestine large intestine Accessory Organs: pancreas liver gall bladder 1 8/26/2016 Organs of the digestive system Digestive System Functions ingestion mechanical and chemical digestion Absorption concentration excretion 2 8/26/2016 Digestive System Histology Mucosa is the inner epithelium of the digestive tract. Glands in the lamina propria secrete through ducts opening into the epithelium stratified squamous epithelium Buccal cavity Pharynx Esophagus anus Remainder lined by simple columnar epithelium Digestive System Histology Submucosa - loose connective tissue surrounding the muscularis mucosae (smooth muscle). Large blood vessels and lymphatic vessels . Submucosal plexus Network of nerves that regulates contractions/secretions 3 8/26/2016 Layers of the gastrointestinal tract Digestive System Histology Muscularis Externa: smooth muscle cells inner circular layer outer longitudinal layer. Serosa is a serous membrane covering the muscularis externa inside the peritoneal cavity. There is no serosa covering the muscularis external of the oral cavity, pharynx, esophagus and rectum. Autonomic reflexes of the myenteric plexus which is between the inner and outer smooth muscle layers. Parasympathetic stimulation increases muscluar tone and activity and sympathetic stimulation promotes muscular inhibition and relaxation. 4 8/26/2016 Organization of the enteric nervous system Digestive System Pacesetter cells in the smooth muscle of the digestive tract trigger waves of contraction. Peristalsis circular muscles contract behind digestive contents. longitudinal muscles contract shortening adjacent segments Segmentation muscles churn and fragment digestive materials to mix them with intestinal secretions. Does not move material in a particular direction. 5 8/26/2016 Buccal Cavity Analysis of potential food. Mechanical digestion (mastication). Lubrication by mucus and salivary secretions (parotid, sublingual, submandibular glands). Chemical Digestion. Bolus: partially digested food for deglutition. Buccal Cavity labia vestibule gingivae hard palate soft palate lingual frenulum uvula 6 8/26/2016 Structures of the mouth (oral cavity) Salivary Glands Produce 1-1.5 L of saliva daily Bolus secretion 99.4% water with ions, buffers, and enzymes lubricate the mouth dissolve chemicals in food & stimulate the taste buds make deglutition possible Background secretion flushes the oral surfaces salivary IgA and lysozymes control oral bacteria. 7 8/26/2016 The three major salivary glandsparotid, sublingual, and submandibular Salivary Glands Parotid glands produce a secretion rich in salivary amylase. Submandibular and sublingual glands produce saliva that contains less enzymes but more buffers and mucus. Salivary production can reach 7 ml per minute during eating When eating the pH rises from about 6.7 to about 7.5. Salivary secretions are normally controlled by the ANS 8 8/26/2016 Teeth Neck of the tooth marks the boundary between the root and the crown. Crown is covered by enamel containing a crystalline form of calcium phosphate. Calcium phosphate is the hardest biologically manufactured substance. Bud of the tooth consists of dentin - a mineralized matrix containing no living cells. A typical tooth 9 8/26/2016 Teeth Pulp cavity receives blood vessels and nerves through the root canal at the root of the tooth. The root sits in the alveolus. Collagen fibers of the periodontal ligament extend from the dentin of the root to the surrounding bone. Cementum covers the dentin of the root providing protection and anchoring the periodontal ligament. Epithelial cells in the gum form tight attachments to the tooth preventing bacterial access to the cementum. Teeth Incisors Cuspids (canine) Bicuspids (premolar) Molars Deciduous teeth (20) are replaced by secondary dentition. Periodontal ligaments and roots of the deciduous teeth erode and are pushed out by the eruption of the secondary teeth (32). Third molars are the last teeth to appear. 10 8/26/2016 Teeth Pharynx & Esophagus The pharynx is the passageway of solid food, liquids, and air. Pharyngeal muscle contractions during swallowing propel the bolus into the esophagus. 11 8/26/2016 Pharynx & Esophagus The esophagus is about 25 cm Upper 1/3 contains skeletal muscle. Lower third contains smooth muscle. Center third is a mixture. Passes through the mediastinum in the thoracic cavity and enters the peritoneal cavity through the esophageal hiatus in the diaphragm. Lined with a stratified squamous epithelium protects from abrasion, temperature extremes and chemicals. Lubricated with mucous glands Histology of the esophagus 12 8/26/2016 Deglutition Oral Phase begins with compression of the bolus against the hard palate and the movement into the pharynx. Under conscious control. Pharyngeal phase begins when bolus comes in contact with sensory receptors around the pharynx and posterior pharyngeal wall and initiates involuntary swallowing reflex. Followed by the closure of the glottis by the epiglottis. Esophageal Phase begins with the passage through the upper esophageal sphincter then peristalsis through the cardiac sphincter. Deglutition (swallowing) 13 8/26/2016 ????? What is the importance of the mesenteries? Whick would be more efficient in propelling intestinal contents from one place to another peristalsis or segmentation? What effect would a drug that blocks parasympathetic stimulation of the digestive tract have on peristalsis? What is occuring when the soft palate and larynx elevate and the glottis closes. Stomach ULQ of abdominopelvic cavity. Functions Bulk storage of ingested food. Mechanical digestion. Chemical digestion (acid and enzymes). Production of intrinsic factor. Structure cardia fundus body Pyloris. 14 8/26/2016 External and internal anatomy of the stomach Stomach Third layer of muscle, the inner oblique, adds strength. Visceral peritoneum of the stomach is continuous with mesenteries greater omentum lesser omentum Rugae are ridges and folds in a relaxed stomach. Bolus is turned to chyme. Can contain 1-1.5 L 15 8/26/2016 Peritoneal Folds Gastric Wall of stomach Is lined by a mucous epithelium. Alkaline mucus protects the stomach lining from HCl and enzymes. Gastric pits open into the gastric surface and contain mucous cells. Gastric glands produce about 1500 ml of gastric juice per day from secretory cells G cells produce hormone stimulated gastric acid secretion, gastrin Parietal cells make intrinsic factor and HCl. Chief cells secrete pepsinogen which is converted to pepsin by HCl. 16 8/26/2016 Histology of the stomach Gastric Activity Cephalic phase parasympathetic fibers under the control of the vagus nerve innervate mucous cells , parietal cells, chief cells and endocrine cells of the stomach. Gastric Phase which begins with the arrival of the bolus in the stomach. Intestinal Phase begins when chyme begins to enter the small intestine. 17 8/26/2016 Gastric Activity Intestinal Phase begins when chyme begins to enter the small intestine. Controls the rate of gastric emptying to ensure secretory, digestive, and absorptive function of the small intestine. Local endocrine control reduces gastric activity. Secretin cholecystokinin (CCK) gastric inhibitory peptide (GIP) Inhibitory reflexes depress gastric activity when the proximal small intestine becomes too full, or acidic. slows stomach wall contractions inhibits PNS, stimulates SNS Small Intestines Small intestines includes the duodenum, the jejunum, and the ileum. The ileocecal valve is the sphincter between the small and large intestines. Plicae are the transverse folds of the intestinal mucosa. Villi are small projections into the lumen of the small intestines. Microvilli cover the villi to increase surface area. 18 8/26/2016 Anatomy of the small intestine Small Intestine Villi contain a network of capillaries to transport respiratory gases and carry absorbed nutrients to the hepatic portal circulation to the liver. At the bases of the villi are entrances to intestinal glands that secrete alkaline intestinal juice and intestinal hormones. 19 8/26/2016 Histology of the duodenum and ileum Small Intestines Intestinal juice includes mucus and hormones to buffer acids and dissolve digestive enzymes. Intestinal hormones include cholecystokinin (CCK) and gastric inhibitory peptide (GIP) to control the rate of the intestinal phase. 90% of absorption occurs in the small intestines. 20 8/26/2016 Small Intestine Duodenum - 25 cm. Receives chyme from the stomach and exocrine secretions of the pancreas and liver Jejunum - 2.5 m supported by a sheet of mesentery attached to the dorsal body wall. Most chemical digestion and nutrient absorption. Ileum - 3.5 m ends at the ileocecal valve. Histology of the small intestine 21 8/26/2016 Histology of the small intestine Small Intestine Weak peristaltic contractions move material as it is being absorbed. Gastroenteric reflex is initiated by distension of the stomach and accelerates glandular secretion and peristaltic activity in all segments. Gastroileal reflex is a response to circulating levels of gastrin which relaxes the ileocecal valve. Transit time through small intestines is about 5 hours. 22 8/26/2016 Small Intestine 1.8 liters of intestinal juice is produced each day. Hormones and CNS regulate secretions. Secretions are increased through parasympathetic stimulation of the vagus nerve. Intestinal Hormones Secretin released when the pH falls in the duodenum Increases the secretion of bile and buffers by the liver and pancreas Cholecystokinin (CCK) Gastric inhibitory peptide (GIP) Secreted when chyme arrives with lipids and partially digested proteins Targets the pancreas and gall bladder Released when fats and carbohydrates enter the small intestine Inhibits gastric activity and stimulates insulin secretion 23 8/26/2016 ????? Which muscle regulates the flow of chyme from the stomach to the small intestine? When a person suffers from chronic ulcers in the stomach treatment sometimes involves cutting the branches of the vahus nerve to the stomach. Why? How would a meal high in fat affect the level of cholecystokinin in the blood? Pancreas The pancreas is about 15cm long & about 80g. Pancreatic islets secrete insulin and glucagon account for 1% of the pancreas. Is connected to the duodenum by the pancreatic duct and secretes the exocrine hormones. Pancreatic acini are sacs where small ducts originate. Enzymes and buffers are secreted by the acinar cells. Pancreatic lipases, carbohydrases, nucleases and proteases do most of the digestion in the small intestine. 24 8/26/2016 Pancreas Secretes about 1000 ml of pancreatic juice per day. Primarily controlled by hormones from the duodenum. Acid chyme stimulates the duodenum to release secretin that triggers pancreatic production of fluid with a pH of 7.5-8.8. CCK controls the production and secretion of pancreatic enzymes. Relation of the pancreas to the liver, gallbladder, and duodenum 25 8/26/2016 Relation of the pancreas to the liver, gallbladder, and duodenum Liver The liver is in the right hypochondriac and epigastric abdominopelvic regions. Four lobes: L, R, caudate, quadrate Falciform ligament divides the L and R lobes. The hepatocytes secrete bile. Bile is released into bile canaliculi common hepatic duct common bile duct duodenum or the cystic duct into the gallbladder. 26 8/26/2016 Liver Metabolic Regulation: Hematological Regulation: All blood leaving the absorptive areas of the digestive tract flow through the liver before reaching the general circulation. Largest blood reservoir in the body Kupffer cells synthesize plasma proteins synthesize clotting factors Synthesizes bile made of ions, bilirubin, cholesterol & lipids (bile salts). Bile dilutes and buffers acids in chyme Bile salts are synthesized from cholesterol in the liver and serve to emulsify fat Histology of the Liver 27 8/26/2016 Histology of the Liver Gallbladder Muscular pear shaped organ Stores and concentrates bile Liver cells produce about 1 L of bile /day. The gallbladder can store 40-70 ml. Bile secretion occurs continuously but is released into the duodenum only under stimulation of CCK. CCK relaxes the hepatopancreatic sphincter of the common bile duct. Gallbladder contractions eject bile. 28 8/26/2016 ????? Narrowing of the ileocecal valve would hamper the movement of materials to what organ? The digestion of which nutrient would be most impaired by damage to the exocrine pancreas? How would a decrease in the amount of bile salts in bile affect the digestion and absorption of fat? Large Intestine The large intestine is 1.5 m and 7.5 cm in diameter. Sections Cecum Colon Rectum Reabsorption, compaction and storage. Bacteria in the large intestine break down undigested food liberate some nutrients and produce gas. 29 8/26/2016 Large Intestine About 1500 ml of watery material arrives in the colon per day. About 1300 ml of water is reabsorbed. About 200 ml of feces is ejected. Average feces is 75% water, 5% bacteria, and 20% indigestible materials, inorganic matter, and epithelial cells. Large Intestines Cecum collects and stores material from the ileum and begins the process of compaction. The veriform appendix is attached to the cecum, about 9 cm long. Walls of the appendix are dominated by lymphoid nodules. Colon has thinner wall than the small intestine. Haustra are pouches in the colon produced by tension in the teniae, longitudinal bands of smooth muscle. 30 8/26/2016 Large Intestines Rectum Anal canal contains anal columns. The opening of the anal canal becomes keratinized. Internal anal sphincter is the circular muscle layer of the muscularis externa. External anal sphincter consists of skeletal muscle and guards the exit of the anorectal canal. Anatomy of the large intestine 31 8/26/2016 Large Intestine The gastroileal and gastroenteric reflexes move material into the cecum while you eat. Movement from the cecum to the transverse colon is slow. Movement from the transverse colon through the rest of the large intestine results from powerful peristaltic contractions called mass movements. Large Intestine Mass movements are stimulated by distension of the stomach and duodenum. Contractions are a result of the intestinal nerve plexuses. Fecal material is forced into the rectum. 32 8/26/2016 Histology of the large intestine Histology of the large intestine 33 8/26/2016 The gastric phase of digestion Daily volumes of fluid ingested, secreted, absorbed, and excreted from the GI tract 34 8/26/2016 Defecation Distension of the rectal wall triggers the defecation reflex resulting in a series of local peristaltic contractions in the colon & rectum. The movement of feces through the anal canal requires relaxation of the internal anal sphincter which shuts the external sphincter automatically. Release of feces requires conscious effort of open the external sphincter voluntarily. If the command does not arrive peristaltic contractions cease until additional rectal expansion triggers the defecation reflex a second time. Absorption of digested nutrients in the small intestine 35 8/26/2016 Clinical Application • inflammation of Hepatitis the liver • most commonly caused by viral infection • can be caused by reactions to drug, alcoholism or autoimmunity Hepatitis A – not washing hands or Signs and Symptoms eating raw shellfish • headache Hepatitis B – chronic; serum • low fever Hepatitis C – serum • fatigue • vomiting Hepatitis D – very severe; only • rash produces symptoms if infected with • foamy urine B; serum • pale feces Hepatitis E, F, G – more rare • jaundice • pain 36