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23 PART 1 The Digestive System Overview of the Digestive System • Organs are divided into two groups • Alimentary canal • Mouth, pharynx, and esophagus • Stomach, small intestine, and large intestine • Accessory digestive organs • Teeth and tongue • Gallbladder, salivary glands, liver, and pancreas • Accessory organs are connected to the alimentary canal by ducts • Secretions contribute to breakdown of foodstuffs Abdominal Regions • Four lines divide abdominal wall into nine regions • Midclavicular lines—vertical lines of grid • Subcostal plane—superior horizontal line • Connects inferior points of costal margin • Transtubercular plane—inferior horizontal line • Connects tubercles of iliac crests Abdominal Quadrants • A simpler method of sectioning the anterior abdominal wall • Right upper quadrant • Left upper quadrant • Right lower quadrant • Left lower quadrant The Peritoneal Cavity and Peritoneum • Peritoneum—a serous membrane • Visceral peritoneum—surrounds digestive organs • Parietal peritoneum—lines the body wall • Peritoneal cavity—a slitlike potential space The Peritoneal Cavity and Peritoneum • Mesentery—a double layer of peritoneum • Holds organs in place • Sites of fat storage • Provides a route for circulatory vessels and nerves The Peritoneal Cavity and Peritoneum • Ventral mesenteries • Falciform ligament • Binds anterior part of liver to anterior abdominal wall • Lesser omentum • Attaches liver to the lesser curvature of stomach The Peritoneal Cavity and Peritoneum • Dorsal mesenteries • Greater omentum • Connects greater curvature of stomach to posterior abdominal wall • A “fatty apron” • Transverse mesocolon • Holds transverse colon in place • Sigmoid mesocolon • Connects the sigmoid colon to posterior pelvic wall The Peritoneal Cavity and Peritoneum • Retroperitoneal organs • Behind the peritoneum • Peritoneal organs • Digestive organs that keep their mesentery Secondarily Retroperitoneal Organs • Initially formed within peritoneum • Become retroperitoneal • Fuse to posterior abdominal wall Digestive Processes • Ingestion—occurs in the mouth • Propulsion—movement of food • Peristalsis—major means of propulsion • Mechanical breakdown—prepares food for chemical digestion • Chewing, churning food in stomach, segmentation • Segmentation is rhythmic local constrictions of intestine Digestive Processes • Chemical digestion—complex molecules broken down to chemical components • Mouth • Stomach • Small intestine • Absorption—transport of digested nutrients • Defecation—elimination of indigestible substances as feces Peristalsis • Major means of propulsion • Adjacent segments of the alimentary canal relax and contract Segmentation • Rhythmic local contractions of the intestine • Mixes food with digestive juices Histology of the Alimentary Canal Wall • Same four layers from esophagus to anus • The mucosa—innermost layer • Consists of • Epithelium • Lamina propria • Muscularis mucosae • The submucosa—external to the mucosa • Contains blood and lymphatic vessels, nerve fibers Histology of the Alimentary Canal Wall • The muscularis externa—external to the submucosa • Two layers • Circular muscularis—inner layer • Longitudinal muscularis—outer layer • The serosa—the outermost layer • Is the visceral peritoneum Smooth Muscle • Primarily found in walls of viscera • Fibers elongated • Have one centrally located nucleus • Grouped into sheets • Longitudinal layer—parallel to long axis of organ • Circular layer—deeper layer, fibers run around circumference of organ Smooth Muscle Contraction • Mechanism of contraction • Myofilaments operate by interaction with cytoskeleton • Dense bodies—correspond to Z-discs of skeletal muscle • Sliding myofilaments shorten the muscle cell by pulling on cytoskeleton • Entry of Ca2+ into sarcoplasm stimulates contraction Smooth Muscle Contraction • Contraction is slow and sustained • Takes 30x longer to contract and relax • Resistant to fatigue • Smooth muscle of arteries and visceral organs must sustain contraction over long periods • Energy requirements are low • Mitochondria are not abundant Innervation of Smooth Muscle • Innervated by ANS • Few fibers per sheet innervated • • Sheet of smooth muscle contracts as a unit • Called single-unit innervation Multiunit innervation • Each smooth muscle cell innervated • Iris of eye and arrector pili muscles Nerve Plexuses • Myenteric nerve plexus • Lies between circular and longitudinal muscularis • Controls peristalsis and segmentation • Submucosal nerve plexus • Lies in submucosa • Signals glands to secrete • Innervation • Sympathetic and parasympathetic motor fibers • Visceral sensory fibers 23 PART 2 The Digestive System The Mouth and Associated Organs • Mouth (oral cavity) • Mucosa-lined cavity • Boundaries are • Lips anteriorly • Cheeks laterally • Palate superiorly • Tongue inferiorly • Fauces of oropharynx posteriorly The Mouth and Associated Organs • The mouth—oral cavity • Mucosal layer • Stratified squamous epithelium • Lamina propria • The lips and cheeks • Formed from orbicularis oris and buccinator muscles, respectively Anatomy of the Mouth • The labial frenulum • Connects lips to gum • The palate • Forms the roof of the mouth • Boundaries of the fauces • Palatoglossal arches • Palatopharyngeal arches The Tongue • Interlacing fascicles of skeletal muscle • Grips food and repositions it • Helps form some consonants • Intrinsic muscles—within the tongue • Extrinsic muscles—external to the tongue • Lingual frenulum • Secures tongue to floor of mouth The Superior Surface of the Tongue • Tongue papillae • Filiform papillae—no taste buds • Fungiform papillae • Vallate papillae • Sulcus terminalis • Marks border between mouth and pharynx • Posterior one-third of tongue lies in oropharynx • Lined with lingual tonsil The Teeth • Deciduous teeth—20 teeth • First appear at 6 months of age • Permanent teeth—32 teeth • Most erupt by the end of adolescence • Dental formula—shorthand • Formula for adult dentition indicates number and position of teeth • 2I, 1C, 2P, 3M Vessels and Nerves to the Teeth • Upper teeth • Innervation is superior alveolar nerves branching from maxillary division of CN V • Lower teeth • Innervation is inferior alveolar nerves branching from mandibular branch of CN V • Arterial supply to teeth • Superior and inferior alveolar arteries (branching from maxillary arteries) Tooth Structure • Crown—exposed surface • Root—in tooth socket • Outer layer is enamel • Dentin—underlies enamel • Pulp cavity—center of tooth The Salivary Glands • Produce saliva • Compound tubuloalveolar glands • • • Parotid glands • Parotid duct—parallel to zygomatic arch • Contain only serous cells Submandibular glands • Lie along medial surface of mandible Sublingual glands • Lie in floor of oral cavity • Contain primarily mucous cells The Pharynx • Oropharynx and laryngopharynx • Passages for air and food • Lined with stratified squamous epithelium • External muscle layer • Consists of superior, middle, and inferior pharyngeal constrictors The Esophagus • Gross anatomy—muscular tube • Begins as a continuation of the pharynx • Joins the stomach inferior to the diaphragm • Cardiac sphincter—closes lumen to prevent stomach acid from entering esophagus The Esophagus • Microscopic anatomy • Epithelium is stratified squamous epithelium • When empty, mucosa and submucosa are in longitudinal folds • Mucous glands—primarily compound tubuloalveolar glands The Esophagus • Muscularis externa • Skeletal muscle • First one-third of length • Mixture of skeletal and smooth muscle • Middle one-third of length • Smooth muscle • Inferior one-third of length • Adventitia • Most external layer of esophagus The Stomach • Site where food is churned into chyme • Secretion of pepsin begins protein digestion • Functions under acidic conditions • Food remains in stomach approximately 4 hours • Regions of the stomach • • • • Cardial part Fundus Body Pyloric part Microscopic Anatomy of the Stomach • Muscularis has three layers • Circular and longitudinal layers and oblique layer • Epithelium is simple columnar epithelium • Mucosa dotted with gastric pits • Gastric glands—deep to gastric pits Microscopic Anatomy of the Stomach • Gastric glands of fundus and body • Mucous neck cells • Secrete a special mucus • Parietal (oxyntic) cells • Secrete hydrochloric acid and gastric intrinsic factor • Chief (zymogenic) cells • Secrete pepsinogen • Pepsinogen is activated to pepsin when it encounters acid in the gastric glands The Small Intestine—Gross Anatomy • Longest portion of the alimentary canal • Site of most enzymatic digestion and absorption • Three subdivisions • Duodenum • Jejunum • Ileum • Innervation • Parasympathetic fibers from vagus nerve • Sympathetic from thoracic splanchnic nerves The Duodenum • Receives digestive enzymes and bile • Main pancreatic duct and common bile duct enter duodenum • Sphincters control entry of bile and pancreatic juices The Small Intestine—Microscopic Anatomy • Modifications for absorption • Circular folds • Transverse ridges of mucosa and submucosa • Villi • Fingerlike projections of the mucosa • Covered with simple columnar epithelium • Microvilli • Further increase surface area for absorption Histology of the Intestinal Wall • Absorptive enterocytes • Uptake digested nutrients • Goblet cells • Secrete mucus that lubricates chyme • Enteroendocrine cells • Secrete hormones • Intestinal crypts • Epithelial cells secrete intestinal juice 23 PART 3 The Digestive System The Large Intestine • Digested residue contains few nutrients • Small amount of digestion by bacteria • Main functions • Absorb water and electrolytes • Mass peristaltic movements force feces toward the rectum Gross Anatomy of Large Intestine • Subdivided into • Cecum, appendix, colon, rectum, anal canal • Special features of large intestine • Teniae coli • Thickening of longitudinal muscularis • Haustra • Puckering created by teniae coli • Epiploic appendages • Fat-filled pouches of visceral peritoneum Gross Anatomy of Large Intestine • Cecum • Blind pouch • Beginning of large intestine • Appendix • Contains lymphoid tissue • Neutralizes pathogens • Colon • Divided into distinct segments • Ascending, transverse, descending, and sigmoid colon Gross Anatomy of Large Intestine • Rectum • • Descends along the inferior half of the sacrum Anal canal • The last subdivision of the large intestine • Lined with stratified squamous epithelium Vessels and Nerves of the Large Intestine • First half of large intestine • Arterial supply—superior mesenteric artery • Innervation • Sympathetic innervation—superior mesenteric and celiac ganglia • Parasympathetic innervation—vagus nerve Vessels and Nerves of the Large Intestine • Distal half of large intestine • Arterial supply—inferior mesenteric artery • Innervation • Sympathetic innervation—inferior mesenteric and hypogastric plexuses • Parasympathetic innervation—pelvic splanchnic nerves Microscopic Anatomy of Large Intestine • Villi are absent • Contains numerous goblet cells • Intestinal crypts—simple tubular glands • Lined with simple columnar epithelial tissue • Epithelium changes at anal canal • Becomes stratified squamous epithelium The Liver • Largest gland in the body • Performs over 500 functions • Digestive function • Bile production • Performs many metabolic functions Microscopic Anatomy of the Liver • Hepatocyte—functional cells of the liver • Portal triad composed of • Bile duct tributary • Branch of hepatic portal vein • Branch of hepatic artery • Hepatic macrophages—destroy bacteria Microscopic Anatomy of the Liver • Some functions of hepatocytes • Rough ER manufactures blood proteins • Smooth ER produces bile salts, detoxifies poisons • • • • • Peroxisomes detoxify poisons (alcohol) Golgi apparatus packages secretory products Mitochondria provide energy for liver processes Glycosomes store sugar Great capacity for regeneration The Gallbladder • Stores and concentrates bile • Expels bile into duodenum • Bile emulsifies fats • Cholecystokinin—released from enteroendocrine cells in response to fatty chyme 23 PART 4 The Digestive System The Pancreas • Has both exocrine and endocrine functions • Endocrine function • Produces insulin and glucagon • Regulates blood sugar • Exocrine function • Produces most enzymes that digest food in the small intestine The Pancreas • Is secondarily retroperitoneal • Main pancreatic duct • Extends the length of the pancreas • Joins bile duct to form the hepatopancreatic ampulla • Empties into duodenum • Arterial supply is from hepatic, splenic, and superior mesenteric arteries The Pancreas • Exocrine function • Acinar cells make, store, and secrete pancreatic enzymes • Enzymes are activated in the duodenum Peptic Ulcers • Are erosions of the mucosa of a region of the alimentary canal • Gastric ulcers • Occur in pyloric region of the stomach • Duodenal ulcers • Occur in duodenum of the small intestine Peptic Ulcers • Caused by Helicobacter pylori • H. pylori • • Acid-resistant Binds to gastric epithelium • Induces oversecretion of acid and inflammation Disorders of the Digestive System • Intestinal obstruction • Mechanical obstructions • Adhesions, tumors, or foreign objects • Nonmechanical obstruction • Halt in peristalsis • Trauma • Intestines touched during surgery Disorders of the Digestive System • Inflammatory bowel disease • Inflammation of intestinal wall • Crohn’s disease • Ulcerative colitis • Viral hepatitis • Jaundice and flulike symptoms • Major types—A, B, C, and G Disorders of the Digestive System • Cystic fibrosis and the pancreas • Pancreatic ducts become blocked with mucus • Clogged ducts prevent pancreatic juices from entering small intestine • Leads to malabsorption of fats and other nutrients Embryonic Development of the Digestive System • Alimentary canal is formed in week 3 • Encloses tubular portion of yolk sac • Vitelline duct • Landmark dividing into three regions • Foregut • Midgut • Hindgut The Digestive System in Later Life • Middle age • Gallstones and ulcers • Old age • Activity of digestive organs declines • Fewer digestive juices and enzymes are produced • Absorption is less efficient • Dehydration of fecal mass leads to constipation • Diverticulosis and cancer of digestive organs