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Anatomy of the Digestive System Chapter 25 Gastrointestinal (GI) Tract Walls of the GI Tract 1. Mucosa – inner lining of tract – secretes mucous – Highly folded – increased surface area for absorption • Microvilli 2. Submucosa – Mainly connective tissue – Contains: • Exocrine glands – Secretes acids & enzymes Walls of the GI Tract 3. Muscularis – Smooth muscle • 2 layers – – – Longitudinal (outer) Circular (inner) Move particles by peristalsis Walls of the GI Tract 4. Serosa – – – Outermost layer Connective tissue and peritoneum (visceral layer) Mesentery connects the parietal & visceral portions of the peritoneum Walls of the GI Tract – Cell Modifications • Although the layers remain the same throughout the GI tract, the cell types may change – Ex: Mucosa layer of the esophagus is composed of stratified squamous cells to resist abrasion, but transitions to simple columnar cells for absorption and secretion Mouth • Also called the oral or buccal cavity • Composed of: – Lips, cheeks, tongue, hard palate, soft palate Tongue • Skeletal muscle covered by mucous membrane • Helps in chewing (mastication), swallowing (deglutition) and speech • Papillae cover upper portion of the tongue – Vallate: posterior portion of tongue; contain taste buds – Fungiform: sides and tip of tongue; contain taste buds – Filiform: anterior 2/3 of tongue; do not contain taste buds Tongue • Frenulum – anchors tongue to floor of the mouth – Ankyloglossia: frenulum is too short; results in speech problems; “tongue-tied” • Floor of mouth and underside of tongue are very vascular – Sublingual drugs (nitroglycerin, morphine) Salivary Glands • Parotid – Upper jaw; anterior & inferior to external ear – watery saliva containing enzymes • Sublingual – floor of the mouth – Mostly mucous saliva • Submandibular – Opens on either side of the frenulum – Mixture of watery (enzymes) and mucous secretions Teeth • Organs of mastication • Increase surface area that digestive enzymes can work on food • 3 main sections: – Crown: exposed portion; covered by enamel – Neck: surrounded by the gingivae (gums) – Root: fits into the alveolar process of the jaw (gomphosis) Teeth • Children - 20 teeth – deciduous or primary • 16 teeth per jaw - 32 total (adult) – Incisors (4) • blade shaped - used to tear food – Canines (Cuspids) (2) • Pointed teeth - used to tear food – Premolars (Bicuspids) (4) • 2 points - used to tear and grind food – Molars (6) • 4 points - used for grinding • Last set called wisdom teeth Esophagus • Collapsible, muscular, mucous-lined tube • 25cm; extends from pharynx to stomach • Posterior to trachea • Upper esophageal sphincter (UES) – prevents air from entering during respiration • Lower esophageal sphincter or cardiac sphincter Esophagus - Problems • Esophageal hiatus – hole in the diaphragm through which the esophagus enters the abdominal cavity – Enlargement results in lower portion of esophagus and stomach bulging upward into the chest hiatal hernia • Gastroesophageal reflux disease (GERD) – backward flow of stomach acid through the cardiac sphincter into the lower esophagus Stomach • Elongated, pouch-like structure • Mostly in LUQ • After eating the stomach walls distend; when empty size of large sausage • In adults holds 1-1.5 liters Stomach Landmarks (fig 25-10) • • • • Fundus – enlarged upper left portion Body – central portion Pylorus – lower portion Lower esophageal sphincter (also cardiac sphincter) • Pyloric sphincter • Lesser curvature • Greater curvature Modifications of the Stomach Wall • Gastric Muscosa – – Arranged into folds which allow for distention (Rugae) Contains gastric glands which secrete gastric juice • 3 major secretory cells: 1. Chief cells: secrete enzyme of gastric juice 2. Parietal cells: secrete hydrochloric acid (HCl) 3. Endocrine cells: secrete ghrelin (stimulate hypothalamus to increase appetite) and gastrin (influences digestive functions) Modifications of the Stomach Wall • Gastric Muscle – Muscularis layer is composed of 3 smooth muscle layers – Superficial to deep • Longitudinal • Circular • Oblique – Allows stomach to contract at many different angles Modifications of the Stomach Wall • Serosa Layer – Visceral layer forms the greater omentum (over intestines) and lesser omentum (connects stomach to liver) Small Intestine • 1 inch in diameter; 20 feet in length • 3 divisions: – Duodenum • 10 inches • C shaped – Jejunum • 8 feet • Begins where the sm. intestine turns forward and downward – Ileum • 12 feet Walls of the Small Intestine • Mucosa lining has circular folds plicae • Small projections called villi (singular – villus) cover plicae • Villi and microvilli increase surface area for absorption Walls of the Small Intestine • Goblet cells are located on villi and in crypts – Secrete mucus • Secretory cells in each crypt produce an enzyme that prevents bacterial growth in the small intestine Large Intestine • 2.5 inches in diameter; 5-6 feet in length • Divisions – Cecum – Colon – rectum Divisions of the Large Intestine • Cecum – First 2-3 inches of the large intestine – Blind pouch in right quadrant Divisions of the Large Intestine • Colon (4 divisions) – Ascending • Vertical position in right quadrant • Ileum joins superior to cecum • Ileocecal valve allows material to pass into the large intestine – Transverse • Horizontal position below liver, stomach & spleen • Extends from the hepatic flexure to the splenic flexure Divisions of the Large Intestine – Descending • Vertical position in the left quadrant • Extends to the level of the iliac crest – Sigmoid colon • Below iliac crest • Means “s-shaped” • Bends from L to R Divisions of the Large Intestine • Rectum – Last 7-8 inches of the large intestine – Anal canal is the last inch • Mucous lined vertical folds anal columns – Opening = anus Walls of the Large Intestine • Intestinal mucous glands – Secrete mucous that coats feces • Longitudinal muscles are grouped into tape-like strips called taeniae coli • Circular muscles are grouped into rings which form pouches haustra • Circular muscles in the rectum form rectal valves Peritoneum • Continuous sheet of serous membrane – Lines walls of abdominal cavity (parietal layer) – Outer layer of abdominal organs (visceral layer) • Binds abdominal organs together – Mesentery: projection of the parietal layer • Attached to small intestine • Allows free movement without becoming tangled (volvulus) – Greater omentum: continuation of the stomach’s serosa layer • Covers small intestines – Lesser omentum • Attaches from the liver to the stomach Vermiform Appendix • Attached to the cecum in the RLQ • 3-4 inches in length • “breeding ground” for intestinal or normal flora – Nonpathogenic bacteria – Aids in digestion and absorption Appendicitis • Mucous lining becomes inflamed • Fecal matter or food becomes trapped causing irritation and inflammation • Rupturing of the appendix results in infectious materials in the abdominal cavity – May cause infection of the peritoneum and/or other abdominal organs Appendicitis • S/S – Nausea/vomiting – RLQ pain (McBurney’s Point) – Rebound tenderness • An enlarged appendix can be removed through a laparoscopic surgical procedure Liver • Largest gland in the body • Weighs 3-4 pounds • RUQ Anatomy of the Liver • Two lobes connected by the falciform ligament – Left lobe 1/6 the size of the right lobe – 3 divisions of the right lobe • Right lobe proper, caudate lobe and quadrate lobe (seen inferiorly) (fig 25-22) Anatomy of the Liver Anatomy of the Liver • Hepatic lobules – anatomical units of the liver – Pentagon-shaped cylinders • Blood enters the lobules from the hepatic artery & hepatic portal vein – Arterial blood oxygenates – Venous blood passes for inspection • Kupffer cells remove bacteria, old RBCs, dissolved toxins • Venous blood continues to the inferior vena cava – Bile formed by hepatic cells passes through the lobules to the bile ducts Fig 25-23, page 758 Bile Ducts • Small bile ducts merge to form R and L hepatic ducts – R and L hepatic ducts form common hepatic duct – Cystic duct and common hepatic duct form common bile duct – Common bile duct opens into the duodenum – Fig 25-25 Bile Ducts Liver Functions • Detoxification • Bile secretion (aids in the digestion & absorption of lipids) • Protein, fat and carbohydrate metabolism • Hematopoisesis (blood cell production) Gallbladder • • • • • • Pear-shaped sac 3-4 inches long Can hold 30-50mL of bile Located on inferior surface of the liver Rugae (similar to stomach) Functions: – Stores and concentrates bile – Contracts and ejects bile into duodenum during digestion Cholecystitis • Inflammation of the gallbladder • Often caused by gallstones (cholelithiasis) – Solid precipitants; mostly cholesterol – High incidence in obese individuals and those undergoing rapid weight loss • Treatment: – Laparoscopic cholecystectomy – Ultrasound lithotripsy – Oral medications (Actigall) Pancreas • 6-9 inches long • LUQ; behind stomach extending to the spleen • Endocrine & Exocrine tissue • Exocrine tissue arranged in a compound acinar formation (grapelike) – Release digestive enzymes into microscopic ducts which join to the main pancreatic duct – Pancreatic duct empties into the duodenum