topic 10 - digestive system

• Main functions of the digestive system

  • Ingest food
  • Break it down into nutrient molecules
  • Absorb molecules into the bloodstream
  • Rid body of any indigestible remains
• Organs of the digestive system

  • Alimentary canal (gastrointestinal or GI tract) - muscular tube that runs from the mouth to anus (approx. 9 m long)
  • Accessory digestive organs - teeth, tongue, gallbladder, digestive glands (salivary glands, liver, pancreas)
• Digestive processes


  1. Ingestion: eating

  2. Propulsion: movement of food through the alimentary canal

  3. Swallowing - mix of voluntary and involuntary

  4. Peristalsis - involuntary

  5. Mechanical breakdown

  6. Digestion: chemical, uses enzymes to break down food

  7. Absorption: passage of digested fragments from lumen of GI tract into blood or lymph

  8. Defecation: elimination of indigestible substances via anus in form of feces
• Peristalsis and segmentation


  • Peristalsis: major means of propulsion of food, involves alternating waves of contraction and relaxation
  • Segmentation: local constriction of intestine mixes food with digestive juices, repeated movement across intestinal wall, mechanical breakdown, limited propulsion
• Mechanical breakdown


  • Increases surface area of food, for enzymatic breakdown
  • Chewing
  • Churning
• Abdominal GI organs


  • Abdominopelvic cavity is lined with moist serous membranes: parietal peritoneum, peritoneal cavity, visceral peritoneum
  • Mesenteries: double layer of peritoneum that support and stabilize GI tract organs, store fat, contain blood & lymphatic vessels, nerves
• Tunics of the abdominal GI tract

  • Mucosa: secrete mucus, digestive enzymes, hormones, absorb into blood, protect against infection
  • Submucosa: areolar connective tissue, nerve, lymphatic vessels, blood vessels, elastic fibers
  • Muscularis externa: inner circular layer forms sphincters, outer longitudinal layer for peristalsis and segmentation
  • Serosa: areolar connective tissue, visceral peritoneum
• Mouth and associated organs

  • Mouth: where food is chewed and mixed with saliva, begins digestion
  • Tongue: gripping, repositioning, mixing of food, formation of bolus, initiation of swallowing, speech, taste
  • Salivary glands: parotid, submandibular, sublingual - functions include cleansing, dissolving food chemicals, moistening, beginning chemical breakdown
  • Teeth: lie in alveolar processes, covered by enamel, mastication (chewing)
• Deglutition


  Swallowing, bolus travels through pharynx to esophagus
• Esophagus


  • Passageway for food from pharynx to stomach, posterior to larynx and trachea, through mediastinum, propels bolus to stomach via peristalsis, gastroesophageal sphincter connects to stomach
• Stomach


  • Physical and chemical degradation of food, digestion via hydrochloric acid and pepsin, holds food, produces chyme (slurry of food and liquid)
  • Gross anatomy: cardia, fundus, body, pyloris, smooth muscle pyloric sphincter
  • Histology: mucosa (epithelium, lamina propria, muscularis mucosae), submucosa, muscularis externa, serosa
• Small intestine


  • Major organ of digestion and absorption, 3 sections: duodenum, jejunum, ileum
  • Histology: circular folds, villi, microvilli
• Large intestine


  • Ileocecal valve to anus, shorter than small intestine but greater diameter, site of absorption of remaining water, contains mutualistic bacteria
  • Gross anatomy: cecum, appendix, colon (ascending, transverse, descending, sigmoid), rectum, anal canal
• Accessory digestive organs


  • Liver
  • Gallbladder
  • Pancreas
• Liver


  • 4 lobes, produces bile for fat digestion, stores/processes nutrients, detoxification, hepatic portal circulation
• Gallbladder


  • Inferior surface of liver, stores and concentrates bile, cystic duct connects to common bile duct
• Pancreas


  • Exocrine functions: secretes digestive enzymes (proteases, amylase, lipases, nucleases) and bicarbonate
  • Endocrine functions: secretes insulin and glucagon
• Bile and pancreatic secretion into small intestine


  Bile duct and pancreatic duct unite in wall of duodenum, fuse together in hepatopancreatic ampulla, hepatopancreatic sphincter controls entry