GI system- Functional Anatomy

• Gastrointestinal System Processes


  • Consume nutrients too large to enter directly into the bloodstream
  • Nutrients undergo digestion before absorption
  • Motility and secretion aid digestion and absorption
• Division of the gastrointestinal system

  • GI tract
  • Accessory glands
• GI tract


  Hollow tube, 15 feet long, from mouth to anus
• GI tract


  1. Mouth

  2. Pharynx

  3. Esophagus

  4. Stomach

  5. Small intestine

  6. Colon

  7. Rectum

  8. Anus
• Wall of the GI tract

  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa
• Mucosa


  Lining of the lumen, separates GI lumen from internal environment
• Epithelial cell types in mucosa


  • Enterocytes: absorptive cells
  • Exocrine cells
  • Endocrine cells
• Layers of mucosa


  • Epithelial lining
  • Lamina propria: connective tissue
  • Muscularis mucosae: thin smooth muscle
• Lamina propria


  • Connective tissue, small blood vessels, lymphatic vessels, nerves, lymphoid tissue
• Muscularis mucosae


  • Thin layer of smooth muscle, longitudinal and circular fibers, function in mixing lumenal contents
• Submucosa


  • Thick layer of connective tissue, distensibility, elasticity, large blood vessels, lymphatic vessels, submucosal plexus
• Muscularis externa


  • Two layers of smooth muscle, inner circular layer changes diameter, outer longitudinal layer changes length, myenteric plexus
• Serosa


  • Inner layer: connective tissue, outer layer: epithelial tissue (mesothelium)
• Functional anatomy of gastrointestinal tract organs


  • Mouth, pharynx, and esophagus
  • Stomach
  • Small intestine
  • Colon
  • Rectum and anus
• Mouth


  1. Digestion begins with mastication (chewing)

  2. Secretion of saliva: lubricant, salivary amylase digests starch and glycogen
• Pharynx


  Throat, common passageway for air and food, food enters via the esophagus
• Esophagus


  • Muscular tube from pharynx to stomach, upper one-third skeletal muscle, lower two-thirds smooth muscle, upper and lower esophageal sphincters
• Stomach


  • Stores food after swallowing, secretes gastric juice, releases food into intestine slowly, fundus, body, antrum, contractions mix chyme, gastric emptying, stomach folds and rugae
• Secretory products of stomach

  • Pepsinogen: precursor for pepsin
  • Hydrogen ions: maintain acidic environment
  • Intrinsic factor: necessary for B12 absorption
  • Gastrin: hormone
  • Mucus
• Acidic environment of stomach

  pH=2, necessary for activating pepsinogen, denatures proteins, kills bacteria
• Gastric mucosal barrier


  • Protective layer of mucus and bicarbonate, secreted from neck cells and goblet cells
• Small intestine


  • Coiled, hollow tube, 8-10 feet long, between stomach and large intestine, primary site of digestion and absorption, three divisions: duodenum, jejunum, ileum
• Secretions into duodenum


  1. Pancreatic juice: digestive enzymes, bicarbonate

  2. Bile: secreted from liver, contains bile salts that aid in fat digestion
• Absorption in small intestine

  • Absorption is completed within first 20% of the intestine length, anatomically arranged to provide a large surface area for absorption
• Small intestine wall


  • Villi increase surface area of epithelium, contain blood vessels and lacteal for absorption of nutrients, microvilli increase surface area of epithelial cells, form brush border, crypts of Lieberkuhn secrete bicarbonate-rich fluid
• Hepatic portal system


  Vasculature that delivers absorbed nutrients to the liver before nutrients enter the general circulation
• Large intestine


  • Consists of cecum, colon, and rectum, ileocecal sphincter, teniae coli, functions: concentration of wastes into feces, absorption of most water, storage of feces until defecation, four sections: ascending, transverse, descending, sigmoid
• Rectum and Anus


  • Colon empties into rectum, feces excreted through anus, internal and external anal sphincters
• Accessory glands


  • Salivary glands: secrete saliva
  • Pancreas: secretes pancreatic juice
  • Liver: secretes bile
• Saliva


  • Rich in bicarbonate ions, contains mucus, enzymes: salivary amylase, lysozyme
• Pancreas


  • Has exocrine and endocrine portions, exocrine portion produces pancreatic juice rich in bicarbonate and enzymes: amylase, lipases, proteases, nucleases
• Liver


  • Secretes bile, processes nutrients (glucose to glycogen, amino acids to fatty acids, triglycerides and cholesterol synthesis), removes old red blood cells, catabolizes hemoglobin, generates bilirubin, removes wastes and toxins, synthesizes plasma proteins, secretes and modifies hormones
• Biliary system


  • Liver synthesizes bile, gallbladder stores bile, common bile duct transports bile from liver and/or gallbladder to duodenum, joins with pancreatic duct to form ampulla of Vater, sphincter of Oddi regulates flow
• Hepatocytes


  Specific cell types of the liver that secrete bile into bile canaliculi