Digestive 3

• Small intestine (SI)


  The same 4 layers are in the SI that are in the rest of the GI tract (mucosa, submucosa, muscular layer, serosa)
• Cells and epithelium in the SI


  • Work in secretion of enzymes, absorption of nutrients, and destroy pathogens
• Intestinal juice


  Provides a vehicle for absorption of substances from chyme as it comes in contact with the villi
• Brush border enzymes


  Found on the surfaces of the microvilli of absorptive cells, break down food products
• Mechanical digestion in the SI

  1. Segmentation: Localized contractions that mix chyme with digestive juices

  2. Peristalsis: Propulsive contractions
• Segmentation


  • Occurs most rapidly in the duodenum, about 12x per minute
• Peristalsis


  • Occurs after most of the meal has been absorbed
  • Then entire process from stomach to large intestine is about 3-5hrs
• Digestion of carbohydrates, proteins, lipids, and nucleic acids occurs in the SI
• Digestive activities in the pancreas, liver, gallbladder, and small intestine


  • Pancreas: Delivers pancreatic juice into duodenum
  • Liver: Produces bile (bile salts) necessary for emulsification and absorption of lipids
  • Gallbladder: Stores, concentrates, and delivers bile into duodenum
  • Small intestine: Major site of digestion and absorption of nutrients and water
• Structures and activities in the small intestine


  • Intestinal glands: Secrete intestinal juice to assist absorption
  • Absorptive cells: Digest and absorb nutrients
  • Goblet cells: Secrete mucus
  • Enteroendocrine cells: Secrete secretin, cholecystokinin, and glucose-dependent insulinotropic peptide
  • Paneth cells: Secrete lysozyme and phagocytosis
  • Duodenal glands: Secrete alkaline fluid to buffer stomach acids and mucus
  • Circular folds: Increase surface area for digestion and absorption
  • Intestinal villi: Fingerlike projections that are sites of absorption
  • Microvilli: Membrane-covered projections that contain enzymes and increase surface area
• Mechanical digestion in the colon


  1. Haustral churning: Distension reaches a point and walls contract to squeeze contents onward

  2. Peristalsis: Propulsive contractions at 3-12 per minute

  3. Mass peristalsis: Strong wave that drives contents into rectum, typically 3-4x per day
• The total time for a meal to pass through the colon depends on gastric emptying time
• The large intestine (colon) is around 5ft long in the average adult
• Structure of the colon


  • Consists of cecum, appendix, ascending, transverse, and descending sections
  • Haustra allow for last stop absorption of liquid and nutrients
• Functions of the colon


  • Haustral churning, peristalsis, and mass peristalsis drive contents into rectum
  • Bacteria convert proteins, break down amino acids, and produce some B vitamins
  • Absorption of some water, ions, and vitamins
  • Formation of feces
  • Defecation
• Histology of the colon


  • Same 4 layers as GI tract, microvilli but not villi, main role is final absorption and fecal matter creation
• Glands and cell types in the colon


  • Absorptive cells absorb final bits of water
  • Goblet cells secrete mucus to lubricate passage