LI 2

• Large intestine


  Terminal part of the gastrointestinal tract, 1.5 meters in length

  
• Major functions of the large intestine

  • Absorption of water and electrolytes
  • Secretion of electrolytes
  • Production of certain vitamins by intestinal microflora
  • Formation of feces
  • Expulsion of feces from the body

  
• Cecum


  Initial part of the large intestine, separated from ileum by the ileocecal valve

  
• Colon


  Longest portion of the large intestine, subdivided into ascending, transverse, descending, and sigmoid colon
• Rectum


  Terminal portion of the large intestine, opens to the exterior with the anus
• Anus


  Controlled by an internal anal sphincter (smooth muscle) and an external anal sphincter (skeletal muscle)
• Appendix


  Small, finger-like projection extending from the cecum, contains lymphoid nodules that participate in immune responses
• Inflammation of the appendix, termed appendicitis, occurs when its lumen is obstructed
• If appendicitis occurs, the appendix should be surgically removed (appendectomy) as the inflamed appendix may rupture, releasing bacteria into the abdominal cavity
• Structural features that markedly increase the absorptive area of the small intestine

  • Villi
  • Crypts
  • Microvilli

  
• The wall of the large intestine does not have as many structural features that increase surface area for absorption compared to the small intestine

  
• There are no villi in the large intestinal cells
• Colonic folds, crypts, and microvilli are present in the large intestine

  
• Consequently, much less absorption occurs in the large intestine than in the small intestine
• Absorptive cells in the large intestine absorb and secrete water and ions, and goblet cells secrete mucus that lubricates the contents of the colon

  
• In contrast to the small intestine, the large intestine is not involved in absorption of nutrients. It can absorb only water and electrolytes

  
• Absorption of nutrients in the large intestine may occur only during the neonatal life

  
• Ileocecal sphincter


  Regulates the passage of chyme from the ileum into the cecum
• Haustra


  Pouches in the large intestine formed by contractions of the circular muscle
• Nonpropulsive segmentation in the large intestine

  Contractions of the circular muscle that contribute to mixing of the colonic contents
• Peristalsis in the large intestine


  Propels colonic content in the distal direction, occurs at a much slower rate than in the small intestine (one to three times a day)
• Mass movement in the large intestine

  Strong wave of contraction that begins in the middle of the transverse colon and quickly drives the contents of the colon into the rectum
• Mass movement is typically triggered by the presence of food in the stomach, called the gastrocolic reflex
• External anal sphincter


  Contains striated muscle, voluntarily controlled, receives somatic innervation from the spinal cord
• Internal anal sphincter


  Contains smooth muscle, involuntarily controlled, receives autonomic (parasympathetic) innervation from the sacral part of the spinal cord
• Defecation reflex


  Distension of the rectum by fecal material stimulates stretch receptors, which initiate a reflex that empties the rectum
• Defecation can be assisted by the Valsalva maneuver, which involves voluntary contraction of abdominal muscles and a forced expiration against a closed glottis
• The Valsalva maneuver increases intra-abdominal pressure, which compresses the sigmoid colon and rectum, facilitating defecation
• Functions of microflora in the large intestine

  • Fermentation of nonabsorbed nutrients
  • Metabolism of bilirubin
  • Production of vitamins
• The glands in the large intestine secrete mucus, but no enzymes

  
• The chemical digestion in the large intestine is provided by bacteria that populate its lumen

  
• Bacteria digest any remaining carbohydrates, and release gases such as hydrogen, CO2, and methane, which contribute to flatus in the colon

  
• Bacteria also convert any remaining proteins to amino acids, and break down the amino acids into simpler substances such as indole, skatole, and hydrogen sulfide, which are eliminated into feces and contribute to its odor
• Bacteria convert bilirubin to simpler pigments such as urobilinogen and stercobilin, which gives feces its brown color
• Bacteria produce B vitamins (thiamine, folate, biotin, riboflavin, panthothenic acid) and vitamin K
• Absorption in the large intestine

  Absorbs Na+, Cl-, K+, and water

  
• Mechanism 1 for Na+ absorption in the large intestine

  Na+-H+ exchanger, operates in the proximal colon
• Mechanism 2 for Na+ absorption in the large intestine

  Epithelial Na+ -channel, operates in the distal colon
• Absorption of Na+ and secretion of K+ in the large intestine is controlled by aldosterone

  
• Cl- absorption in the large intestine


  Occurs via paracellular and transcellular pathways