LI Physio

• 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) to prevent rupture and release of bacteria into the abdominal cavity
• Haustra

  Pouches in the large intestine formed by contractions of the circular muscle

  
• Types of movement in the large intestine

  1. Nonpropulsive segmentation

  2. Peristalsis

  3. Mass movement

  
• Gastrocolic reflex


  Mass movements in the large intestine are typically triggered by the presence of food in the stomach
• Defecation reflex


  1. Distension of rectum by fecal material stimulates stretch receptors, which initiates the reflex to empty the rectum

  2. Involves contraction of the longitudinal muscle in the wall of the rectum and relaxation of the internal anal sphincter
• Defecation can be assisted by the Valsalva maneuver, which increases intra-abdominal pressure to facilitate defecation
• Functions of microflora in the large intestine

  • Fermentation of nonabsorbed nutrients
  • Metabolism of bilirubin
  • Production of vitamins
• Fermentation by intestinal microflora

  • Bacteria digest remaining carbohydrates and release gases like hydrogen, CO2, and methane
  • Bacteria also convert remaining proteins to amino acids and break them down further
• Bilirubin metabolism

  Bacteria convert bilirubin to simpler pigments like urobilinogen and stercobilin, which gives feces its brown color
• Bacteria produce B vitamins (thiamine, folate, biotin, riboflavin, panthothenic acid) and vitamin K
• Absorption of Na+ in the large intestine

  1. Via Na+-H+ exchanger in the proximal colon

  2. Via epithelial Na+ -channel in the distal colon

  
• Absorption of Cl- in the large intestine

  1. Via paracellular and transcellular pathways

  2. Paracellular absorption is passive, driven by the electrochemical gradient created by Na+ absorption

  3. Transcellular absorption involves Cl--HCO3- exchanger

  
• Absorption of K+ in the large intestine

  1. Only occurs in the distal portion of the colon via transcellular pathway

  2. Involves H+-K+ pump at the luminal membrane of enterocyte

  
• Water absorption in the large intestine

  • Depends on osmotic gradients and active transport processes
  • Results in the chyme becoming solid or semisolid, forming feces

  
• Composition of normal feces

  • Unabsorbed materials
  • Indigestable parts of food
  • Water
  • Bacteria
  • Stercobilin
• Secretion of bicarbonate in the large intestine

  Occurs via HCO3- secretion into the intestinal lumen

  
• Secretion of K+ in the large intestine

  1. Via passive paracellular pathway driven by the negative potential in the intestinal lumen

  2. Via active transcellular pathway involving Na+-K+ pump and Na+-K+-2Cl- co-transporter, stimulated by aldosterone