Functions of the Colon

1. Colonic Motility

• Segmentation (Haustrations):

  • Location: Throughout the colon (from the caecum to the sigmoid colon).
  • Contractions occur in small segments (~2.5 cm), almost occluding the lumen.
  • Frequency increases from:
    • 2/min in the caecum to
    • 6/min in the sigmoid colon.

• Peristalsis:

  • Location: Throughout the colon.
  • Weak, slow movements that help propel contents over a long period.

• Mass Movements:

  • Location: Typically start in the transverse colon and move towards the rectum.
  • Occur 1-3 times/day, usually after meals.
  • Triggered by:
    • Gastrocolic reflex (stomach stretch after meals).
    • Duodenocolic reflex (duodenal stretch after food enters the small intestine).
  • Propels fecal matter into the rectum.

2. Defecation Reflex

• Rectal Distension:

  • Location: Rectum.
  • Triggers peristalsis in the descending and sigmoid colon, and rectum.
  • Causes relaxation of the internal anal sphincter (involuntary).

• Conscious Control:

  • Location: External anal sphincter (striated muscle).
  • Controlled by the pudendal nerves, enabling voluntary relaxation of the external sphincter during defecation.
  • Valsalva maneuver increases intra-abdominal pressure to aid defecation.

• Fullness Perception:

  • Location: Sensory signals travel from rectal stretch receptors to the brain, conveying the urge to defecate.

3. Fluid and Electrolyte Absorption

• Volumes of Secretion and Absorption:

  • Fluid secretions occur in the:

    • Stomach (gastric juice),
    • Liver (bile),
    • Pancreas (pancreatic juice),
    • Small intestine (intestinal juice).

  • Absorption Sites:

    • Small Intestine (Ileum): Absorbs most of the fluid (~8.5 L/day).
    • Large Intestine (Colon): Absorbs the remaining fluid (~1.4 L/day).
    • Excreted in Feces: About 0.1 L/day.

• Water and Electrolyte Reabsorption:

  • Small Intestine (Ileum):

    • Location: Ileum (last part of the small intestine).
    • Uses a Na+/K+ pump in the basolateral membrane for sodium absorption.
    • Na+ enters cells with amino acids and sugars (primary route for salt absorption).
    • This mechanism remains functional even in the presence of bacterial toxins.

  • Large Intestine (Colon):

    • Location: Colon.
    • Electrolyte exchange (Na+/H+ and HCO3-/Cl- exchangers) similar to the small intestine, though no amino acid or sugar carriers are present.
    • Na+ is absorbed by a different Na+ carrier.
    • NaCl absorption is inhibited by bacterial toxins in this area.

4. Colon Secretions & Chloride Secretion Mechanism

• Chloride Secretion:

  • Location: Intestinal crypts of the colon.
  • Cl- is secreted via the CFTR (Cystic Fibrosis Transmembrane Regulator) channel.

• Impact of Cholera Toxin:

  • Location: Colon.
  • Cholera toxin (from Vibrio cholerae) activates the CFTR channel, resulting in excessive NaCl and water secretion.
  • This can produce up to 20 L/day of watery stool (leading to severe diarrhea).
  • Toxin stimulates secretion of salt and water while inhibiting absorption.
  • However, sodium and glucose co-transport systems are unaffected, allowing for treatment with oral rehydration solutions containing sodium and glucose.