II. Physiology

  1. Stool propelled via colon to Rectum
    1. Reaches cecum 4 hours after ingestion
    2. Reaches Rectum 7-8 hours after ingestion
  2. Stool enters Rectum
    1. Stool distends Rectum
    2. Rectum contracts reflexively
    3. Stool forced into anal canal
    4. Stool crosses 90 degree angle
      1. Canal straightened with hip flexion (squatting)
  3. Stool enters anal canal
    1. Internal anal sphincter (involuntary control)
      1. Automatically relaxes on stool entrance
      2. Stretch receptors (S2-4) signal stool in anal canal
    2. External anal sphincter (voluntary control)
      1. Voluntary relaxation results in Defecation
  4. Defecation
    1. External anal sphincter opens
    2. Intra-abdominal pressure increases with valsalva
    3. Stool moved out
  5. Results of chronically ignoring urge to defecate
    1. Sensory Receptor reflex arc less responsive
    2. Stool collects in distal colon
    3. Rectosigmoid colon dilates
    4. Stool desiccates and worsens Constipation

III. References

  1. Sartor in Dornbrand (1992) Ambulatory Care, p. 221-5
  2. Arce (2002) Am Fam Physician 65(11):2283-90 [PubMed]

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