Rectum - Anatomy & Physiology

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Structure

• Exists dorsal to the reproductive organs, bladder and urethra.
• The cranial portion of the rectum is attached to the dorsal body wall by a short mesorectum which is a continuation of the mesocolon.
• The mesorectum is reflected to continue with the parietal peritoneum of the pelvic cavity and to cover the urogenital organs ventrally.
  • This forms the rectogenital pouch.
  • Therefore the most distal part of the rectum is retroperitoneal.
    • This distal, retroperitoneal part is directly attached to the vagina in the female and to the urethra in the male.
    • The retroperitoneal space is filled with soft tissue rich in fat.
• The anal canal joins the bowel to the exterior.
  • This is a short passage derived from the proctodeum (formed by invagination of the surface ectoderm).
  • Before joining the anal canal, the rectum becomes dilated to form the rectal ampulla.
• At the rectoanal junction, the lumen is constricted by longitudinal folds in the mucosa.
  • These are normally pressed together to occlude the lumen.
• There are two anal sphincters:
  • Internal anal sphincter, formed by thickening of the circular smooth muscle of the gut and under autonomic control.
  • External anal sphincter, formed from striated skeletal muscle and under voluntary control.

Histology

Rectum

• Epithelium is columnar.
• Goblet cells are present in the mucosa.

Recto-Anal Junction

• Termination of the lamina muscularis and longitudinal layer of the tunica muscularis.
• The circular layer of the tunica muscularis forms the internal anal sphincter.
• The external anal sphincter is formed from skeletal muscle.
• Epithelium changes from columnar to stratified squamous non-keratinised.