Rectum and anal canal

Gross Anatomy - The Final Stretch

• Rectum: Begins at S3, featuring:
  • Sacral flexure: Follows the posterior curve of the sacrum and coccyx.
  • Anorectal flexure: An ~80° forward bend maintained by the puborectalis sling.
  • Transverse folds (of Houston): Three shelf-like folds inside the rectum.
• Anal Canal: Extends from the pelvic diaphragm to the anus.
  • Anatomical canal ends at the pectinate line; surgical canal extends to the anal verge.
• Key Relations:
  • Anterior: Prostate, seminal vesicles (♂); Vagina (♀).
  • Posterior: Sacrum, coccyx.
  • Lateral: Levator ani muscles.

⭐ The anorectal flexure, created by the tonically contracted puborectalis muscle (pelvic diaphragm), is a primary mechanism for maintaining fecal continence at rest.

Vasculature - Red & Blue Highways

• Arterial Supply

-   **Portal System (Above Pectinate Line):** Superior Rectal Vein → IMV → Portal Circulation.
-   **Systemic System (Below Pectinate Line):** Middle & Inferior Rectal Veins → Internal Iliac/Pudendal Veins → Systemic Circulation (IVC).

⭐ Internal hemorrhoids (superior rectal vein) are painless visceral dilatations, often linked to portal hypertension. External hemorrhoids (inferior rectal vein) are painful somatic thromboses.

Pectinate Line - The Great Divide

This embryological landmark dictates the neurovascular and lymphatic supply of the anal canal.

-   **Internal Hemorrhoids**: Above pectinate line. Painless (visceral innervation). Risk of adenocarcinoma.
-   **External Hemorrhoids**: Below pectinate line. Painful (somatic innervation). Risk of squamous cell carcinoma.

⭐ The pectinate line is a site of portocaval anastomosis. Dilated submucosal veins above this line in portal hypertension are anorectal varices, not internal hemorrhoids.

Innervation & Clinicals - Control & Chaos

• Autonomic Control (Involuntary):
  • Sympathetic: Superior hypogastric plexus → maintains rectal tone & internal sphincter contraction.
  • Parasympathetic: Pelvic splanchnics (S2-S4) → sense distension; trigger peristalsis & internal sphincter relaxation for defecation.
• Somatic Control (Voluntary):
  • Pudendal Nerve (S2-S4): Motor to external anal sphincter (continence) & sensory from perianal skin.

• Common Clinicals:
  • Anal Fissure: Longitudinal tear, typically posterior midline due to ↓ blood flow.
  • Hemorrhoids: Engorged vascular cushions. Internal (painless) vs. External (painful).
  • Anorectal Abscess/Fistula: Abscess can form a fistula-in-ano. 📌 Goodsall's Rule predicts fistula track.

⭐ Pectinate Line: Divides visceral innervation (autonomic, painless) above from somatic innervation (pudendal nerve, painful) below. This is why internal hemorrhoids are painless, while external hemorrhoids and anal fissures are exquisitely painful.

• The pectinate line is the key watershed for blood supply, innervation, and lymphatic drainage.
• Above: Internal hemorrhoids (painless, visceral nerve), adenocarcinoma, internal iliac nodes, portal venous drainage.
• Below: External hemorrhoids (painful, pudendal nerve), squamous cell carcinoma, superficial inguinal nodes, caval venous drainage.
• Superior rectal artery (from IMA) supplies the proximal rectum; the middle/inferior rectal arteries supply distal areas.
• Anal fissures are typically located at the posterior midline due to poor perfusion.