Anal Canal

The anal canal is the terminal part of the large intestine, extending from the rectum to the external environment at the anal orifice. It is a short but highly specialized segment that plays a critical role in the voluntary and involuntary control of defecation. The canal is surrounded by complex sphincteric musculature, receives dual blood and nerve supply above and below a key embryological landmark called the pectinate line, and exhibits distinct differences in epithelial lining, vascular drainage, and lymphatic flow. Its structural complexity makes it anatomically and clinically significant, especially in conditions like hemorrhoids, fissures, fistulas, and continence disorders.

Length and Boundaries

• Length: Approximately 3–4 cm in adults
• Upper limit: Anorectal junction (at the level of the puborectalis sling)
• Lower limit: Anal verge (external opening at the perianal skin)

Division by Pectinate Line

The anal canal is divided into two anatomical and functional zones by the pectinate (dentate) line:

• Above the pectinate line: Derived from endoderm (hindgut)
• Below the pectinate line: Derived from ectoderm (proctodeum)

Consequences of this division:

Feature	Above Pectinate Line	Below Pectinate Line
Epithelium	Columnar/transition zone	Stratified squamous (keratinized near anus)
Innervation	Autonomic (visceral, insensitive to pain)	Somatic (inferior rectal nerve — highly sensitive)
Blood supply	Superior rectal artery (IMA)	Inferior rectal artery (internal pudendal)
Venous drainage	Superior rectal vein (portal system)	Inferior rectal vein (systemic circulation)
Lymph drainage	Internal iliac nodes	Superficial inguinal nodes

Internal Features

• Anal columns (of Morgagni): Longitudinal mucosal folds in the upper canal
• Anal valves: Horizontal folds joining the columns inferiorly
• Anal sinuses: Depressions just above the valves; may trap fecal material
• Pectinate line: Wavy line formed by the anal valves; separates upper and lower canal
• Anal pecten: Zone below the pectinate line lined by non-keratinized squamous epithelium
• White line of Hilton: Marks transition to keratinized skin near anal verge

Sphincters of the Anal Canal

Internal Anal Sphincter

• Thickened continuation of circular smooth muscle of rectum
• Involuntary control
• Tonically contracted at rest; relaxes reflexively during defecation
• Controlled by:
  • Sympathetic fibers (maintain tone)
  • Parasympathetic fibers (inhibit tone)

External Anal Sphincter

• Striated muscle under voluntary control
• Three parts: deep, superficial, and subcutaneous
• Surrounds entire length of anal canal
• Innervated by inferior rectal nerve (branch of pudendal nerve, S2–S4)

Musculature Supporting the Canal

• Puborectalis muscle: Forms a sling at the anorectal junction, maintaining the anorectal angle (~80°) to aid continence
• Levator ani: Supports the superior canal and resists downward pressure

Blood Supply

• Superior rectal artery: From inferior mesenteric artery; supplies upper canal
• Middle rectal artery: From internal iliac; variable contribution
• Inferior rectal artery: From internal pudendal artery; supplies lower canal and perianal skin

Venous Drainage

• Superior rectal vein: Drains to inferior mesenteric vein → portal system
• Middle and inferior rectal veins: Drain to internal iliac → systemic circulation
• Clinical note: This forms a portosystemic anastomosis — site of internal hemorrhoids in portal hypertension

Lymphatic Drainage

• Above pectinate line: Internal iliac lymph nodes
• Below pectinate line: Superficial inguinal lymph nodes

Innervation

Above the pectinate line

• Visceral afferents via pelvic splanchnic nerves (S2–S4)
• Insensitive to pain, touch, and temperature

Below the pectinate line

• Somatic afferents via inferior rectal nerve (pudendal nerve)
• Highly sensitive to pain and touch

Embryological Origin

Segment	Origin
Above pectinate line	Endoderm (hindgut)
Below pectinate line	Ectoderm (proctodeum)

Clinical Significance

• Hemorrhoids:
  • Internal: Painless, above pectinate line; dilated submucosal veins of superior rectal plexus
  • External: Painful, below pectinate line; involve inferior rectal veins
• Anal fissure: Tear in the anoderm, often posterior midline, below the pectinate line; causes severe pain during defecation
• Anal fistula: Abnormal tract connecting anal canal to perianal skin; often follows crypt abscess infection
• Incontinence: Damage to sphincters or pudendal nerve can lead to fecal leakage
• Imperforate anus: Congenital defect where anal opening is absent or misplaced; associated with failure of anal membrane rupture
• Anal carcinoma: Tumor behavior and prognosis differ above and below pectinate line due to embryological and lymphatic differences