Overview
The anal canal is the distal continuation of the rectum and terminal segment of the gastointestinal tract. It is located within the anal triangle of the perineum, between the left and right ischioanal fossae. It functions to regulate release of excrement by two muscular sphincter complexes (internal and external).
Origin
Continuous with rectum superiorly from the anorectal ring, narrows in the pelvic floor
- Anorectal ring – junction of the rectum and the anal canal, formed by the fusion of the internal anal sphincter, external anal sphincter and puborectalis muscle.
- Anorectal junction – pulled forward by puborectalis (part of the levator ani), creating a 90 degree flexure
Termination
The anus is the aperture at the terminal portion of the anal canal.
Structure
Above pectinate line
- Lined by intestinal mucosa (columnar epithelium) in longitudinal folds (anal columns)
- The columns unite inferiorly in crescentic folds (anal valves) with depressions in between (anal sinuses) which contain glands that secrete mucous
- The anal valves collectively form an irregular circle (pectinate or dentate line)
- Derived from embyronic hind gut
Below pectinate line
- Inferior to pectinate line, transition zone lined by non-keratinised squamous epithelium (anal pecten)
- Anocutaneous (white) line – where anal pecten ends and lining becomes true skin (keratinised stratified squamous epithelium)
Internal anal sphincter
- Involuntary control
- Spans the upper two-thirds of the anal canal.
- Formed by a thickening of the circular smooth muscle in the bowel wall, encircled by deep and superficial parts of external sphincter
External anal sphincter
- Voluntary control
- Acts to keep the orifice of the anal canal closed and relaxes upon defecation
- Deep part: encircles upper anal canal, no bony attachment
- Superficial part: encompasses middle part of anal canal, and lower portion of internal sphincter, attaches to coccyx behind, and perineal body in front
- Subcutaneous part: located below internal sphincter in perianal space, encompasses
lower part of anal canal, no bony connections
Arterial Supply
Above pectinate line
- Superior rectal artery – branch of inferior mesenteric artery
- Middle rectal artery – branch of anterior division of internal iliac artery
Below pectinate line
- Inferior rectal artery – branch of internal pudendal artery (derived from anterior division of internal iliac artery)
Venous Drainage
Above pectinate line
- Superior rectal vein drains into inferior mesenteric vein and ultimately into the portal vein
- Middle rectal vein drains into internal iliac vein and ultimately into the inferior vena cava
Below pectinate line
- Inferior rectal veins drains into internal pudendal vein
Lymphatic Drainage
Above pectinate line
- Internal iliac nodes
Below pectinate line
- Superficial inguinal nodes
Innervation
Internal sphincter
- Sympathetic supply: pelvic plexus
- Parasympathetic supply: pelvic splanchnic
External sphincter
- Somatic and sensory: Inferior rectal branches of pudendal nerve (S2-4), and branches from S4
Anatomical Variants
- Imperforated anus
- Ectopic anus
- Persistent cloaca