Anal canal

Canalis analis

Also known as: anus

The anal canal is the terminal 3–4 cm of the gut, running from the anorectal junction to the anus. It is surrounded by the internal and external anal sphincters and divided by the pectinate line into embryologically distinct upper and lower parts.

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Overview

Length≈3–4 cm

Internal sphincter share of resting pressure≈70–85%

The canal runs downwards and backwards from the puborectalis sling to the anus through the perineum, between the ischio-anal fossae. It is kept collapsed except during defecation.

Its upper half has 6–10 vertical anal columns (of Morgagni) containing terminal branches of the superior rectal vessels. Their lower ends are joined by crescentic anal valves that enclose small anal sinuses, into which the anal glands open. The valves form the scalloped pectinate (dentate) line. Below this is the smooth anal pecten (transitional, non-keratinised stratified squamous epithelium), then the intersphincteric groove (white line of Hilton), and finally the anal verge with hair-bearing skin.

Continence depends on several structures working together. The internal anal sphincter is involuntary smooth muscle and provides most of the resting pressure. The external anal sphincter is voluntary skeletal muscle with subcutaneous, superficial and deep parts, the deep part blending with puborectalis; it provides the squeeze. The puborectalis sling maintains the anorectal angle. The anal cushions are vascular submucosal pads, classically at 3, 7 and 11 o'clock in the lithotomy position, that make a watertight seal.

Location: Perineum (anal triangle), from the level of the pelvic diaphragm to the anus, about 4 cm in front of and below the tip of the coccyx.

Parts & subdivisions

Pectinate (dentate) line
The watershed between endoderm-derived and ectoderm-derived anal canal.
Internal anal sphincter
Thickened inner circular smooth muscle; involuntary.
External anal sphincter
Voluntary skeletal muscle (subcutaneous, superficial and deep parts) supplied by the inferior rectal nerve (pudendal, S2–S4). It contracts on voluntary command or on a cough.
Anal columns, valves and sinuses
Mucosal folds above the pectinate line. Anal glands open into the sinuses and are the origin of most anorectal abscesses and fistulae.
Anal cushions
Submucosal arteriovenous plexuses that seal the canal at rest. They contribute about 15–20% of resting pressure.

Anatomy

Innervation: Above the pectinate line: autonomic (inferior hypogastric plexus), sensitive only to stretch. Below: somatic inferior rectal nerve (pudendal, S2–S4), sensitive to pain, touch and temperature. The external sphincter is supplied by the inferior rectal nerve and the perineal branch of S4.
Arterial supply: Above the pectinate line: superior rectal artery (IMA). Below it: inferior rectal arteries (internal pudendal). Middle rectal arteries anastomose with both.
Venous drainage: Above: superior rectal vein → IMV → portal vein. Below: inferior rectal veins → internal pudendal → internal iliac → IVC.
Lymphatic drainage: Above the pectinate line: internal iliac (and inferior mesenteric) nodes. Below: superficial inguinal nodes.
Histology: Simple columnar (colorectal-type) epithelium above the anal columns; a transitional zone; non-keratinised stratified squamous epithelium in the pecten; keratinised skin below the white line.

Function

Faecal continence at rest and during raised abdominal pressure.
Discriminates flatus from liquid or solid contents (sensitive transition zone sampling).
Controlled evacuation during defecation.

Clinical significance

Haemorrhoids

Enlarged, prolapsing anal cushions. Internal haemorrhoids arise above the pectinate line and cause painless bright-red bleeding on the paper or pan and prolapse (graded I–IV). External haemorrhoids lie below the pectinate line and are painful when thrombosed. Risk factors are straining, constipation, pregnancy and low fibre. Portal hypertension produces rectal varices, which are a distinct entity.

Anal fissure

A longitudinal tear of the anoderm, usually in the posterior midline. It causes severe pain on defecation and bright bleeding. Spasm of the internal sphincter reduces blood flow and perpetuates it. Treatment is topical GTN or diltiazem, botulinum toxin or lateral internal sphincterotomy.

Anorectal abscess and fistula

Infection of the anal glands (cryptoglandular origin) spreads through the intersphincteric space to perianal, ischio-anal or supralevator spaces. A chronic tract to the skin is a fistula-in-ano. Goodsall rule predicts where the internal opening lies. Crohn disease should be suspected if fistulae are complex.

Test yourself

Lymph from the anal canal below the pectinate line drains first to the:

Internal iliac nodes
Inferior mesenteric nodes
Para-aortic nodes
Superficial inguinal nodes

Answer: Superficial inguinal nodes

The ectodermal part of the anal canal drains to the superficial inguinal nodes, like the perineal skin. Anal squamous carcinoma therefore spreads to the groin.