Anal canal !

The anal canal is the last 3-4 cm of the bowel, from the rectum to the anus; its sphincters keep stool and gas in and open to let the bowel empty.

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What it is

The last 3-4 cm of the bowel, from the end of the rectum to the anus, surrounded by the internal and external anal sphincters.

What it does

Keeps stool and gas in and opens to let the bowel empty.

Key facts

Where: Last 3–4 cm of the gut, from the anorectal junction at the puborectalis sling to the anus; the pectinate line divides an upper (endodermal) and a lower (ectodermal) part
Blood supply: Above the pectinate line: superior rectal artery (inferior mesenteric); below: inferior rectal artery (internal pudendal); veins form a portosystemic anastomosis
Nerve: Above the line: autonomic, stretch only (internal hemorrhoids painless); below: inferior rectal nerve (pudendal, S2–S4), pain-sensitive; external sphincter also pudendal
Function: Continence by the internal (smooth, resting tone) and external (striated, voluntary) sphincters with puborectalis; lymph above the line to internal iliac nodes, below to superficial inguinal

Relations

Above: Rectum, at the anorectal junction where the puborectalis sling pulls the gut forward and forms the anorectal angle
In front: Perineal body and, in the male, the bulb of the corpus spongiosum and the membranous urethra, at risk in anterior anorectal surgery
Behind: Anococcygeal body, a fibrous band between the canal and the coccyx
Lateral: Ischioanal fossae, filled with fat and crossed by the inferior rectal vessels and nerves; abscesses spread into them and can pass behind the canal from one side to the other (horseshoe abscess)

How to find it

Examined with the person lying on the left side, hips and knees bent. Inspection shows the anal verge, skin tags, fissures or external openings; on digital rectal examination the finger passes the sphincters and meets the anorectal ring (puborectalis) at the top of the canal.

Examination

Digital rectal examination: resting tone reflects mainly the internal sphincter and voluntary squeeze the external anal sphincter and puborectalis; weak tone points to sphincter damage or a pudendal nerve or sacral root lesion
Anal wink (anocutaneous reflex): stroking the perianal skin makes the external anal sphincter contract; an absent response suggests a lesion of S2–S4 or the pudendal nerve
Anoscopy: shows internal haemorrhoids, the base of a fissure and the internal opening of a fistula

On imaging

Endoanal ultrasound shows the internal sphincter as a dark (hypoechoic) ring and the external sphincter as a brighter outer layer, and finds sphincter defects. Pelvic MRI is the reference for mapping complex fistulas and their relation to the sphincters.

Common problems

Haemorrhoids (swollen anal cushions).
Anal fissure (a small, painful tear).
Abscess and fistula.

How it is diagnosed

Examination; anoscopy.

Treatment

Fibre, fluids and creams; small procedures or surgery when needed.

Clinical notes

Internal haemorrhoids are graded by prolapse: I none, II prolapse that returns by itself, III needs pushing back, IV stays out
Fistulas are classed by their path through the sphincters (Parks): intersphincteric, transsphincteric, suprasphincteric, extrasphincteric; the more sphincter crossed, the greater the risk of incontinence after fistulotomy
Goodsall rule: an external opening behind a transverse line through the anus usually tracks in a curve to the posterior midline; one in front usually runs straight to the nearest crypt
Anal cancer below the pectinate line spreads to the superficial inguinal nodes, so the groin is examined; tumours above drain to the internal iliac nodes

Development

The upper canal comes from the endodermal hindgut (cloaca) and the lower from the ectodermal proctodeum; the anal membrane between them breaks down in the embryo and its line persists as the pectinate line. Failure of this process gives imperforate anus and other anorectal malformations.

Test yourself

Think of the answer, then tap the question to see it.

How does the pectinate line divide the anal canal?

Above: columnar mucosa, autonomic stretch-only nerves (adenocarcinoma, painless lesions). Below: squamous anoderm, pudendal pain fibres (squamous cancer, painful lesions).

Where do most anal fissures lie, and what does a fissure off the midline suggest?

In the posterior midline, where blood supply is poorest; off the midline suggests Crohn disease, infection or cancer.

How are internal haemorrhoids graded?

By prolapse: I none, II prolapses and returns by itself, III needs pushing back, IV stays out.

What does Goodsall rule predict about anal fistulas?

An external opening behind a transverse line through the anus usually curves to the posterior midline; one in front runs straight to the nearest crypt.

Where does lymph drain from the anal canal above and below the pectinate line?

Above: internal iliac nodes. Below: superficial inguinal nodes, so the groin is examined in anal cancer.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): canalis analis
Official Spanish (DeCS): Canal anal
English (TA2): Anal canal

Sources: Anatomy, Abdomen and Pelvis: Anal Canal.

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Representative model, for educational purposes only.