Anal Canal | Brief Anatomy | Easy

The anal canal is the most terminal part of the lower GI tract/large intestine, which lies between the anal verge (anal orifice, anus) in the perineum below and the rectum above. The description in this topic is from below upwards, as that is how this region is usually examined in clinical practice. Images depicting the anal canal can be seen below.

Except during defecation, the anal canal is collapsed by the internal and external anal sphincters to prevent the passage of faecal material.

Anal Sphincters
The anal canal is surrounded by internal and external anal sphincters, which play a crucial role in the maintenance of faecal continence:

Internal anal sphincter – surrounds the upper 2/3 of the anal canal. It is formed from a thickening of the involuntary circular smooth muscle in the bowel wall.
External anal sphincter – voluntary muscle that surrounds the lower 2/3 of the anal canal (and so overlaps with the internal sphincter). It blends superiorly with the puborectalis muscle of the pelvic floor.
At the junction of the rectum and the anal canal, there is a muscular ring – known as the anorectal ring. It is formed by the fusion of the internal anal sphincter, external anal sphincter and puborectalis muscle, and is palpable on digital rectal examination.

The superior aspect of the anal canal has the same epithelial lining as the rectum (columnar epithelium). However, in the anal canal, the mucosa is organised into longitudinal folds, known as anal columns. These are joined at their inferior ends by anal valves. Above the anal valves are small pouches which are referred to as anal sinuses – these contain glands that secrete mucus.

The anal valves collectively form an irregular circle – known as the pectinate line (or dentate line). This line divides the anal canal into upper and lower parts, which differ in both structure and neurovascular supply. This is a result of their different embryological origins:

Above the pectinate line – derived from the embryonic hindgut.
Below the pectinate line – derived from the ectoderm of the proctodeum.
Inferior to the pectinate line, the anal canal is lined by non-keratinised stratified squamous epithelium (known as the anal pecten). It is a pale and smooth surface, which transitions at the level of the intersphincteric groove to true skin (keratinised stratified squamous).

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