Analkanal

Analkanal, or anal canal, is the terminal portion of the large intestine. It extends from the anorectal junction, where the rectum ends and the canal begins, to the anal verge at the opening of the anus. In adults it is about 3 to 5 cm long. The canal is bounded by the internal and external anal sphincters and lies within the perineal region.

Structure and histology

The anal canal is traditionally described as having an upper (columnar) part and a lower (non-keratinized stratified squamous) part, separated by the pectinate (dentate) line. Above the pectinate line, the mucosa is derived from endoderm and resembles rectal mucosa; below it, the mucosa changes to non-keratinized squamous epithelium continuous with the skin of the anal verge. The region contains anal columns and glands in the upper part and transitions to a skin-lined canal distally. The anorectal ring marks the junction with surrounding muscular structures.

Blood supply, innervation, and lymphatics

The upper anal canal receives blood from the superior rectal artery, a branch of the inferior mesenteric artery, while the lower canal is supplied by the middle and inferior rectal arteries from the internal iliac and internal pudendal systems. Innervation above the pectinate line is visceral and relatively insensitive to pain, whereas below it is somatically innervated by the pudendal nerve, providing acute pain sensation. Lymphatics drain above the pectinate line to internal iliac and inferior mesenteric nodes, and below to superficial inguinal nodes.

Clinical relevance

Pathologies include internal hemorrhoids (above the pectinate line) and external hemorrhoids (below). Anal fissures typically occur below the pectinate line and are often painful. Anal canal cancers are commonly squamous cell carcinomas in the lower region and are associated with HPV infection.