Anal Canal

Anal canal is the terminal part of the large intestine, situated below the level of the pelvic diaphragm between the rectum and the anus, in the center of the anal triangle. It is about 1.5 inches long and its lateral walls are kept in apposition by anal sphincters and levatores ani muscles, except during defecation.

Structure:

Structurally, the upper half of anal canal is remarkable different than the lower half, as they have different embryological origins.

Upper half of anal canal:

It has following important features:

  • The mucous membrane is derived from the hindgut endoderm
  • It is lined by simple columnar epithelium
  • It has vertical folds called anal columns that are joined together at their lower ends by semilunar folds called anal valves. Anal valves are the remains of the proctodeal membrane.

Lower half of anal canal:

It has following important features:

  • The mucous membrane is derived from ectoderm of the proctodeum
  • It is lined by stratified squamous epithelium that gradually merges at the anus with the perianal epidermis.
  • There are no anal columns or anal valves

Pectinate line of anal canal:

It is the line that indicates the level where the upper half of anal canal joins the lower half, i.e. it represents the hindgut-proctodeum junction.

Anal sphincters:

Anal sphincters are rings of muscular tissue that surround and close the anal canal. There are two components: involuntary internal anal sphincter and voluntary external anal sphincter.

Blood supply:

The upper and lower halves of the anal canal have different embryological origins, therefore they have different blood supply, lymph drainage and nerve supply.

Blood supply of upper half of anal canal:

  • Arterial supply is through the superior rectal artery, which is a branch of the inferior mesenteric artery. It is good to remember that the inferior mesenteric artery also supplies terminal parts of the large intestine.
  • Venous drainage is by way superior rectal vein, which is a tributary of the inferior mesenteric vein.

Blood supply of the lower half of anal canal:

  • Arterial supply is through the inferior rectal artery, which is a branch of the internal pudendal artery.
  • Venous drainage is by way of inferior rectal vein, a tributary of the internal pudendal vein.

Lymph drainage:

  • Lymph drainage of upper half of anal canal: The lymphatics pass upward along the inferior rectal artery to the pararectal nodes and eventually into the inferior mesenteric nodes.
  • Lymph drainage of lower half of anal canal: Downward into the medial group of superficial inguinal lymph nodes

Nerve supply:

  • Nerve supply of upper half of anal canal: Upper half is innervated by branches from the hypogastric plexuses and thus it is sensitive only to stretch.
  • Nerve supply of lower half of anal canal: Lower half is innervated by the somatic inferior rectal nerve, and therefore it is sensitive to pain, temperature, touch and pressure.