The Anal Canal
The anal canal is the last part of the gastrointestinal tract. It is about 3 to 4 cm long and lies completely extraperitoneally. It begins at the anorectal junction distally from the perineal flexure and ends at the anus. The anal canal may be subdivided into the columnar, intermediate and cutaneous zone.
- Columnar zone: The lumen has folds of mucous membrane (anal columns) produced by arterial cavernous bodies (anal cushions) in the submucosa. These columns are connected to each other at their distal ends by transverse folds (anal valves). Behind the anal valves lie crypts (crypts of Morgagni) into which the excretory ducts of the anal glands open. All anal valves together form the dentate (or pectinate) line, a serrated line where the intestinal mucosa merges with the squamous epithelium of the anal canal.
- Intermediate zone: Distally from the dentate line lies a 1 cm long zone with anal mucosa (anoderm).
- Cutaneous zone: This zone below the anal verge (anocutaneous line), a hollow between the internal and external anal sphincter, has regular perianal skin. The tension of the corrugator cutis ani muscle gives it its fan-like look.
The columnar zone derives from the endoderm whereas both the intermediate and cutaneous zone develop from the proctodeum (cloaca). As a result of the different embryologic origins the zones have separate supplying structures. Hereby the dentate line serves as an important marker.
- Above the dentate line: The arterial blood is supplied by the superior rectal artery (branch of the inferior mesenteric artery), the venous blood flows through the internal hemorrhoidal plexus into the superior rectal vein (→hepatic portal system). The lymph drains into the lumbar (paraaortic) lymph nodes. The sympathetic innervation is carried by the inferior mesenteric plexus, the parasympathetic innervation by the pelvic splanchnic nerves and the inferior hypogastric plexus.
- Below the dentate line: The blood supply comes from the medial (branch of the internal iliac artery) and inferior rectal arteries (branch of the pudendal artery from the internal iliac artery). The venous blood drains via the external hemorrhoidal plexus into the medial and inferior rectal veins (→body circulation). The lymph flows into the inguinal lymph nodes. The pudendal nerve is responsible for the sensory innervation.
In the anal canal the intestinal mucosa (colorectal zone) changes to anal mucosa (anoderm) through a transitional zone and finally merges with the perianal skin (cutaneous zone).
- Colorectal zone: equates the histological structure of the rectum with simple columnar epithelium. Folds of mucous membrane filled with small arteries form cushions in the wall.
- Transitional zone: resembles the histological structure of the rectum but it presents both simple columnar epithelium and stratified squamous epithelium. Characteristic are transverse mucous folds with crypts and anal glands (tubular epithelial ducts).
- Anoderm (“white zone”): has stratified squamous non-keratinized epithelium which is associated with the internal anal sphincter. This zone features densely packed sensory neurons registering the status of the intestinal content (e.g. solid, liquid or gaseous).
- Cutaneous zone: is equal to the histological structure of the skin with stratified squamous keratinized epithelium. It is strongly pigmented and has all typical skin-associated structures such sweat and sebaceous glands, Pacinian corpuscles and hair follicles.
The anal canal is an important part of the continence organ. It is surrounded by a muscular sphincter system which tightly closes the lumen. The internal anal sphincter is permanently contracted through the sympathetic tonus and relaxes under parasympathetic influence. The external anal sphincter surrounds the anal canal like a clamp. It is in close relationship to the puborectalis muscle (part of levator ani muscle) which encircles the rectum from behind (puborectal sling) and thus forms a bending closure. Both the external anal sphincter and the puborectalis muscle are voluntarily controlled.
The anal cushions play an important role in the fine control. Physiologically they are filled with arterial blood. During defecation the internal anal sphincter relaxes so that the blood in the cushions drains away allowing a smooth passage of the stool through the anal canal. A pathological enlargement of the anal cushions leads to hemorrhoids.
The human anal glands are rudimentary. Their secreted scent does not a play role for humans anymore. For many animals the scent still fulfills important functions, e.g. territory marking or sexual stimulation.