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.
It is divided into three anatomical zones; columnar, intermediate and cutaneous. The pectinate (dentate) line divides the anal canal into upper (two-thirds) and lower (one-third) parts, each one being supplied by completely different neurovascular structures. The functions of the anal canal include the maintenance of fecal continence and defecation. This is achieved with the help of the anal sphincters and the neighbouring puborectalis muscle.
This article will describe the anatomy and function of the anal canal.
|Extraperitoneal, between the anorectal junction and anus
Colorectal (columnar) zone: anal columns, anal cushoins, anal valves, anal sinunses/crypts, pectinate line
Intermediate zone: anoderm
Squamous (cutaneous) zone: perianal skin
Above pectinate line.(superior 2/3): superior rectal artery
Below pectinate line (inferior 1/3): middle and inferior rectal arteries
Above pectinateline.(superior 2/3): superior rectal vein followed by hepatic portal circulation
Below pectinate line (inferior 1/3): middle and inferior rectal veins followed by the vena cava circulation
Above pectinatel ine.(superior 2/3): inferior mesenteric plexus, pelvic splanchnic nerves, inferior hypogastric plexus
Below pectinate line (inferior 1/3): pudendal nerve
Colorectal zone: simple columnar epithelium
Transitional zone: simple columnar and stratified squamous epithelium
Anoderm: stratified squamous non-keratinized epithelium
Cutaneous zone: stratified squamous keratinized epithelium
|Fecal continence and defecation
- Macroscopic anatomy
- Microscopic anatomy
The anal canal may be subdivided into the columnar, intermediate and cutaneous zone.
- Colorectal (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 anal crypts/sinuses (of Morgagni) into which the excretory ducts of the anal glands open. All anal valves together form the pectinate line, a serrated line where the intestinal mucosa merges with the squamous epithelium of the anal canal.
- Intermediate zone - Distally from the pectinate line lies a 1 cm long zone with anal mucosa (anoderm).
- Squamous (cutaneous) zone - This zone below the anal verge (anocutaneous line) is a hollow between the internal and external anal sphincter and has regular perianal skin. The tension of the corrugator cutis ani muscle gives it its fan-like look.
Blood supply and innervation
The colorectal zone derives from the endoderm whereas both the intermediate and squamous zone develops from the proctodeum (cloaca). As a result of the different embryologic origins, the zones have separate supplying structures. Hereby the pectinate line serves as an important marker.
Above the pectinate 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, while the parasympathetic innervation by the pelvic splanchnic nerves and the inferior hypogastric plexus.
Below the pectinate line
The blood supply comes from the middle (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 middle and inferior rectal veins (→body circulation). The lymph flows into the inguinal lymph nodes. The pudendal nerve is responsible for the sensory innervation.
For more details about the anal canal, take a look at the videos, articles, illustrations and quizzes in the following study unit:
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: It 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: It 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'): It 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).
- Squamous (cutaneous) zone: It 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 canal is only one component of the gastrointestinal tract. Is your knowledge of the rest of the digestive system up to scratch? Find out with our free digestive system quizzes and learning tools.
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.
Now that you've learned everything about the anal canal, why not put that knowledge to the test with the quiz below!
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. Overall, it is an important part of the continence organ. Anatomically it is subdivided into three zones:
- Colorectal: The anal valves together form the or pectinate line, influencing the blood supply, innervation and lymphatic drainage to/from the anal canal.
The blood supply, innervation and lymphatic drainage is influenced by the pectinate line.
Histologically, the anal canal consists of the following zones:
- Colorectal - simple columnar epithelium
- Transitional - simple columnar and stratified squamous epithelium
- Anoderm - stratified squamous non-keratinized epithelium
- Cutaneous - stratified squamous keratinized epithelium
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