Anal Canal
The terminal part of the gastrointestinal (GI) tract or large intestine is known as anal canal. It is situated in between anus (anal verge) and rectum.
Anal canal extends from anorectal junction to anus. It is 2.5 cm to 4 cm in length directed downwards and backwards. It is surrounded by sphincters that keep its lumen closed in the form of a slit.
It is divided into three parts. The upper half of the canal, also known as Zona columnaris, is lined by Simple Columnar Epithelium. The upper and lower halves of anal canal are separated from each other through a line called “pectinate line or dentate line”. Anal canal’s lower half is further separated in two parts by “Hilton’s White Line”. These zones are: Zona Hemorrhagica and Zona Cutanea. Zona Hemorrhagica and Zona Cutanea are lined by stratified squamous non keratinized and stratified squamous keratinized epithelium respectively.
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Anal Canal
DIVISIONS OF ANAL CANAL
Anal canal is unequally divided into two sections; Upper and Lower.
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Upper Division
2/3 of this canal is upper division and has longitudinal folds of Tunica Mucosa. Its lower ends are joined by folds of mucus membrane known as Anal Valves. This 2/3 upper region is supplied by Superior Rectal Artery, which is a branch of Inferior Mesenteric Artery.
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Lower Division
The remaining 1/3 of this canal is lower division and is lined by Stratified Squamous Epithelium which blends with skin. This region is supplied with Inferior Rectal Artery, which is a branch of the Internal Pudendal Artery.
EMBRYOLOGY
Anal canal develops from the Proctodeum below the dentate line while above the dentate line it develops from Hindgut’s endoderm.
NERVES
The anal canal skin doesn’t have skin appendages i.e. hairs and sweat glands, but has somatic nerve endings which are sensitive to pain.
MUSCLES
The anorectal ring is formed by innermost fibers of levator ani muscle extending from Pubic Bone, Obturator Fascia and Ischial Spine (Puborectalis).
ANAL CANAL CANCER
There are many cases of death from anal, anal canal, and anorectal cancer in USA. It is curable. The three major factors for it are Site,(anal canal vs. perianal skin) Size(primary tumors) and Nodal Status. The basic risk factor for this type of cancer is certain sexual practices like anal intercourse or a person with having a high lifetime number of partners. HPV (Human Papillomavirus) infections are also strongly related with anal cancer.
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