The anal canal is the part that connects the
rectum to the
anus, located below the level of the
pelvic diaphragm. It is located within the
anal triangle of the
perineum
The perineum in humans is the space between the anus and scrotum in the male, or between the anus and the vulva in the female. The perineum is the region of the body between the pubic symphysis (pubic arch) and the coccyx (tail bone), includi ...
, between the right and left
ischioanal fossa
The ischioanal fossa (formerly called ischiorectal fossa) is the fat-filled wedge-shaped space located lateral to the anal canal and inferior to the pelvic diaphragm. It is somewhat prismatic in shape, with its base directed to the surface of the ...
. As the final functional segment of the
bowel
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
, it functions to regulate release of
excrement by two muscular
sphincter
A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the h ...
complexes. The anus is the aperture at the terminal portion of the anal canal.
Structure
In humans, the anal canal is approximately long, from the anorectal junction to the
anus.
It is directed downwards and backwards. It is surrounded by inner involuntary and outer voluntary sphincters which keep the
lumen closed in the form of an anteroposterior slit.
The canal is differentiated from the rectum by a transition along the internal surface from
endodermal
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gastru ...
to skin-like
ectodermal
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
tissue.
The anal canal is traditionally divided into two segments, upper and lower, separated by the
pectinate line (also known as the dentate line):
* upper zone (zona columnaris)
** mucosa is lined by
simple columnar epithelium
** features longitudinal folds or elevations of tunica mucosa which are joined inferiorly by folds of
mucous membrane
A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It i ...
known as
anal valves
The anal valves are small valve-like folds at the lower ends of the anal sinuses in the rectum. The anal valves join the lower ends of the anal columns. The anal sinuses are located between them. The valves and sinuses form the pectinate line.
See ...
** supplied by the
superior rectal artery
The superior rectal artery (superior hemorrhoidal artery) is an artery that descends into the pelvis to supply blood to the rectum.
Structure
The superior rectal artery is the continuation of the inferior mesenteric artery. It descends into the p ...
(a branch of the
inferior mesenteric artery)
* lower zone
** divided into two smaller zones, separated by a white line known as the
Hilton's line:
*** zona hemorrhagica - lined by
stratified squamous non-keratinized epithelium
*** zona cutanea - lined
stratified squamous keratinized epithelium, which blends with the surrounding perianal skin
** supplied by the
inferior rectal artery (a branch of the
internal pudendal artery
The internal pudendal artery is one of the three pudendal arteries. It branches off the internal iliac artery, and provides blood to the external genitalia.
Structure
The internal pudendal artery is the terminal branch of the anterior trunk of ...
)
The anal verge refers to the
distal end of the anal canal, a transitional zone between the
epithelium of the anal canal and the
perianal skin. It should not be confused with the pectinate line between the upper and lower zones within the anal canal.
The
Anal gland
Anal may refer to:
Related to the anus
*Related to the anus of animals:
** Anal fin, in fish anatomy
** Anal vein, in insect anatomy
** Anal scale, in reptile anatomy
*Related to the human anus:
** Anal sex, a type of sexual activity involvi ...
secretes lymphal discharge and built-up
fecal matter
Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...
from the
colon lining. In some animals this gland expungement can be done routinely every 24–36 months to prevent infection and
fistula
A fistula (plural: fistulas or fistulae ; from Latin ''fistula'', "tube, pipe") in anatomy is an abnormal connection between two hollow spaces (technically, two epithelialized surfaces), such as blood vessels, intestines, or other hollow or ...
formation.
Relations
*The
ischioanal fossa
The ischioanal fossa (formerly called ischiorectal fossa) is the fat-filled wedge-shaped space located lateral to the anal canal and inferior to the pelvic diaphragm. It is somewhat prismatic in shape, with its base directed to the surface of the ...
are on each side of the anal canal.
*The
perianal space surrounds the anal canal below the white line.
*The submucous space of the canal lies above the white line between the mucous membrane and
internal anal sphincter muscle
The internal anal sphincter, IAS, (or sphincter ani internus) is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal; its inferior border is in contact with, but quite separate from, the external anal sphincter. It is ...
.
Function
The
external anal sphincter
The external anal sphincter (or sphincter ani externus ) is a flat plane of skeletal muscle fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus.
Anatomy
The external anal sphincter measures about 8 ...
muscle is the voluntary muscle that surrounds and adheres to the anus at the lower margin of the anal canal. This muscle is in a state of
tonic contraction, but during
defecation, it relaxes to allow the release of
feces.
Movement of the feces is also controlled by the involuntarily controlled internal anal sphincter, which is an extension of the circular muscle surrounding the anal canal. It relaxes to expel feces from the rectum and anal canal.
Additional images
Image:Anorectum-en.svg , Anatomy of the anus and rectum
Image:Gray404.png, Left levator ani from within
Image:Gray1080.png, The interior of the anal canal and lower part of the rectum
Image:Gray1228.png, Median sagittal section of male pelvis
Image:Gray1230.png, Median sagittal section of female pelvis
See also
*
Anal columns
Anal may refer to:
Related to the anus
*Related to the anus of animals:
** Anal fin, in fish anatomy
** Anal vein, in insect anatomy
** Anal scale, in reptile anatomy
*Related to the human anus:
** Anal sex, a type of sexual activity involving s ...
*
Anal sinuses
The anal sinuses (rectal sinuses) are furrows in the anal canal, that separate the anal columns from one another. The anal sinuses end below in small valve
A valve is a device or natural object that regulates, directs or controls the flow o ...
*
Anal sex
References
External links
*
* — "The rectum and anal canal in the male pelvis"
{{Authority control
Digestive system
Canal
Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface flo ...