The anal canal is the part that connects the

rectum The rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...

to the

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

, located below the level of the

pelvic diaphragm The pelvic floor or pelvic diaphragm is an anatomical location in the human body which has an important role in urinary and anal continence, sexual function, and support of the pelvic organs. The pelvic floor includes muscles, both skeletal and ...

. It is located within the

anal triangle The anal triangle is the posterior part of the perineum. It contains the anus in mammals. Structure The anal triangle can be defined either by its vertices or its sides. * ''Vertices'' ** one vertex at the coccyx bone ** the two ischial tuber ...

of the

perineum The perineum (: perineums or perinea) in placentalia, placental mammals is the space between the anus and the genitals. The human perineum is between the anus and scrotum in the male or between the anus and vulva in the female. The perineum is ...

, 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 tract is the largest of the body's systems, after the cardiovascular system. T ...

, it functions to regulate release of

excrement Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) 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 ...

by two muscular

sphincter A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the human bo ...

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

. 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 The pectinate line (dentate line) is a line which divides the upper two-thirds and lower third of the anal canal. Developmentally, this line represents the hindgut- proctodeum junction. It is an important anatomical landmark in humans, and fo ...

(also known as the dentate line): * upper zone (zona columnaris) ** mucosa is lined by

simple columnar epithelium Simple columnar epithelium is a single layer of columnar epithelial cells which are tall and slender with oval-shaped nuclei located in the basal region, attached to the basement membrane. In humans, simple columnar epithelium lines most organs ...

** 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 ...

known as

anal valves An anal valve is a small horizontal fold uniting the inferior ends of any two adjacent anal columns of the anal canal. An anal valve forms the inferior boundary of an anal sinus. The anal valves demarcate the level of the pectinate line The p ...

** 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 ...

(a branch of the

inferior mesenteric artery In human anatomy, the inferior mesenteric artery (IMA) is the third main branch of the abdominal aorta and arises at the level of L3, supplying the large intestine from the distal transverse colon to the upper part of the anal canal. The region ...

) * 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 The inferior rectal artery (inferior hemorrhoidal artery) is an artery that supplies blood to the lower third of the anal canal below the pectinate line. Structure The inferior rectal artery arises from the internal pudendal artery as it passes ab ...

(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 ...

) The anal verge refers to the

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

end of the anal canal, a transitional zone between the

epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

of the anal canal and the

perianal skin The perineum (: perineums or perinea) in placental mammals is the space between the anus and the genitals. The human perineum is between the anus and scrotum in the male or between the anus and vulva in the female. The perineum is the region o ...

. It should not be confused with the pectinate line between the upper and lower zones within the anal canal. The

anal gland The anal glands or anal sacs are small glands near the anus in many mammals. They are situated in between the external anal sphincter muscle and internal anal sphincter muscle. In non-human mammals, the secretions of the anal glands contain most ...

secretes lymphal discharge and built-up

fecal matter Feces (also known as faeces or fæces; : faex) 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 relatively small amount of ...

from the colon lining. In some animals this gland expungement can be done routinely every 24–36 months to prevent infection and

fistula In anatomy, a fistula (: fistulas or fistulae ; from Latin ''fistula'', "tube, pipe") is an abnormal connection (i.e. tube) joining two hollow spaces (technically, two epithelialized surfaces), such as blood vessels, intestines, or other h ...

formation.

Relations

*The

are on each side of the anal canal. *The

perianal space In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

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.

Function

The

external anal sphincter The external anal sphincter (or sphincter ani externus) is an oval tube of skeletal muscle fibers. Distally, it is adherent to the skin surrounding the margin of the anus. It exhibits a resting state of tonical contraction and also contracts duri ...

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 Defecation (or defaecation) follows digestion and is the necessary biological process by which organisms eliminate a solid, semisolid, or liquid metabolic waste, waste material known as feces (or faeces) from the digestive tract via the anus o ...

, it relaxes to allow the release of

feces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) 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 ...

. 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

References

External links

* * — "The rectum and anal canal in the male pelvis" {{Authority control Digestive system