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In humans, the anal canal is approximately 2.5 to 4 cm (0.98 to 1.57 in) long, from the anorectal junction to the anus. [2] [3] [4] 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 sl
The rectoanal inhibitory reflex (RAIR), also known as the anal sampling mechanism, anal sampling reflex, rectosphincteric reflex, or anorectal sampling reflex, is a reflex characterized by a transient involuntary relaxation of the internal anal sphincter in response to distention of the rectum. [1]
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.It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.
The anus is the site of potential infections and other conditions, including cancer (see anal cancer). [4] With anal sex, the anus can play a role in sexuality. Attitudes toward anal sex vary, and it is illegal in some countries. [5] The anus is often considered a taboo part of the body, [5] and is known by many, usually vulgar, slang terms.
This results in feces moving along the colon to the rectum. [4] The presence of stool in the rectum causes reflexive relaxation of the internal anal sphincter (rectoanal inhibitory reflex), so the contents of the rectum can move into the anal canal. This causes the conscious feeling of the need to defecate.
The inferior rectal nerves (inferior anal nerves, inferior hemorrhoidal nerve) usually branch from the pudendal nerve but occasionally arises directly from the sacral plexus; they cross the ischiorectal fossa along with the inferior rectal artery and veins, toward the anal canal and the lower end of the rectum, and is distributed to the sphincter ani externus (external anal sphincter, EAS) and ...
The human rectum is a part of the lower gastrointestinal tract. The rectum is a continuation of the sigmoid colon, and connects to the anus. The rectum follows the shape of the sacrum and ends in an expanded section called an ampulla where feces is stored before its release via the anal canal.
It is an important anatomical landmark in humans, and forms the boundary between the anal canal and the rectum according to the anatomic definition. [1] Colorectal surgeons instead define the anal canal as the zone from the anal verge to the anorectal ring (palpable structure formed by the external anal sphincter and the puborectalis muscle). [1]