Function of the Anus: Definition, Anatomy and Functions of This Part of the Digestive Tract

It opens the lower gastrointestinal tract and connects to the rectum.

This, in turn, connects to the colon, and on a backward journey, it relates to the small intestine, then to the stomach, from there to the esophagus, and finally to the mouth.

It is distinguished from the rectum by the transition of its inner surface from a layer of mucous membrane (endodermal) to one of skin tissue (ectodermal).

Many people undergo a colonoscopy and assume that this will also help them detect anal cancer.

But colonoscopies bypass the anus entirely, and their images are much higher up the intestinal tract.

While some colorectal specialists look at the digestive tract and find anal lesions, they often do not specifically look for them, as they primarily look higher up in the colon.

The unique imaging test for diagnosing the anus is a high-resolution anoscopy.


Anatomy of the anus

The anal canal has a length of 2.5 to 4 cm; its diameter is narrower than that of the rectum to which it is immediately connected anatomically.

The inner wall of the anal canal is first covered by moist, smooth skin devoid of hair or glands; then, it becomes a tough (keratinized) layer of skin that contains hair and glands. The keratinized layer is continuous with the skin of the anal opening and the external body.

The upper part of the anus, or that part that connects to the rectum, is

the squamocolumnar junction. This is where the rectum’s columnar or glandular epithelial cells become the squamous cells of the anus.

The lower portions of the anal columns are joined by small concentric circular folds of the mucous membrane known as the anal valves.

Between these anal valves are small anal sinuses that open to the lymphatic ducts and glands; these sometimes become abscesses and become infected, especially in people suffering from chronic diarrhea, constipation, or diabetes mellitus.

A line corresponds to columns, also known as the dentate line, which can be seen with the naked eye and is just below the squamocolumnar junction or towards the outside.

This is followed by the anal canal, which leads to the anal rim, the junction on the outside of the anus of hairy and hairless skin, similar to the corner of the lip of the mouth with the skin of the face.

The rim of the anus is somewhat irregular and dynamic, which means that it can be seen as a fluctuating undulating rim.

Beyond the edge that travels outward by 2 inches is the perianal skin, also known as the anal margin.

The squamocolumnar junction is the area most commonly affected by the human papillomavirus and where many of the lesions are likely to arise.

Historically, a distinction has been made between the anal canal or internal cancers (cancers located within or within the anus) and the anal margin or perianal area.


The lower anal canal and the anal opening are two muscular constrictions that regulate the fecal passage, the sphincters.

There are two sphincter muscles: an internal sphincter muscle, which can feel like a muscle ring, beyond which is the rectum, and an external sphincter muscle.

The internal sphincter is part of the inner surface of the canal; It is made up of concentric layers of circular muscle tissue and is not under voluntary control.

The external sphincter is a layer of voluntary (striated) muscle that surrounds the outer wall of the anal canal and the anal opening.

The anal opening is keratinized skin that has multiple folds while contracted. When open, the folds allow the skin to stretch without tearing.

Blood supply of the anus

The anal canal connects to the rectum at the point where it passes through a muscular pelvic diaphragm. The upper region has 5 to 10 rectal columns; each column contains a small artery and vein.

Numerous blood vessels surround the anal canal. These terminal portions of the blood vessels supplying the rectal and anal areas are susceptible to enlargement and rupture.

This condition is commonly known as hemorrhoids and can cause pain, bleeding, and projection of the vessels from the anus.

The mucous membrane of the upper portion is similar to that of the rest of the large intestine and contains absorbent and mucus-producing cells.

The function of the anus

Both the upper and lower portions of the anal canal have circular and longitudinal muscle layers that allow expansion and contraction of the channel: the sphincters.

The sphincters can contract and expand at will, except during the first years of life when it is not yet fully developed.

The nerves in the anal canal cause the sphincter response and pain sensation. The lower part of the channel is susceptible to heat, cold, cuts, and abrasion.

The primary function of the anus is the evacuation of waste products from the digestive canal.

Sphincters prevent stool from coming out until it relaxes during a bowel movement.

The stool passes into the anal canal from the rectum. The nervous responses of the rectum cause the internal sphincter to relax while the external one contracts; shortly after, the external sphincter also relaxes and allows fecal discharge.

The pelvic diaphragm and longitudinal muscles draw the anus and rectum on the passing stool to not spread out of the anal opening with the chair.

In the skin around the anal opening, but not immediately adjacent to it, some glands emit secretions and lubricate, allowing the passage of stool.