Anal cancer

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The anus

The anus is part of the digestive system. Stool leaves the body through the anus. It is the opening at the end of the large intestine after the rectum.

Diagram of the digestive system

Structure

The area around the anus is called the anal region. It is made up of the anal canal and the perianal skin.

Anal canal

The anal canal is a tube about 4 cm long. It connects the anus to the rectum, which is the holding area for stool. The junction between the rectum and anal canal is called the anorectal line.

The anal canal is surrounded by ring-like muscles called the anal sphincter (made up of the internal anal sphincter and the external anal sphincter). These muscles relax to let stool pass out of the body.

The anal canal is lined by a mucous membrane, which is a thin layer of moist tissue. It contains gland cells that make mucus (a thick, slippery fluid). This mucus helps stool move easily out of the body. The upper part of the anal canal contains folds of epithelium called anal columns. The lower part of the anal canal contains squamous cells. The dentate line marks the place where the anal columns change to squamous cells. At the anus, the tissue starts to change from moist to dry and more like skin on the rest of the body.

Perianal skin

The perianal skin is the skin around the anus. The perianal skin is made up of squamous cells and is similar to skin found anywhere else on the body.

Diagram of the anal canal and structures

Diagram of the perianal area

Function

The anus is the end of the gastrointestinal (GI) tract. When the rectum is full, your body feels the urge to have a bowel movement. The internal anal sphincter relaxes and pushes the stool from the rectum into the anal canal. Once the external anal sphincter relaxes, the stool is pushed out of the body through the anus.

digestive system

The group of organs that work together to take in food and liquid, break them down, absorb nutrients and pass waste from the body.

The digestive system includes the organs of the gastrointestinal (GI) tract, or digestive tract. The organs of the GI tract are the mouth, pharynx (throat), esophagus, stomach, small intestine and large intestine. The digestive system also includes other organs of digestion, which are the teeth, tongue, salivary glands, liver, pancreas and gallbladder.

epithelium

A tissue made up of epithelial cells. It makes up the surface of the skin. It also lines hollow areas of the body (called cavities), glands and the passages of the respiratory, digestive, reproductive and urinary systems.

squamous cell

A type of epithelial cell that is thin and flat and looks like a fish scale.

Squamous cells are found in the epithelium that makes up the surface of the skin. They are in the epithelium lining of organs such as the mouth, pharynx, esophagus, anus, cervix and vagina. Squamous cells also make up the lining of blood vessels and hollow areas of the body (called cavities).

gastrointestinal (GI)

Referring to or having to do with the digestive organs.

The gastrointestinal (GI) tract, or digestive tract, includes the mouth, pharynx (throat), esophagus, stomach, small intestine and large intestine.

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