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    The anus is the external part of the body that serves as the opening of the rectum. During puberty, most individuals will start growing pubic hair around the anus—starting off fine and sparse and filling out as the individual matures.

    The primary function of the anus is to allow feces to exit the body. Muscular tissue around that area, called sphincter muscles, control the anus’s opening and closing to allow stool to pass. From a medical standpoint, the anus is also a site of numerous potential health complications including anal fissures and anal cancer.

    The Role of the Anus in Defecation

    As the colon fills up with fecal matter, pressure builds inside the rectum, forcing the feces to push up against the walls of what is called the anal canal. Sometimes additional pressure created by the abdominal muscles and pelvic muscles will exacerbate this process. At a certain point, the accumulation of pressure caused by feces in the body will trigger the urge to have a bowel movement in order to achieve relief.

    Then, an involuntary muscle called the internal anal sphincter will relax, letting the accumulating feces pass through the anal canal. The rectum becomes shortened at this point, as stool is pushed by peristaltic waves out of the body. After the fecal matter is out of the body, the associated muscles will relax and a muscle called the levator ani will pull up the anus over the feces, resulting in a closing effect. The individual will clean the outside of the anus with toilet paper or water to prevent infection from residual feces.

    Anal Care

    It’s important to wash the anus regularly with soap and water. Using scented soaps can sometimes cause irritation and itching, as can using toilet paper too roughly. Stripping the anal tissue of its natural oils can also lead to irritation; the goal is to keep skin surrounding the anus healthy and elastic. If itching of and around the anus is persistent, you should contact a medical doctor right away to check whether you have pinworms (a type of parasitic roundworm commonly found along the gastrointestinal (GI) tract).

    During sexual activity, penetration can tear or otherwise damage an individual’s anal tissue. Use lubrication to avoid this, and be careful not to damage the anal sphincter.  Generally, damage to the anus is very painful, can result in conditions such as fecal incontinence or leaking, and can require surgery to repair.

    Anal Diseases

    • Anal Cancer—although a more rare form of cancer, anal cancer can be pretty easily detected during routine physical or rectal exams.
    • Abscess—abscesses occur when pus collects and accumulates in a cavity formed by infection.
    • Warts—roughly growths that look very similar to blisters, usually caused by a viral infection.
    • Anal Fistula—abnormal tissue connection between the anal canal and surrounding skin tissue.
    • Anal Fissure—breaks or tears in the lining of the anus, usually caused by severe bowel movements or sexual activity.  Anal fissures can cause bleeding, like any cut to the surface of the skin.
    • Hemorrhoids—an anal hemorrhoid is an enlarged or swollen vein located beneath the skin of the anus, which causes sensitivity, pain and sometimes bleeding in that area.
    • Sexually-Transmitted Diseases—because of its active role in human sexuality, the anus is also a site of many STDs.

    Reviewed 12/12/2011 by David M. Nolan, M.D.
    Diplomate of the American Board of Internal Medicine, 2011
    Currently a Fellow of Gastroenterology, at UCI 2011-2014