Anus
The anus is the terminal structure of the digestive system, facilitating the expulsion of feces from the body. It comprises the anal canal and the anal sphincter, which includes two rings of muscle: the internal and external sphincters. These muscles play a crucial role in controlling defecation, allowing individuals to manage the urge to expel waste until they are ready. As feces accumulate in the rectum, pressure triggers the urge to defecate, leading to the relaxation of the sphincters and the eventual passage of fecal matter.
The anus can be affected by various disorders and injuries, with hemorrhoids being the most common condition, characterized by swollen veins that can either protrude or remain within the anal canal. Other issues include anal fissures, which are tears in the anal tissue, and anal abscesses, which are painful pockets of infection. Additionally, conditions such as anal fistulas and pruritus can arise, often requiring medical intervention. While primary cancer of the anus is relatively rare, it can occur, primarily manifesting as squamous cell carcinoma. Understanding the anatomy and potential health issues related to the anus is important for maintaining overall digestive health.
Subject Terms
Anus
Anatomy or system affected: Gastrointestinal system, intestines
Definition: The anus ends the gastrointestinal tract, the point most distant from the mouth. It controls defecation.
Structure and Functions
The anus terminates the long tube through the body where digestion takes place, and that begins with the mouth. It consists of the anal canal, located immediately below the rectum, and the anal sphincter. The sphincter itself has a double ring of strong muscles, the interior and external sphincters. In the anus, the tissue type changes from that of the intestinal tract to the external skin.
The anus controls defecation: the expulsion of the semisolid waste matter of digestion (feces) from the body. The anal sphincter is controlled by the autonomic nervous system, which keeps it shut. As feces fill the rectum, pressure on its walls creates the urge to defecate. The muscles of the anal sphincter then relax. Most adults and children, but not infants, can force the lower sphincter to remain tight until they are ready to defecate and then aid the process by consciously relaxing the muscles involved. During defecation, a muscular contraction pulls the anus back over the feces until the sphincter shuts.
Disorders and Diseases
The anus is subject to several common mechanical injuries and infections. The most common of all is hemorrhoids, which are varicose (swollen) veins. They can protrude from the anus (external hemorrhoids) or lie inside the anal canal (interior hemorrhoids). Pregnancy, exertion while lifting heavy objects, and straining during defecation are the typical causes. Hemorrhoids are also associated with certain cancers and Irritable Bowel Syndrome. Sometimes a hard or large bowel movement or anal sex causes a tear in the anus, called an anal fissure. Both fissures and hemorrhoids can cause pain and light bleeding during defecation.
An anal abscess is a pocket of bacteria that has filled a mucus-secreting gland in the anal wall. It can cause severe pain and damage to surrounding tissue. An anal fistula is a tube that opens between one part of the anal wall to another or to a separate organ, such as the vagina. All these defects can be repaired surgically. Additionally, pruritus (itching) can occur from a variety of causes, such as bacteria, poor hygiene, allergic reactions, parasites, and the anxiety-itch-anxiety cycle.
Injury to the anus or infection may impair or prevent control of the anus, which leads to fecal incontinence (uncontrollable bowel movements). Primary cancer in the anus is rare, occurring in less than 10,000 people annually in the United States. It usually involves squamous cell carcinoma in the skin near the anus or in the transition between the anus and the rectum.
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