Human anus

The human anus is the external opening of the rectum. It is located within the intergluteal cleft, which is the space between the buttocks. This structure serves as the final part of the gastrointestinal tract. Its primary biological function is to manage the exit of feces from the body. This process is known as defecation. Because it is the final exit point for waste, it is a vital part of human anatomy and health.

To control the passage of waste, the anus uses two circular muscles called sphincters. These muscles act like gates that can open or close. The first is the internal anal sphincter, which is an involuntary muscle. This means it works automatically without you thinking about it. The second is the external anal sphincter, which is a voluntary muscle. You can consciously control this muscle to hold or release waste. These two rings maintain constriction of the orifice normally, but they relax when the body needs to function.

Inside the anal canal, there is a specific boundary called the pectinate line. This line is a transition point between different types of tissue. Above this line, the lining is a pseudostratified columnar epithelium, which is part of the mucosa. Below this line, the tissue changes into stratified squamous epithelium, which is more like regular skin. This transition also marks the division between the internal and external anus. The area is surrounded by folds called anal valves that converge at this line. The anus receives blood from the inferior rectal artery and signals from the pudendal nerve.

Defecation happens through a specific sequence of physical steps. First, intra-rectal pressure builds as the rectum fills with feces. This pushes the feces against the walls of the anal canal. At the same time, contractions of the abdominal and pelvic floor muscles increase intra-abdominal pressure. This pressure causes the involuntary internal sphincter to relax. As the rectum shortens, peristaltic waves push the feces into the anal canal. Finally, the internal and external sphincters relax, and the levator ani muscles pull the anus up over the exiting feces.

Various medical conditions can affect this area due to its exposure to feces or physical stress. Hemorrhoids are common, consisting of visible blood vessels from the internal or external venous plexuses. They can cause pain, itching, or bleeding and are often linked to constipation or pregnancy.

Another condition is an anal fissure, which is a painful tear in the mucosal lining. Fissures can cause bleeding or discharge and may be treated with creams or surgery. Abscesses can also form when glands become infected, sometimes leading to an anorectal fistula. These are abnormal connections that often require surgical drainage and antibiotics.

During puberty, the body undergoes hormonal changes that affect the appearance of the anus. Testosterone triggers the growth of androgenic hair in the pubic region. While hair may start out sparse, it typically fills in by the end of puberty. In some genetic populations, this hair growth is less common. The area also has a high concentration of nerve endings, making it a sensitive erogenous zone. The pudendal nerve, which supplies the external sphincter, also branches to the clitoris and penis. This connection can lead to indirect stimulation of nearby organs during sexual activity.

Human culture and biology intersect in many ways regarding the anus. For instance, scientists measure the anogenital distance, which is the space between the anus and the genitals. This distance is typically smaller in women than in men.

Hygiene practices also vary globally, with some cultures using bidets and others using toilet paper. Because the anus is often considered a taboo part of the body, it is frequently associated with many slang terms. Understanding the anatomy and function of the anus is essential for maintaining overall health and hygiene.