Table of Contents
- 1 Anus Function
- 2 What is Anus?
- 3 Anus Functions
- 4 How Anus Work
- 5 Anus Parts
- 6 Anus Diseases
- 7 Anus Facts
- 8 References
- What is at stake in pondering about the anus?
- Why are infants unable to control bowel and bladder?
- Is anal triangle found only in males?
- How is the treatment of imperforate anus a big challenge?
- What anus disorders can be prevented through changes in lifestyle?
Culturally considered a source of shame, anxiety, irritation and humiliation, the anus or anal orifice is the posterior-most organ of the human gastrointestinal tract. In the book “Anal Pleasure and Health”, Morin frequently makes use of the word “discomfort”, particularly referring to psychic discomfort, painful bowel movements, and hemorrhoids.
Here you will learn about the anus definition, anus function, parts, diseases with symptoms and therapies, and fun facts about the anus.
What is Anus?
Allan (2016), calls anus one of the key organs of the body and a remarkably complex structure with significant symbolic potential in the socio-cultural context. Owing to its alignment with abjection, shame, and dirtiness, this organ makes people rather uncomfortable while thinking or discussing about it. That is the very reason that couth, well-mannered, proper and respectable individuals hesitate to talk about the anus and anus functions.
Biologically, the anus can be defined as the terminal part of the alimentary canal or the gastrointestinal tract. The anal canal begins below the pelvic flour and terminates by an opening to the exterior, called the anal orifice.
Located at the terminal end of the gastrointestinal tract, the anus forms an integral part of the gastrointestinal system. On the other hand, is also serves as a complementary part of the human excretory system. So, it can be said that anus contributes to the structure and function of both the gastrointestinal and excretory systems.
Here it seems pertinent to talk a bit about the gastrointestinal tract. Also called the alimentary canal, the gastrointestinal tract forms one of the two main groups of the digestive system organs. Extending from the mouth to the anus, this 30 feet long continuous tube runs through the ventral cavity of your body and includes the organs like the oral cavity, the oropharynx, the esophagus, the stomach, the small intestine, the large intestine and the rectum.
The anus is composed of several structurally and functionally distinct parts, like the anal canal, the internal and external anal sphincters, the mucous membrane, the anal valves, the anal orifice, and so on.
Do you know the anus is much more than just an opening for defecation? It also plays a role in the regulation of feces and producing erotic (sexual) pleasure. In the absence of the anus, you wouldn’t have been able to delay defecation for social convenience.
The anus diseases include hemorrhoids, anal stenosis, anal fissure, constipation, perianal Cohn’s disease, anal carcinoma, and so on.
In addition to serving as a voluntarily controlled orifice for the passage of feces, the anus also serves several other jobs, including detection of rectal contents, trigging “call to stool”, preventing constipation and producing erotic sensation, etc.
Some of the key anus functions are described in brief as under:
Elimination of Waste
The elimination of solid waste or feces is called defecation. After the removal of water and salts, the contents of the large intestine are called feces. As an excretory system organ, the primary job of the anus is to expel the digestive waste out of the body. Everything you eat cannot be digested and utilized in the body. So, the indigestible part of the food has to be converted into feces and eliminated. As an opening for the feces to pass, the anus plays a very important role in the process of defecation.
The excretion of waste products is one of the characteristics of simple cells. The inability to excrete the waste products is likely to lead to the loss of homeostasis and cause a disruption in the cellular process.
As a major organ of the excretory system, the large intestine solidifies the digestive waste matter by facilitating the reabsorption of water into the bloodstream.Finally, the last part of the bowel (the anal canal and orifice) allows for its elimination. You will be surprised to learn that even after the reabsorption of water in the bowel, the feces still consist of 60 to 70% of water by weight.
Sometimes, babies are born with imperforate anus as a birth defect. So, they are unable to defecate. The condition is to be treated through a surgical operation.
Do you know before reaching the age of 2 to 3 years, the human babies are unable to control bowel? Actually, the external anal sphincter (composed of skeletal muscle) is not under the conscious control in children.
The anus is lined by bands of muscle, called anal sphincters, which are two in number – the internal anal sphincter and the external anal sphincter. It is owing to your conscious control over the skeletal sphincter muscles that you are able to control and regulate the passage of feces through the anal opening.
Defecation requires the voluntary relaxation of external anal sphincter as it is under conscious control in adults. Other factors that contribute to defecation include a forcible exhalation and the voluntary contractions of the abdominal muscles.
Detecting Rectal Contents
The sensitivity of the anal canal to different states of matter allows it to differentiate between the solid, liquid or gaseous contents. In other words, you are able to detect if the rectal contents are liquid, solid or gas.
On detecting gas, the sphincters relax to allow its release at flatulence. On the other hand, when there is liquid in the anal canal, you feel the desperate desire to defecate. This desire can be countered through a conscious effort to tighten the sphincters. However, in case of solid matter, you can easily delay defecation. For delaying elimination of feces, you need to contract the anal sphincters strongly and the feces shall be pushed back into the rectum.
Triggering “Call to Stool”
As fecal matter enters the anal canal, it triggers a “call to stool” or an urge to defecate.So, you can look for a suitable place for the elimination of feces. The call to stool can also be resisted or temporarily suppressed for social convenience. It is perfectly normal and there is no harm in doing so. It can be harmful only if you do it on regular basis.
It is interesting to note that the anus also contributes to the prevention of constipation. Actually, when you regularly resist the urge to defecation, it makes the rectum tolerant of large volumes of stool. On the other hand, the tolerance of the rectum to large volumes of feces is one of the common causes of constipation.
Renowned for his studies about the structure of the brain and sexual orientation, Simon LeVay (a British-American neurologist) claims in his book “The Sexual Brain” that “anus is without question a sexual organ”. It plays a significant role in the sexual stimulation.
Meanwhile, there is rich innervation of the skin around the anus (perianal skin), the anal sphincters and the anal mucosa. These anal parts have the ability to generate erotic sensation and cause internal stimulation (e.g.of the prostate gland in men). So, they can play an important role in sexual arousal.
How Anus Work
The anus not only serves as an outlet for the elimination of digestive waste, but also controls and regulates the passage of stools.
While trying to understand how anus works, you should keep in mind that the anal canal is a continuation and part of the rectum. While the rectal region forms the last 7 to 8 inches of the gastrointestinal tract, the anal canal consists of the terminal 1 inch of the rectum.
The anus carries out the act of defecation by relaxing the internal and external anal sphincters. Defecation can be defined as the propulsion of feces through the large intestine. After passing through the small intestine in 3 to 10 hours and the large intestine in 18 to 24 hours, the digestive waste enters the rectum. The rectum serves as the reservoir for feces until elimination.
As the rectum is filled with feces, the nerve receptors in the wall of the rectum are stimulated and initiate a defecation reflux. Resultantly, the peristaltic movements get intensified and the internal sphincter muscle (which is not under voluntary control) is relaxed. At this point, if the external anal sphincter muscle (which is under voluntary control) is also relaxed, the peristaltic contractions force the feces through the anal opening. 
If you prevent the defection voluntarily, the defecation reflex will be diminished and the urge to urge to defecate will be reduced until pressure by feces again stimulates the defection reflex.
The innervation of the rectum and the anus is provided by the autonomic and somatic nerves originating from the sacral segments of the spinal cord. The tone of the internal anal sphincter is maintained by the parasympathetic signals. On the other hand, the sacral motor neurons control the functioning of the external anal sphincter. Meanwhile, owing to innervation by the somatic sensory nerve endings, the anus is as sensitive to pain and touch as the skin.
The anus is not just the name of the opening at the posterior end of the gastrointestinal tract. It performs a number of jobs with the help of several distinct structures, called the anus parts. The anus parts include the anal canal, mucous lining, sphincters muscles, glands, valves, pectinate line, orifice and the perineal skin.
A region, identified as the anal triangle is found in both the males and females. The anus forms the central structure of this triangle. On the anterior, the anal canal is attached to the adjacent perineum (perineal body). On the posterior, it is anchored to the anococcygeal raphe (ligament) that attaches the anal canal to the tip of the coccyx.
According to Drake et al. (2009), the perineal body is an ill-defined but functionally important connective tissue which serves as a place of attachment for the pelvic floor and perineum.
Perineum or the perineal body is the part of your body or the region lying between the anus and scrotum in males. In females, on the other hand, it is located between the anus and the posterior vulva junction.
Read on to learn about structure and function of different anus parts.
: An anorectal junction is a region in the gastrointestinal tract which joins rectum with its terminal part, the anus. This junction marks the end of the rectum and the beginning of the anal canal.
The 12 to 15 cm long segment of the alimentary canal, called the rectum, extends from the sigmoid colon to the anus. Lying in front of the sacrum, it is retroperitoneal except anteriorly and proximally. In women, it lies behind the vagina, the uterus and the pouch of Douglas. In men, on the other hand, it lies behind the seminal vesicles and the prostate gland.
On the distal end, the mucosa of rectum forms longitudinal ridges or rectal columns. The intervening furrows of the rectal columns which terminate in small folds at the anorectal junction are termed as the anal valves.
The Anal Canal
The anal canal represents the terminal segment of the rectum. The anus may range from 2.5 cm to 4.0 cm in length. Directed posteriorly, the lumen of the anal canal forms a 70-degree angle with the lumen of the rectum. This angulation assists the anal sphincter in its functioning.
The lumen of the anal canal is lined by the mucous membrane. It is termed as pectin or comb owing to its termination about 0.75 inches above the muco-cutaneious junction in a series of linear processes or the “linea dentata”.
The color of the mucous lining is intermediate between that of the mucous membrane of the intestine proper and of the skin. 
The anus is surrounded by two sphincters, called the internal anal sphincter and the external anal sphincter. The internal anal sphincter is a powerful sphincter which is formed from the circular smooth muscle layer which is continuous with the muscular layer of the rectum.
The distal 2.5 to 4.0 cm condensation of circular muscle layer of rectum represents the internal anal sphincter. Owing to both the extrinsic autonomous neurogenic and intrinsic myogenic properties, the internal sphincter muscle is in a state of continuous maximal contraction. It acts as a natural barrier to the involuntary loss of gas and stool.
Endosonography shows that internal sphincter muscle is a circular band of muscle, measuring 2 to 3 mm in thickness.
Lying external to the internal anal sphincter, the external anal sphincter muscle is made of the striated muscle fibers and envelopes the entire length of the inner tube of the smooth muscles. Looking like an elliptical cylinder, it measures 5 to 8 mm in thickness and appears to be twice as thick as the internal sphincter muscle.
The external anal sphincter muscle is under voluntary control and can temporary block defecation and expel the feces back to the rectum under conscious effort. When it relaxes with the relaxation of the internal anal sphincter, defecation commences.
Also called anal glands, the anal sacs are located between the internal and external anal sphincters on either side of the anus. These anal glands open into small anal sinuses located behind the anal valves.
Also called the valves of Ball, the anal valves are located in front of the anal sinuses into which the anal sacs open. They are arranged along the line, called the pectinate or dentate line. The anal valves are formed from the lower ends of the vertical columns (anal columns or columns of Morgagni) of the mucosal lining.
Also called the pectinate line, the dentate lining represents the junction between the ectoderm and endoderm. The anal valves are located along this dividing line.
The pectinate line serves as the division between the lower one third and upper two-thirds of the anal canal.
The Anal Orifice
The anal orifice is the opening of the anal canal to the exterior of the body. It allows the passage of feces for elimination. It is surrounded by the anal sphincters which help in the control and regulation of bowel movements.
The skin around the anus is called perianal skin. The perianal skin is richly innervated and is capable of generating erotic sensation.
According to Beck et al. (2012), the perianal skin is extremely sensitive. Here the nerve endings are supplied in a pattern similar to that of the genitalia, lips, and fingers.
As the anus is culturally regarded as the source of shame, anxiety, and humiliation, any benign or severe anal disorder will adversely affect the social life of the individual. The benign anus disorders include hemorrhoids, anal stenosis, fissure, abscess, fecal incontinence, and constipation. Anal carcinoma is a neoplasm of the anus.
Basic knowledge about the symptoms and treatment of common anal disorders will help you to timely diagnosis and treat the conditions.
Some of the benign and severe anus diseases are described below along with the associated symptoms and common therapies.
The condition of hemorrhoids refers to the dilation or swelling of the anal venous structure. According to Ehrenpreis (2003), hemorrhoids occur in up to fifty percent (50%) of the adult population. They may be internal or external. The dilations of the venous structures in the internal hemorrhoidal plexus are termed as internal hemorrhoids.
On the other hand, the external hemorrhoids originate from inferior venous plexus. While the internal hemorrhoids arise from above the dentate line, the external hemorrhoids originate from below the dentate (pectinate) line.
Ehrenpreis (2003) lists the following symptoms of the hemorrhoids in the book “Anal and Rectal Diseases Explained”.
- Mild discomfort and sensation of prolapse.
(Note: Prolapse is a condition in which an organ of your body may slip down or forward from the normal position.
- Passage of some quantity of bright red blood with stool.
- Itching of various intensity at the anus or pruritus ani (external hemorrhoids).
- Pain associated with thrombosis (external hemorrhoids).
Perianal examination and gentle palpation can be used to diagnose thrombosis of the external hemorrhoids. The internal hemorrhoids can be diagnosed with the help of anoscopy or sigmoidoscopy.
After a diagnosis of internal hemorrhoids, your healthcare provider may suggest the use of high fiber diet and increased fluids. Sitz baths are used to relieve discomfort. In a sitz bath, you immerse only the buttocks and hips in water.
For external hemorrhoids, the patients may be advised sitz baths 3 to 4 times a day as well as after each bowel movement. The stool softeners, high-fiber diet and use of a laxative may also be beneficial. Sometimes, topical anesthetic creams, like lidocaine or benzocaine, are applied 2 to 4 times a day.
Anal stenosis or the narrowing of the anal canal is an acquired disease associated with a variety of conditions. Prior hemorrhoidectomy is the most common cause of the disease. Other benign causes of the disease include anal sphincter repair, fissurectomy, chronic diarrhea, trauma, rectal foreign body insertion, prior anorectal radiation and Crohn’s disease. On the other hand, the carcinoma or anus or rectum is the malignant cause of anal stenosis.
After hemorrhoidectomy, the utilization of excess anal skin to close the wound may lead to the narrowing of the anal canal or anal stenosis. In case of carcinoma, the narrowing occurs due to annular tumor growth.
The following symptoms are helpful in the diagnosis of anal stenosis.
- Passage of small stools.
- Narrowing of the stool and incomplete evacuation.
- Painful defecation.
- Passage of red blood from the rectum (hematochezia).
- Decreased anal diameter causes difficulty in the passage of the finger into the rectum.
The digital rectal examination may be used for the diagnosis of the disease. Other diagnostic tests include flexible sigmoidoscopy, anoscopy, barium enema, colonoscopy and pelvic imaging (for example, CT scan). Biopsies may be obtained to rule out malignancy.
Medical therapy involves the use of stool softeners, bulking agents and periodic dilation through the use of a digital method or flexible dilators of increasing diameter.
The surgical methods include the removal of scar in combination with sphincterotomy. Anoplasty may be used for moderate-severe cases. In anoplasty, perianal skin is used to cover an area of the anal canal.
There are different subtypes of the anal carcinoma, including cloacal, squamous cell and tumors of the anal margin and perianal skin. The cloacal tumors originate from the transitional epithelium which separates the rectum from the squamous-lined region of the anal canal near the pectinate line.
The squamous cell anal carcinoma arises from the squamous epithelium in the anal canal. On the other hand, the tumor of the anal margin and perianal skin arise from the keratinized, hair-bearing skin proximal to the entrance of the anal canal.
The tumors of the perianal skin and anal margin are more common in men (around 80%), whereas the anal canal tumors are more common in women (around 60%).
Individuals infected with HIV or HPV (human papillomavirus) and homosexual men practicing receptive anal intercourse are at the risk of developing anal carcinoma. Meanwhile, smoking, chronic anal fistula, cervical cancer and perianal Crohn’s disease are some other risk factors.
There is a presumption that previous infection with human papillomavirus (HPV) places the individuals at risk for the condition. Immunosuppression and environmental factors (like cigarette smoking and other sexually transmitted diseases) further promote carcinogenesis.
Around 75% of patients with anal tumors are asymptomatic. However, the most common symptoms include:
- Pain in the anorectal region.
- Rectal bleeding.
- The feeling of fullness or a lump in the anal region.
- Change in bowel habits.
- Pruritus ani.
- Anal discharge and pain in the pelvic region.
Initially, visual inspection is carried out which may include anoscopy, barium study, sigmoidoscopy and digital rectal exam. Full evacuation may require the use of anesthesia. Biopsy of the lesion is very helpful for diagnosis.
A wide local excision is performed for the removal of a lesion that is small and involves only the mucosa and submucosa. On the other hand, an abdominal-perineal resection and colostomy formation is required for the large, advanced lesions.
Sometimes, a combination of radiation and chemotherapy is used to treat the condition. It is called Nigro protocol. In this case, external beam radiation is administered over the period of three weeks. It is accompanied by the use of 5-fluorouracil continuously for the first four days and again on days 29-32. On the first day of treatment, mitomycin-C is also given.
The expected success rate is 85% and the patients undergoing this treatment do not need abdominal-perineal resection or colostomy.
Occurring in about 1 of 5,000 live births, an imperforate anus is a malfunction of the anorectal area of the gastrointestinal system. It may occur in several forms. For example, the rectum may end in a blind pouch that does not connect to the colon or it may have openings (fistulas) between the rectum and the perineum, the urethra in males and the vagina in females.
Occurring during the early stages of fetal development, the malfunctions are usually associated with anomalies in other body systems.
Imperforate anus is a condition when the anus fails to connect to the rectum. The babies born with imperforate anus usually have other development anomalies, like those affecting the kidneys, the heart, the spine and the esophagus.
The symptoms of imperforate anus include:
- Inability to pass the stool. (primary symptom)
- Abdominal cramping.
Depending on its level, the defect of the imperforate anus can be classified as low or high type. Surgical intervention is needed for the treatment of both the low and high types of the defect.
For a high type of defect, the surgery involves a colostomy in the newborn period. The corrective surgery is performed in stages to allow for growth. On the other hand, corrective surgery for the low type of anomaly (which includes a fistula) involves the closure of fistula, creation of an anal opening and the relocation of the rectal pouch into the anal opening.
The major challenge for either type of corrective surgery is locating, using or the creation of adequate nerve and muscle structures around the rectum to provide for the normal process of evacuation (defecation).
- Do you know the anus contributes to the structure and functioning of both the digestive and excretory systems?
- What is the total length of the alimentary canal which ends at anus? It runs for the length of 30 feet (9 meters)!
- Are you able to feel the nature of the rectal contents? Actually, the sensitive lining of the anal canal is specialized to detect if there is solid, liquid or gas in the rectum.
- As the sensitive lining of the anal canal detects the presence of gas, the sphincters are relaxed to allows the gas to escape as flatulence.
- Did you ever realize when there is liquid in the anal canal, you feel a desperate desire to defecate? It can be countered only by a conscious effort to tighten the anal sphincter.
- In case the anal canal contains solid matter, you can either sit down to defecate or strongly contract the anal sphincters to push the feces back into the rectum and postpone defecation.
- Do you know resisting the “call to stool” or urge to defecate can lead to constipation?
- Even after the reabsorption of water in the large intestine, 60 to 70 percent of the weight of feces is water.
- The feces consist mainly of indigestible materials, undigested food and a large number of bacteria.
- It is quite surprising to note that bacteria account for nearly one-third of the dry weight of the feces. 
- Acting as a reservoir for feces, the rectum is wider than the rest of the large intestine and can be further distended.
- Measuring about 2 to 3 mm in thickness, the internal anal sphincter acts a natural barrier to the involuntary loss of stool or gas.
- Do you know 1 of every 5,000 children is born with imperforate anus? 
- One of the common causes of constipation is the tolerance of the rectum to large volumes of feces. The rectum develops tolerance when you frequently delay constipation!
- Your anus is without a question a sex organ as it plays significant role in sexual stimulation.
- Surprisingly, about 75% individuals with anal carcinoma are asymptomatic (without symptoms)!
- The Nigro protocol (combined radiation and chemotherapy) for the treatment of anal carcinoma has an amazing success rate of 85%!
- While performing corrective surgery for imperforate anus, it is a big challenge to locate, use or create adequate nerve and muscle structures around the rectum for the normal process of evacuation.
- The children who have not been toilet trained cannot constrict the external urethral sphincter to delay micturition (urination). It is because the neural connections between the brain and the bladder have not been fully developed in them. 
- Allan, J. A. (2016). Reading from behind: A cultural analysis of the anus. Zed Books Ltd..,  [Ref = Book]
- Hinrichsen, C. (2008). A Synopsis of Regional Anatomy. World Scientific Publishing Company. [Ref = Book]
- Conner, C. & Dawson, D. (2009). Operative anatomy. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. [Ref = Book]
- Drake, R., Vogl, A. W., & Mitchell, A. W. (2009). Gray’s Anatomy for Students E-Book. Elsevier Health Sciences. [Ref = Book]
- (2018, June 04). Retrieved from https://www.thefreedictionary.com/perineum [Ref = Book]
- Rizzo, D. C. (2015). Fundamentals of anatomy and physiology. Cengage Learning., ,  [Ref = Book]
- Shmueli, U. & Clough, J. (2007). Irritable bowel syndrome: answers at your fingertips. London: Class., , , , , , , ,  [Ref = Book]
- Brooker, C., & Waugh, A. (2013). Foundations of Nursing Practice E-Book: Fundamentals of Holistic Care. Elsevier Health Sciences.,  [Ref = Book]
- Chiras, D. (2005). Human biology. Boston: Jones and Bartlett Publishers. , , , ,  [Ref = Book]
- Keshav, S. (2009). The gastrointestinal system at a glance. John Wiley & Sons., , , , ,  [Ref = Book]
- LeVay, S. (1994). The sexual brain. Cambridge, Mass: MIT Press., ,  [Ref = Book]
- Jacob, S. (2007). Human anatomy: a clinically-orientated approach : an illustrated colour text. Edinburgh New York: Churchill Livingstone/Elsevier., ,  [Ref = Book]
- Beck, D. E., Roberts, P. L., Saclarides, T. J., Senagore, A. J., Stamos, M. J., &Nasseri, Y. (Eds.). (2011). The ASCRS textbook of colon and rectal surgery. Springer Science & Business Media., ,  [Book]
- Malcolm, M. (1896). The Practitioner: A Journal of Practical Medicine. Cassell and Company Limited: London, Paris and Melbourne. [Ref = Book]
- Wingerd, B. (2013). The human body: Concepts of anatomy and physiology. Lippincott Williams & Wilkins., ,  [Ref = Book]
- Ehrenpreis, E. (2003). Anal and rectal diseases explained. Remedica., , , , ,  [Ref = Book]
- Ricci, S. & Kyle, T. (2009). Maternity and pediatric nursing. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins., , ,  [Ref = Book]
- Neighbors, M. & Jones, R. (2015). Human Diseases. Australia: United States Cengage Learning.,  [Ref = Book]
- Beck, D., & Beck, D. E. (2012). Handbook of colorectal surgery. JP Medical Ltd. [Ref = Book]
- Coyne, I., Timmins, F., & Neill, F. (Eds.). (2010). Clinical skills in children’s nursing. Oxford University Press. [Ref = Book]