Anus Definition, Parts, Diseases and Functions in Human Digestive System

Did you REALLY know? Anus – a 2” canal – could be the highest-risk factor for AIDS.

And you’d die without this ‘MOST UNDERAPPRECIATED’ part of the body.

Read on to discover the quirks and mysteries surrounding the function of anus, its parts, and diseases with a dose of wow.

Here’re the key functions of anus:

You can read about these anal functions in detail with anus pictures in the succeeding sections. Also, get interesting answers to the FAQs about the anus like:

  • Why do human babies not have bowel control?
  • What is achieved first: anus control or bladder control?
  • How to train children for the voluntary passing of stool?
  • Who first develop control over their bladder and anus, a girl or a boy?
  • What is the function of anus hair?
  • What are anus worms?
  • How can anus inflammation be treated?
  • Which diseases cause anal cramps?
  • How dangerous are the anus tumors?

Simultaneously serving 3 body systems, the anus structure is a complex of internal and external sphincters. It literally means a ring or a circle.

Which one of the anal sphincters helps you consciously hold stools back? Read on to get the answer.

Its thin epithelium and many blood vessels are prone to mild abrasive damages.

Recall if you ever experienced tearing sharp pain during defecation. It could be due to acute or chronic anal fissures.

People may live with and die due to anal diseases like carcinomas without knowing about it!

Did you know? “Soap and shampoo can damage the delicate skin of the anus”, claims Rafael Lugo, a colorectal surgeon from Houston.

Water and wiping are enough for the hygiene of the anus.

Sticks, stones, snow, grass, and leaves are some of the things people used over centuries to wipe their buttocks.

Anus Serving 3 Body Systems

The anus simultaneously serves 3 major body systems, i.e. the digestive, excretory, and reproductive systems.

  1. Digestive System and the Anus
  2. It forms the last piece of the gastrointestinal system.

    The anus function in the digestive system is to provide an outlet for disposing of the undigested matter out of the body.

  3. Excretory System and the Anus
  4. It performs several jobs in its capacity as an excretory organ.

    For example, in addition to being an aperture for defecation, the anus regulates bowel movements.

    Also, it gives the individual conscious control over the process.

  5. Reproductive System and the Anus
  6. Interestingly, the anal hair conducts scents that our ancient selves used for reproductive purposes.

    However, this function of the anus is no more considered useful.

    Anus Definition - Anus Diagram

    Fig. 1: The human anus & nearby body parts.

Anus Definition

The Merriam Webster Dictionary gives different anus definition for the medical students and the English language learners. For medical students, it defines anus as:

"The posterior opening of the digestive tract".

For English language learners, on the other hand, it defines the anal opening as:

"The opening on the body present between the person’s buttocks through which the digestive waste passes from the body."

Wikipedia the Free Encyclopedia gives the definition of the anus in these words:

"The anus is an opening at the opposite end of an animal’s digestive tract from the mouth."

On the, you will find a bit detailed definition:

"The human anus is an opening at the lower end of the alimentary canal for the excretion of the solid waste of the body."

Here is a bit more comprehensive anus definition:

"Playing an important regulatory role in the excretion of digestive wastes, the anal opening is surrounded by a voluntary and an involuntary sphincter, and constitutes the terminal part of the digestive system".

Two round muscular structures line the anus. These muscular structures assist in the opening and closing of the anal aperture.

Anus Definition Biology

The anus definition biology is specifically for the medical students. “The Online Medical Dictionary” provides the biological definition in these words:

"The lower opening of the digestive tract that is associated with the anal sphincter and lies in the cleft between the buttocks to help in the extrusion of the fecal matter."

So, here you also get a bit understanding of the structure and function of this organ of digestive system.

The opening and closing of the hole is under the voluntary control of an individual, particularly, the external sphincter.

It is not completely functional in the human babies. However, as the child advances towards maturity, it comes under their deliberate command.

On the attainment of puberty in an individual, pubis hair starts to grow around the anal opening.

Just imagine, "How would your social life be if you were unable to hold and prolong feces before you reach toilet for defecation?"

When you prolong defecation for a longer duration, the excretory matter goes backwards into colon.

Here there is further absorption of fluids, salts and any other useful substances.

You will learn about the details of the things related to anus definition in the succeeding parts of the article.

Difference Between Rectum and Anus:

Anus and rectum are two terminal parts of the large intestine. Both these organs have different size, structure, function and location in the body.

As you look at the elementary canal diagram and observes it from anterior to the posterior, rectum forms the second last part and anus comes at the end.

In other words, the rectum is the concluding segment of the digestive tract which ends at the anus.

Anus and Rectum

Fig. 2: Anus and Rectum diagram

Rectal region is a bit large in size as compared with the anus. The former measures in length from 10 to 15 cm, while the latter may extend only up to 5 centimeter.

The rectum has the same diameter as that of the sigmoid colon.

The job of rectum is to store the waste products in preparation for their ultimate removal out of the body.

The job of the anus, on the other hand, is to serve as an opening for the elimination of feces.

Anus Functions in Detail:

Seemingly, there is just one anus function – an opening for defection.

In fact, it performs various other functions as well. Some of the major anus functions are listed here.

Did you ever imagine the repercussions of holding the digestive wastes in the body for just a month?

Apart from excretion, the anus also has a close association with sexuality.

The knowledge of anal anatomy makes it easy for you to understand the anus function in reproduction.

The following information will be very useful.

  1. An Aperture for Defecation
  2. The primary anus function is to serve as an aperture for defecation.

    After defecation, the colon and rectum prepare themselves to receive and store the digestive wastes descending along the alimentary canal.

  3. Regulation of Excretion
  4. The internal and external sphincters play a key role in the regulation of excretory process.

    The internal involuntary sphincter operates under the command of autonomous
    nervous system.

    On the other hand, you can deliberately open or forcefully contract the external sphincter.F

  5. Voluntary Control over Stools
  6. Holding feces back for a certain duration is a very important anus function.

    Otherwise, you won’t be able to hold the bowels for some time in order to reach the place of defecation.

  7. Triggering Need for Removal of Feces
  8. The capacity of rectal storage and sensation of waste products arriving in the region inspire the need for their removal.

    Consequently, you feel the desire to defecate and move to a suitable place.

  9. Sexual Arousal
  10. The anus is to do something with sexuality.

    There is a high concentration of nerve endings in the anal region.

    These nerve endings can play a role in sexual arousal.

    Role in sexuality is a secondary anus function. It happens so due to the proximity of the anus with the sensitive parts of male and female genitals.

  11. Conducting Scents
  12. Did you know? Your anus also helps conduct scents.

    Conducting scents is no more considered a useful function of the anus.

    However, humans had been relying upon this function of the organ for reproductive purposes in the distant past.

Anus Diagram and Steps for Excretion

Looking at the anus diagram, you can easily understand various steps for excretion. Different nearby parts of anus experience physical changes as waste matter reaches there. Here is a brief discussion of the 5-step procedure for excretion.

Anus Diagram

Fig. 4: Anus Diagram showing different parts of anal canal.

  1. First Step
  2. After digestion in the stomach and the small intestine, the small food particles are taken into the blood stream for use in the body. On the other hand, the undigested material descends to the large intestine for further absorption of left over water and salts.

    After that, the taste materials reach the rectal region. It is the first step in the process of defecation.

  3. Second Step
  4. In the second step, the waste, semi-solid substances fill the rectum and anal canal. Therefore, the surrounding walls stretch to accommodate it.

  5. Third Step
  6. The nerve endings present at the rectal region deliver the stimulus to the brain. The brain processes this stimulus to generate an appropriate response.

  7. Fourth Step
  8. In the fourth step of the process of defecation, the responding signal reaches the target place. It induces the desire for defecation in the individual. Afterwards, the individual takes appropriate action accordingly.

  9. Fifth Step
  10. The fifth and final step involves the relaxation of the internal and external anal sphincters. The internal sphincter opens through an involuntary action while the external one is under the voluntary control of the individual.

Anus Parts:

This 2-inch canal can be distinguished into 9 different parts.

Here is useful information related to anus anatomy.

Two layers of muscles surround this ring-shaped structure. They form the internal anal sphincter and external anal sphincter.

However, both are not under your deliberate action.

The interior sphincter tightens or relaxes under the involuntary action while you can voluntarily control the action of the external one.

The sphincters vary in their length while running along the anal canal.

The internal muscle extends up to 4 centimeters and the external one reaches the maximum size of 10 centimeters or more.

The outer muscular layer retracts during the process of defecation.

Further, it has two strata, namely, deeper stratum and the superficial one (the main portion).

Here follows a brief description of different parts of the anus structure, starting from the anorectal ring and ending at the anal orifice.

  1. Anorectal Ring
  2. Where does the anal region start? What marks the end of rectum and the start of the anal canal?

    Anorectal Ring Anorectal Junction

    Fig. 5: Anorectal Ring

    It is the anorectal ring or the anorectal flexure which serves as a demarcation between the rectum and the anal region.

  3. Anal Canal
  4. The anal canal is a small terminal part of the large intestine that starts from the rectum and continues till the anal orifice. It ranges from 3 to 5 centimeters in length.

    Anal Canal

    Fig. 6: Anus Canal Diagram.

  5. Longitudinal Folds or Anal Columns
  6. Anal Columns

    Fig. 7: Anal Columns (formerly Rectal Columns)

    The anal canal can be divided into the upper part, lower potion and the anal opening. In the upper part, you will come across the longitudinal folds.

    They are also called the anal columns or rectal columns.

    There are 5 to 10 rectal columns in the upper part.

    The enfolding of the mucous membrane and some of the muscular tissues in the anal canal results in the formation of vertical ridges, called anal columns.

    The description of these columns is found in Gray’s Anatomy (1918).

  7. Blood Vessels
  8. Lots of blood vessels surround the anal canal to supply oxygenated blood to this organ.

    In the upper part of anal canal, each rectal column contains a small artery and vein.

    Blood Vessels of the Anal Canal

    Fig. 8: Blood Vessels in Anorectal Area

  9. Nerve Endings
  10. Sensitive nerve endings are present in the anal region.

    As the waste products enter this region, the nerves take signal to the brain for the generation of an appropriate response.

  11. Anus Glands
  12. The anus glands are present in the anal canal of different mammalian species including cats and dogs.

    They are also present in humans.

    John Aglitis, from the Department of Anatomy at the Ohio State University, published a paper in 1961 discussing the presence and anatomy of the anus glands.

    By nature, the anal glands are the eccrine or exocrine glands.

    The fluid, secreted by these glands, enters the anal canal via the anal ducts.

    Anal Glands

    Fig. 9: Location of Anus Glands in the Anal Canal

    The wall of the anal canal consists of different layers of muscles, blood vessels, lymph vessels, and sensitive nerve-endings.

    Anus glands are also present in this wall at varying lengths.

    According to the findings of Eglitis, these glands are vestigial in man.

    But still, they have got great clinical significance.

    Regarding their size, form, and number, they vary considerably among individuals.

    The obstruction of anal ducts can lead to some serious health issues, such as the formation of anus abscesses and anus fistula.

  13. Anal Ducts
  14. Anal ducts are present in the wall of the anal canal.

    The job of these ducts is to empty the secretions of anus glands into the lumen of the canal.

    Anal Ducts

    Fig. 10: Anal Ducts in the wall of Anal Canal

  15. Anus Sphincter Muscles
  16. A sphincter muscle can also be called an anus ring as it forms a ring-like structure around the anal orifice.

    There are two sphincter muscles: internal anal sphincter and external anal sphincter.

    Anal Sphincters

    Fig. 11: Internal Anal Sphincter and External Anal Sphincter

    The internal sphincter acts involuntarily to regulate the bowel movements.

    On the other hand, the external sphincter is under the voluntary control of the individual.

  17. Anal Valves
  18. The anal valves are small circular folds of the mucous membrane that join the lower portions of the anal columns.

    Small hollow spaces are available between these valves which lead to lymph ducts and the glands.

Anal Valves

Fig. 12: Anal Valves

Anus Diseases

Anus Disease

Fig. 13: Different Anal Disorders

Various conditions of the anal canal and anus orifice are common nowadays.

The anus diseases also affect the anal glands, muscles and sphincters.

Such disorders not only disturb the excretory function of your digestive system, but are also the cause of great embarrassment for the victim in social life.

Some of the disorders are infectious. The others may be viral or cancerous.

Still there are some that arise due to mechanical reasons. Some of the most frequently occurring ailments of excretory orifice include anal abscesses, anus fistula, hemorrhoids, anal fissures, and so on.

The diagnostic symptoms of the acute and chronic conditions of anus may include pain, anal itching, intense sensation of burning, bleeding, cramps, anus inflammation, bowel obstruction, release of pus, chills, fever, and so on.

The suggestion and application of treatment strategy depends upon the type and severity of the condition.

It may involve medication, diet supplementation, or surgical removal of the infected part. However, if you take preventive measures in advance, it can save you from a lot of trouble and embarrassment.

After the onset of the disease, consult your health care provider immediately, so you can save yourself from further hassle.

Here is a brief description of seven different anus diseases with their symptoms and treatments.

  1. Anal Abscesses
  2. The condition of anal abscesses refers to the development of pus near the anus.

    Mostly, there is a perianal abscess, appearing as a bile-like swelling around the anus.

    It is warm to touch and red in appearance.

    Anal Abscesses

    Fig. 14: Different Locations of Anal Abscesses

    An anal abscess can be very painful and may require surgery for its treatment.

    The causes include blockage of the anal gland or a sexually transmitted infection.

    • Anal Abscesses Symptoms
    • The frequently occurring symptoms include:

      • Development and drainage of pus near the anus.
      • Anus pain usually radiating to buttocks.
      • Anal itching.
      • Formation of lump in the anal area.
      • Sensation of anus pain during bowel movements.
      • Fever and fatigue.

    • Treatment
    • In most of the cases, a surgical method is recommended for the drainage and removal of anal abscesses.

      For the drainage of pus, the surgeons make an incision near the anus.

      In case of delay in the treatment, it may lead to fistula formation.

      For the treatment of fistula, again there is the need of surgical operation.

      After the surgery, your healthcare provider may prescribe some medications for pain relief.

      However, the people with diabetes or an immunity disorder may also need the use of antibiotics.

  3. Anal Fistula
  4. An anal fistula is a small tunnel in the anal region that develops an abnormal connection between the skin and the site of an abscess.

    It is a painful condition and can lead to bleeding during the passage of stools.

    Anal Fistula

    Fig. 15: Fistula Formation in the Human Anus

    Fistula formation may occur as a complication of the anal abscess.

    It may cause persistent drainage or result in the recurrent anal abscesses.

    Surgery is the only successful treatment measure for fistula.

    • Anal Fistula Symptoms
    • The possible symptoms for anal fistula include:

      • Constant anus pain which becomes worse on sitting down.
      • Swelling, redness and feeling of tenderness around the anus.
      • Anal itching and irritation.
      • Drainage of pus and anus bleeding.
      • Fever and constipation.
      • Painful bowel movements.

    • Treatment
    • There are usually very rare cases when the condition of anal fistula is healed without a proper treatment.

      Here, the viable option is that of surgery. However, to determine the best therapy, the doctors need to find out the position of the fistula.

      They also need to examine if the fistula is a single channel or branches off in different directions.

      Usually a general anesthetic is used to carry out surgery and the patient is discharged from the hospital within a day.

      Did you ever think why is surgery a better option for fistula treatment?

      It is because surgical therapy can heal the condition without inflicting a damage to the sphincter muscles.

      A damage to the sphincter muscles, on the other hand, can result in the loss of bowel control.

  5. Anal Fissures
  6. A shallow tear or ulceration in the tissues along the wall of anal canal is given the name of anal fissure.

    In addition to irritation in the region, an anal fissure can also trigger a spasm in the anal sphincter.

    The spasms are very painful and may involve anus bleeding.

    Anal Fissures

    Fig. 16: Anal Fissures

    The passage of large, hard stools and a secondary complication of anal surgery may be the causes of development of anal fissures.

    Meanwhile, constipation, repeated use of enema nozzles and multiple pregnancies increase the risk for the onset of this condition.

    Here enema is a fluid used to clean out the terminal part of the large intestine, especially before a surgical procedure.

    • Anal Fissures Symptoms
    • The commonly occurring mild and painful symptoms of the condition include:

      • A Sensation of sharp pain.
      • Anus burning during or immediately after the bowel movements.
      • Discharge of blood with stools.
      • Rectal itching and irritation.
      • Painful spasms in the anal sphincter region.

    • Treatment
    • You can prevent any anal disorders by taking a high-fiber diet, maintaining good hygiene in the anal area and drinking plenty of liquids.

      However, in case of becoming an unfortunate victim of anal fissures, initially you should wait for some days for the natural healing of the condition.

      In case the condition persists, your healthcare provider may advise stool softeners to ease and smoothen the bowel movements.

      To relieve spasms, nitroglycerin ointment is considered effective.

      For treating chronic anal fissures, doctors may prescribe local injections of borox.

      If the anal fissures become persistent or recurrent, the patients may need to go for surgical removal.

  7. Anus Cancer
  8. Several different types of tumors can grow in the anal region.

    Some of the anus tumors are benign while others are linked with anus cancer.

    However, sometimes a benign anus tumor may become cancer at the later stage.

    Anal Cancer

    Fig. 17: Anal Cancer

    Do you know what cancer is in the literal sense?

    In biology, cancer refers to the uncontrolled growth of cells in any part of the body.

    Cancer cells crowd out the normal cells and can also spread to other organs of the body.

    The chances of anal cancer increase with advancing age. According to a research, more than 80% of the cases are of those people with the age above 60.

    Meanwhile, as compared to the married men, the rate of anus cancer is six times higher in the single men.

    An anus infection with human papillomavirus and diseases of immune system, such as AIDS, makes people prone to anal cancer.

    • Anal Cancer Symptoms
    • Listed below are some of the symptoms of the anus cancer:

      • Irregularity in the bowel habits.
      • Appearance of a hard mass or lump near the anus.
      • Unusual discharge of liquid from the anal orifice.
      • Severe pain in the affected area.
      • A sensation of pressure buildup in the anal region.

    • Treatment
    • Digital rectal exam, anoscopy and endorectal ultrasound can help in the diagnosis of the condition.

      Afterwards, biopsy will lead to the confirmation of the disease.

      For the treatment of anal cancer, doctors use chemotherapy and radiation in combination.

      Such a treatment measure has been in vogue since 1974.

      If this therapy does not show positive results, the doctors need to utilize the option of surgery.

  9. Hemorrhoids/Piles
  10. It is one of the very painful anus diseases.

    The presence of distended varicose veins in the anal region characterizes the condition of hemorrhoids or piles.

    The valves in the veins permit only unidirectional flow of blood – back to the heart.

    However, the weakening of these valves may result in the flow of blood towards wrong direction or its stagnation in veins.

    Anal Piles

    Fig. 18: Anal Piles

    In case of hemorrhoids, there occurs engorging of veins with blood in the anus.

    Though initially painless, the fragile walls of swollen vein may rupture and result in bleeding with stool.

    Clot formation in the vessels is very painful and may require surgical removal.

    • Piles Symptoms
    • Frequently occurring symptoms for piles/hemorrhoids include:

      • Bowel movements cause bleeding.
      • The appearance of bright red blood on the stool, toilet paper or toilet bowl.
      • Itching in the anus.
      • Painful bowel movements.
      • Mucus coming from the anus.

    • Treatment
    • To recommend an appropriate treatment measure, your healthcare provider needs to conduct rectal exam.

      They may conduct colonoscopy and some other tests to determine the cause of the disease.

      You can treat the anus hemorrhoids through the use of over-the-counter products and changes in the lifestyle.

      However, it is better to consult a doctor before using any medicine or ointment.

      The Use of high-fiber diet is very helpful in treating the condition.

      You should also consume plenty of water on daily basis. The option of surgery is also there to remove the swollen veins from the affected area.

  11. Anal Warts
  12. Anus warts are a viral disorder.

    Are you familiar with its unfamiliar name, ‘condyloma acuminata’?

    In the beginning, it affects the area inside or around anus and then spreads to genital parts.

    A wart initially appears as small as the head of a pin. However, the warts may grow much larger to cover the entire anal area.

    Anal Warts

    Fig. 19: Anal Warts

    Early stages cause no symptoms. But the advanced stages can give rise to bleeding, itching and discharge of mucus with stools.

    The causative agent is Human Papilloma Virus (HPV) that an affected individual may communicate to others through sexual contact.

    However, it may be transmitted without having anal intercourse.

    If there is delay in the diagnosis and start of treatment, it is likely to worsen to become anal cancer.

    The available curative measures include the application of topical medication.

    However, depending upon the severity of the condition, your health care provider may also recommend the use of liquid nitrogen to freeze the warts or remove them through surgical therapy.

    • Anal Warts Symptoms
      • The appearance of tiny spots or growths inside and around anal opening.
      • Itching in the affected area.
      • Bleeding with stools.
      • Discharge of mucus.
      • Feeling of a mass in anus.
      • Tiny growths may become much larger with time.

    • Treatment
    • The timely treatment of anus warts is necessary because they can grow larger and multiply.

      Also persistent warts can increase the risk of anus cancer.

      Different topical medications are available in the market which can freeze or burn the anus warts or heal them slowly.

      In case, topical medications do not work the patients need to undergo surgery.

      It involves the surgical removal of warts. Here the choice of anesthesia depends on the number and location of the warts that are to be removed.

      If there is a large number of warts, the surgery may be performed in stages.

  13. Pruritus Ani
  14. Affecting men more commonly than women with a 4:1 ratio, pruritus ani is a common medical problem of anus.

    From 1% to 5% of individuals in the general population may be suffering from this condition.

    Literally meaning "itchy anus", it gives rise to a discomforting sensation in the rectal opening, thus triggering a desire in the victim to scratch the affected region.

    The injury of skin due to scratching makes irritation more intense and widespread.

    Scratching also induces the desire to scratch even more.

    Depending on severity of the condition, pruritus ani can be classified into primary and secondary forms.

    Referred to as idiopathic, the primary form may appear without an identifiable cause, while in the secondary case, the cause is identifiable and treatable.

    Because of the unknown causative agent in the majority of the cases, the treatment is not easy.

    Taking fibrous foods and use of bowel medications is useful for the thickening and complete evacuation of stool.

    Maintaining hygiene is very necessary.

    The administration of antibiotics can cure infections.

    Meanwhile, hair dryer can be helpful to keep the skin dry.

    • Pruritus Ani Symptoms
      • Unpleasant sensation around rectal opening, arousing desire in the victim to scratch.
      • Development of red spots.
      • Leakage of mucus with stools.
      • Loose stools.
      • Injury resulting from scratching.
      • Cracks, wounds and thickening of skin.

    • Treatment
    • The first step to relieve the symptoms of pruritus ani is to avoid the contact of anus with heat, wool, moisture and pressure.

      To suggest an appropriate treatment measure, your doctor needs to discover the underlying cause.

      The causes include different skin conditions, such as psoriasis, eczema, lichen sclerosis, allergic dermatitis, and so on.

      For treatment, different topical medications, such as steroid and antifungal creams, may be very effective.

      Meanwhile, your healthcare provider may prescribe certain antibiotics to treat infection of the anus, if any.

About the Author

Posted by: M. Isaac / Senior writer

A graduate in biological sciences and a PhD scholar (NCBA&E University, Lahore), M. Isaac combines his vast experience with a keen and critical eye to create practical and inherently engaging content on the human body. His background as a researcher and instructor at a secondary school enables him to best understand the needs of the beginner level learners and the amateur readers and educate them about how their body works, and how they can adopt a healthier lifestyle.

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