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Anus
Anus of a dog
Formation of anus in proto- and deuterostomes
Details
PrecursorProctodeum
SystemAlimentary
ArteryInferior rectal artery
VeinInferior rectal vein
NerveInferior rectal nerves
LymphSuperficial inguinal lymph nodes
Identifiers
Latinanus
TA98A05.7.05.013
TA23022
Anatomical terminology

In mammals, invertebrates and most fish,[1][2] the anus (pl.: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the exit end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facilitate the expulsion of waste that remains after digestion.

Bowel contents that pass through the anus include the gaseous flatus and the semi-solid feces, which (depending on the type of animal) include: indigestible matter such as bones, hair pellets, endozoochorous seeds and digestive rocks;[3] residual food material after the digestible nutrients have been extracted, for example cellulose or lignin; ingested matter which would be toxic if it remained in the digestive tract; excreted metabolites like bilirubin-containing bile; and dead mucosal epithelia or excess gut bacteria and other endosymbionts. Passage of feces through the anus is typically controlled by muscular sphincters, and failure to stop unwanted passages results in fecal incontinence.

Amphibians, reptiles and birds use a similar orifice (known as the cloaca) for excreting liquid and solid wastes, for copulation and egg-laying. Monotreme mammals also have a cloaca, which is thought to be a feature inherited from the earliest amniotes. Marsupials have a single orifice for excreting both solids and liquids and, in females, a separate vagina for reproduction. Female placental mammals have completely separate orifices for defecation, urination, and reproduction; males have one opening for defecation and another for both urination and reproduction, although the channels flowing to that orifice are almost completely separate.

The development of the anus was an important stage in the evolution of multicellular animals. It appears to have happened at least twice, following different paths in protostomes and deuterostomes. This accompanied or facilitated other important evolutionary developments: the bilaterian body plan, the coelom, and metamerism, in which the body was built of repeated "modules" which could later specialize, such as the heads of most arthropods, which are composed of fused, specialized segments.

In comb jellies, there are species with one and sometimes two permanent anuses, species like the warty comb jelly grows an anus, which then disappear when it is no longer needed.[4]

Development

[edit]
Main articles: Protostome and Deuterostome

In animals at least as complex as an earthworm, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the growth of the gut. In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes were so named because it was thought that in their embryos the dent formed the mouth first (proto– meaning "first") and the anus was formed later at the opening made by the other end of the gut. Research from 2001 shows the edges of the dent close up in the middles of protosomes, leaving openings at the ends which become the mouths and anuses.[5]

See also

[edit]
  • The dictionary definition of anus at Wiktionary
  • Anal canal – Functional segment of the large intestine
  • Anorectal manometry – Medical functional test of the anus and rectum
  • Imperforate anus – Birth defect of malformed rectum

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Anus

View on Grokipedia
from Grokipedia
The anus is the external opening of the , the terminal portion of the , measuring approximately 2.5 to 4 centimeters in length and serving as the conduit for by expelling fecal matter from the body. It is anatomically divided by the pectinate (dentate) line into an upper portion lined with columnar derived from the and a lower portion lined with from ectodermal origin, facilitating both continence and controlled release of waste. The structure is supported by the , an involuntary layer that maintains resting tone to prevent leakage, and the , a voluntary that allows conscious control over bowel movements, typically developing full functionality between 18 and 24 months of age. Additionally, the anus features anal columns, sinuses, and valves in its upper region, which aid in secretion to lubricate passage, while the surrounding perianal provides barrier protection against . Functionally, the anus plays a critical role in maintaining fecal continence by distinguishing between solid, liquid, and gaseous contents through sensory mechanisms in the , while coordinating with the to store and propel waste during . This process involves relaxation of the sphincters triggered by rectal distension, ensuring efficient elimination without involuntary loss, and the anus remains closed under normal conditions to block bacterial entry from the external environment. Embryologically, the superior anal canal arises from endodermal tissue, while the inferior portion develops from invading during cloacal division around the seventh week of , establishing its dual epithelial lining and innervation patterns—visceral above the pectinate line and somatic below. Common conditions affecting the anus highlight its clinical importance, including , which are swollen veins either above (internal, often painless) or below (external, painful) the ; anal fissures, tears in the lining causing bleeding and pain; and abscesses or fistulas from s. can result from sphincter damage or nerve issues, while anal stenosis narrows the canal, impeding passage, and malignancies like are frequently linked to human papillomavirus (HPV) . Preventive measures emphasize a high-fiber diet to soften stool, gentle to avoid , and safe sexual practices to reduce risks, with prompt medical evaluation recommended for symptoms such as bleeding, itching, or discharge.

Anatomy

Gross anatomy

The anus is the terminal opening of the through which are expelled from the body. It is located in the , situated between the and posterior to the . The serves as the short passage connecting the to the anus, measuring approximately 4.4 cm in length in males and 4.0 cm in females, and beginning at the anorectal ring where it meets the . This canal is positioned within the , supported by the muscle, and bordered laterally by the ischiorectal fossae, which provide space for fat and facilitate expansion during . Key external landmarks include the anal verge, which marks the distal end of the and the transition to the perianal skin, and the (also known as the dentate line), located approximately 2 cm proximal to the anal verge.

Microscopic anatomy

The anal canal's epithelial lining undergoes a characteristic transition along its length, reflecting its functional adaptation to digestive and protective roles. In the proximal portion, adjacent to the , the mucosa is lined by , which facilitates absorption and secretion similar to the intestinal tract. This transitions at the dentate (—located approximately 2 cm from the anal verge—to stratified squamous non-keratinized in the distal (anoderm), providing a protective barrier against mechanical stress and pathogens. This epithelial shift occurs within the anal transitional zone (ATZ), a short segment (about 1 cm) proximal to the dentate line, where intermediate epithelial types blend, often appearing as cuboidal or low columnar cells before fully converting to squamous layers. Embedded within the anal canal's submucosa are anal glands and sinuses, which contribute to its secretory functions. The anal glands, numbering 6–12 on average, are compound tubuloacinar structures that open into the anal sinuses (crypts of Morgagni) at the dentate line; their ducts extend into the and often penetrate the . These glands secrete to lubricate the canal during , aiding smooth passage of and reducing . However, the sinuses and gland ducts serve as potential reservoirs for bacterial accumulation, predisposing to infections such as perianal abscesses and fistulas when obstructed. Above the dentate line, the mucosa features prominent longitudinal folds known as anal columns (of Morgagni), which are vertical projections of the supported by underlying vascular cushions. These columns, typically 6–10 in number and extending 1–1.5 cm proximally, contain branches of the rectal arteries and veins that form the anal cushions, enhancing structural integrity. Connecting the bases of these columns are transverse anal valves, thin folds of mucosa that form the irregular dentate line and house the openings of the anal glands. These folds increase surface area for secretion and sensation while contributing to continence by creating a valvular mechanism. The of the is a layer rich in elastic fibers, , and lymphoid tissue, providing flexibility and support to the overlying mucosa. It houses the anal glands, their ducts, and the vascular and lymphatic plexuses of the anal cushions, which are erectile-like structures that swell to maintain closure. The muscularis layer specific to the canal includes the , a thin sheet of fibers that underlies the and extends from the ; it thickens in the transitional zone to anchor the mucosal folds and regulate local glandular secretion. Below this, the canal's circular layer transitions into the , but the itself facilitates fine mucosal movements during .

Associated structures

The anal region is supported by a dual sphincter mechanism that ensures fecal continence. The , composed of , provides involuntary tonic contraction responsible for the majority of resting anal pressure, typically over 70%, thereby preventing passive leakage of stool and gas. This structure encircles the upper and maintains baseline closure without conscious effort. In contrast, the consists of arranged in a triple-loop configuration, enabling voluntary contraction to augment closure during activities that increase intra-abdominal pressure, such as coughing or lifting, thus preserving continence under stress. The arterial supply to the anal canal varies by region. Above the , it is supplied by the , a branch of the . Below the , it receives blood from the (branching from the ) and the inferior rectal arteries (branching from the ), which supply the lower , external , and perianal . Venous drainage of the anal canal is region-specific. Above the , blood drains via the superior rectal veins into the and the portal system. Below the , the middle and inferior rectal veins drain into the internal pudendal vein and subsequently the internal iliac veins, forming part of the systemic circulation. Lymphatic drainage from the anus follows a bifurcated pattern based on anatomical landmarks, such as the . Below this line, lymph flows primarily to the , which then progress to the external iliac nodes, supporting immune surveillance of the perianal area. Above the , drainage directs toward the internal iliac nodes, integrating with pelvic lymphatic pathways. Innervation of the anus involves both autonomic and somatic components for coordinated control. Autonomic innervation includes sympathetic fibers from the splanchnic nerves (L1-L2), which reach the internal sphincter via the to modulate tone, and parasympathetic fibers from the (S2-S4), which promote relaxation during . Somatic innervation is provided by the (S2-S4), which supplies sensory and motor fibers to the , enabling voluntary contraction and tactile sensation in the perianal skin.

Physiology

Defecation process

The defecation process is initiated by the accumulation of fecal matter in the rectum, primarily propelled by peristaltic contractions in the colon that move contents distally toward the rectosigmoid junction. This colonic peristalsis, occurring in coordinated waves, delivers feces into the rectum, where distension of the rectal walls activates mechanoreceptors in the rectal mucosa. The resulting rectal distension triggers the rectoanal inhibitory reflex (RAIR), an involuntary response mediated by the enteric nervous system, which causes relaxation of the internal anal sphincter to allow fecal material to enter the upper anal canal. Once the RAIR is activated, voluntary control becomes crucial through the , a that can be consciously relaxed or contracted to permit or delay expulsion. Concurrently, the puborectalis muscle, part of the group, relaxes to straighten the anorectal angle from its normal 90-degree bend to approximately 120 degrees, facilitating the passage of feces by reducing resistance. This relaxation, combined with increased intra-abdominal pressure generated by the (straining with diaphragmatic descent), propels the fecal bolus through the . Several factors influence the efficiency of defecation, including fecal consistency, which determines the ease of passage—soft, formed stools ( types 3-4) facilitate smoother expulsion compared to hard, dry ones affected by or low-fiber diets. Intra-abdominal , augmented by coordinated contraction of abdominal muscles, aids in pushing feces outward, while precise coordination with the muscles ensures perineal descent and relaxation for complete evacuation. In adults, normal defecation frequency ranges from three times per week to three times per day, with a median daily fecal volume of approximately 100-106 grams, varying by diet and individual .

Sensory and motor functions

The anus receives somatic sensory innervation primarily through the , which transmits sensations of touch, , and from the perianal skin and to the sacral segments S2–S4. This nerve's inferior rectal branch specifically innervates the and adjacent mucocutaneous areas, enabling fine discrimination of stimuli such as pressure or irritation. The reflex exemplifies this sensory-motor integration: a brisk contraction of the occurs in response to perianal stimulation, like a pinprick at the , confirming an intact sacral via pudendal afferents and efferents. Absence of this reflex indicates disruption in somatic pathways, though its presence signifies preserved function. Autonomic motor control of the anus is mediated by both parasympathetic and sympathetic divisions, regulating the internal anal 's smooth muscle tone. Parasympathetic fibers from sacral segments S2–S4, via the , provide inhibitory innervation that promotes relaxation of the internal , facilitating defecation through nitrergic and (VIP)-mediated pathways that reduce tone via and cGMP signaling. In contrast, sympathetic fibers from the thoracolumbar outflow (T10–L2), relayed through the inferior mesenteric and hypogastric plexuses, exert excitatory effects via norepinephrine acting on α-adrenoceptors, thereby maintaining basal tone and contributing to continence during rest. This dual autonomic modulation ensures the internal generates over 70% of resting anal pressure, with coordinating slow-wave activity for sustained function. Sensory feedback plays a pivotal role in anal continence by enabling detection and voluntary retention of fecal matter through integrated neural loops. Rectal distension activates mechanoreceptors innervated by parasympathetic afferents (S2–S4), signaling fullness to the and triggering es like the rectoanal inhibitory , which transiently relaxes the internal while contracting the external via pudendal efferents. The upper anal canal's endings, particularly around the anal valves, sample rectal contents by equalizing pressures, allowing discrimination between gas, liquid, and solid material to prevent leakage. The further refines this process locally, while cortical processing in the prefrontal area coordinates voluntary squeeze for retention, with impaired feedback—such as reduced or electrical sensitivity—compromising overall continence. The anus also serves erogenous functions due to its dense innervation, sharing pathways with genital structures that contribute to sexual pleasure. The links perianal sensation to genital arousal, innervating the external sphincter, , and , which facilitates orgasmic responses during anal , particularly in the anterior rectal wall overlying the in males or extending clitoral structures in females. This shared somatic input allows for heightened sensations, with studies reporting orgasm achievement from receptive anal intercourse in up to 36% of men and 19% of women, underscoring the nerve's role in integrating anal and genital erogeneity. Motor contributions from the to perineal muscles further support coordinated contractions during sexual activity, enhancing overall pleasurable feedback.

Embryological development

Early formation

The development of the anus begins with the , a common chamber formed during early embryogenesis that serves as the terminal portion of both the gastrointestinal and urogenital tracts. Around the fourth week of , the cloaca is present as an endodermal cavity lined by and covered caudally by the cloacal membrane, which consists of and endoderm without intervening . By week 6, the urorectal septum, derived from , begins to grow caudally to partition the cloaca. This division is completed by approximately week 7, separating the cloaca into the ventral and the dorsal anorectal canal, thereby establishing the foundational separation of the urinary and gastrointestinal systems. Concomitant with internal partitioning, external structures form through ectodermal invagination. The proctodeum emerges as a shallow depression in the surface caudal to the cloacal , representing the future site of the anal opening. This deepens during weeks 6 to 7, connecting with the anorectal canal after the urorectal fuses with the cloacal , which divides into the urogenital and anal membranes. The proctodeum contributes to the external, inferior portion of the developing . Internally, the endoderm plays a critical role in forming the anorectal canal. The , which extends from the distal , gives rise to the endodermal lining of the and the superior two-thirds of the following cloacal septation. This endodermal contribution ensures the mucosal continuity from the into the anal region. By week 8, the anal membrane, the thin dorsal remnant of the cloacal membrane separating the proctodeum from the anorectal canal, ruptures to establish patency and allow communication between the canal and the exterior. This rupture is essential for normal anorectal development.

Differentiation and maturation

Following the initial division of the by the urorectal septum around weeks 7-8, the undergoes further differentiation through the migration of cells, which originate from the dorsal and travel caudally along the gut axis to populate the (ENS) in the and anal region. These cells differentiate into neurons and glia essential for gastrointestinal motility and sensation in the anorectal area, with their proliferation, migration, and survival regulated by glial cell line-derived neurotrophic factor (GDNF) and to prevent premature differentiation. Between weeks 9 and 12 of , the develops as a specialization of the circular layer from the , while the arises from surrounding mesodermal tissue forming , establishing the dual sphincter mechanism for continence. Concurrently, the pectinate (dentate) line forms as the junction between the endodermal upper and the ectodermal lower portion, corresponding to the site of the ruptured anal membrane and the original cloacal membrane division, marking the transition in epithelial lining and vascular supply. During fetal development, the anus undergoes relative descent in position relative to the , influenced by the expansion of the lower limbs and pelvic structures, which repositions the anal opening caudally for its final adult orientation by the end of the first trimester. Genetic and hormonal factors, particularly the Sonic hedgehog (Shh) signaling pathway expressed in the endodermal epithelium, drive epithelial-mesenchymal interactions that promote maturation, inducing downstream targets like bone morphogenetic protein 4 (Bmp4) and to facilitate epithelial transitions from columnar to stratified squamous types in the lower canal. Additional genes such as Hox clusters and Cdx2 further refine morphology and epithelial differentiation during this phase.

Clinical aspects

Common disorders

Hemorrhoids, also known as piles, are swollen and inflamed veins in the and lower that affect approximately 4.4% of the population annually . They are classified into two main types: internal hemorrhoids, which develop inside the and are typically painless but prone to , and external hemorrhoids, which form under the skin around the and often cause significant , itching, and swelling. Common causes include straining during bowel movements, chronic or , due to increased pressure on pelvic veins, , and prolonged sitting. Symptoms may include painless during , anal itching or irritation, a lump near the that may , and in severe cases, leading to acute and a hard, discolored lump. Anal fissures are small, linear tears in the lining of the , often resulting from trauma caused by the passage of hard or large stools. They are frequently associated with , as the hardened stool stretches the anal mucosa beyond its capacity, leading to the . Other contributing factors include chronic , anal intercourse, and underlying conditions like that promote . Symptoms typically involve sharp pain during and immediately after bowel movements, which can persist for hours due to sphincter , along with bright red blood on the stool or from minor bleeding at the site. Most acute fissures heal within weeks with conservative management, but chronic cases may require medical intervention to prevent recurrence. Perianal abscesses arise from bacterial , usually originating in the anal glands when they become blocked and infected, leading to a collection of in the soft tissues around the anus. These are more common in people with , , or compromised immune systems, and they form a painful, swollen lump that may cause fever, chills, and difficulty sitting. If untreated, an can rupture spontaneously or require drainage, often progressing to an —a chronic abnormal tract connecting the to the skin surface. Fistulas result from persistent where the abscess cavity fails to heal fully, allowing ongoing drainage of , recurrent pain, swelling, and irritation around the anus, with symptoms including foul-smelling discharge and recurrent abscess formation. Fecal incontinence, the involuntary loss of bowel control, affects the anus through dysfunction of the muscles or surrounding nerves, leading to leakage of stool or gas. Key etiologies include damage to the anal from vaginal , particularly with forceps delivery or , which can weaken the muscle ring; surgical interventions such as hemorrhoidectomy or repair that inadvertently injure the ; and neurologic disorders like or that impair sensory and motor signals to the . Other causes encompass chronic straining from , which stretches the rectal walls, and inflammatory conditions like that erode integrity. Symptoms range from occasional soiling to complete loss of control, often accompanied by urgency and social embarrassment, with increasing with age and affecting up to 15% of adults over 70. Anal , or anal stricture, is a narrowing of the that can impede the passage of stool. It is often caused by scarring from previous surgeries (such as hemorrhoidectomy), , , or trauma. Symptoms include , painful bowel movements, narrow or pellet-like stools, and sometimes . This condition is relatively rare but can significantly affect quality of life if severe. Anal cancer is a rare of the anus, with an incidence of about 2.0 per 100,000 people in the United States as of 2024. It is strongly associated with human papillomavirus (HPV) infection, particularly high-risk types, as well as , , and receptive anal intercourse. Common symptoms include , anal pain, itching, a lump or mass near the anus, and changes in bowel habits. Early detection through screening and HPV can prevent many cases.

Hygiene and care

Maintaining proper hygiene of the anal area is essential to prevent , infections, and conditions such as or fissures. After bowel movements, gentle cleaning is recommended using soft, unscented or, preferably, water via a or handheld shower to minimize friction and remove residue without harsh scrubbing. Avoid soaps, fragrances, or wipes containing chemicals, as they can disrupt the skin's natural barrier and cause ; instead, pat the area dry with a soft or use a on low heat. Daily sitz baths in plain warm water for 10-15 minutes can further soothe the perianal skin and promote , particularly for those prone to . Diet plays a key role in anal health by influencing stool consistency and reducing straining during . A high-fiber diet, including fruits, , whole grains, and (aiming for 25-30 grams daily), softens stools and prevents , thereby lowering the risk of anal trauma from hard stools. Adequate hydration, with at least 8 glasses of per day, complements intake by keeping stools moist and facilitating easier passage, which supports overall perianal integrity. Regular screening helps detect potential anal issues early, allowing for timely intervention. Digital rectal examinations (DRE), performed by a healthcare provider during routine check-ups, can identify abnormalities like masses or in the . screening, which includes visualization of the and anus, is recommended starting at age 45 for average-risk adults, with every 10 years. For individuals at higher risk, such as those with a family history of , screening should begin earlier, typically 10 years before the age at which the youngest affected family member was diagnosed. Post-surgical care following anal procedures, such as hemorrhoidectomy or repair, emphasizes gentle and dietary adjustments to aid recovery. Patients should continue high-fiber intake and hydration to avoid , while using sitz baths multiple times daily to clean and reduce swelling in the surgical site. In the , after , perineal care involves applying cold packs to the anal area for 10-20 minutes several times a day in the first 72 hours to minimize swelling and , alongside avoiding through fiber-rich foods and fluids. Follow-up visits at 6-12 weeks postpartum are advised to assess healing, especially if anal injury occurred.

References

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