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Rectum and Anal Canal: Anatomy

The rectum and anal canal are the most terminal parts of the lower GI tract/ large intestine Large intestine The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy that form a functional unit and control defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility. Fecal incontinence Fecal incontinence Failure of voluntary control of the anal sphincters, with involuntary passage of feces and flatus. Pediatric Constipation can occur if this function is disturbed. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis Puborectalis Vagina, Vulva, and Pelvic Floor: Anatomy muscle, and internal and external anal sphincters. Peristaltic waves within the rectal muscularis, involuntary relaxation of the internal anal sphincter (controlled by the ANS ANS The ans is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ans consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System: Anatomy), and voluntary relaxation of the external anal sphincter (controlled by the cerebral cortex Cerebral cortex The cerebral cortex is the largest and most developed part of the human brain and CNS. Occupying the upper part of the cranial cavity, the cerebral cortex has 4 lobes and is divided into 2 hemispheres that are joined centrally by the corpus callosum. Cerebral Cortex: Anatomy) are essential for defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility to occur. The rich plexus of veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology surrounding the anal canal can develop into hemorrhoids Hemorrhoids Hemorrhoids are normal vascular cushions in the anal canal composed of dilated vascular tissue, smooth muscle, and connective tissue. They do not cause issues unless they are enlarged, inflamed, thrombosed, or prolapsed. Patients often present with rectal bleeding of bright red blood, or they may have pain, perianal pruritus, or a palpable mass. Hemorrhoids if dilated.

Last updated: Sep 1, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Development

Gross anatomy

Gross anatomy of the rectum and anal canal

Gross anatomy of the rectum and anal canal

Image by Lecturio.

Rectum

  • A straight, tubular structure in the pelvic cavity
  • Located between the sigmoid colon Sigmoid colon A segment of the colon between the rectum and the descending colon. Colon, Cecum, and Appendix: Anatomy and anal canal
  • Length: approximately 15 cm
  • Relation of the rectum with the peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy:
    • Upper 3rd: covered anteriorly and laterally by the peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy
    • Middle 3rd: covered anteriorly (only) by the peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy
    • Lower 3rd: not covered by the peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy (completely below the pelvic cavity)
  • Rectosigmoid junction:
    • Point between the sigmoid colon Sigmoid colon A segment of the colon between the rectum and the descending colon. Colon, Cecum, and Appendix: Anatomy and rectum
    • Defined by loss of taenia Taenia Taenia belong to the Cestoda class of helminths. Humans are infected with these tapeworms by eating undercooked beef (T. saginata) or pork (T. solium and T. asiatica). Taeniasis is often asymptomatic, but the ingestion of larvae can cause abdominal discomfort, nausea, and constipation or diarrhea. Taenia/Taeniasis coli of the colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy
  • The rectum has 2 flexures:
    1. Sacral flexure: a dorsal concave bend as the rectum courses anterior to the sacrum Sacrum Five fused vertebrae forming a triangle-shaped structure at the back of the pelvis. It articulates superiorly with the lumbar vertebrae, inferiorly with the coccyx, and anteriorly with the ilium of the pelvis. The sacrum strengthens and stabilizes the pelvis. Vertebral Column: Anatomy
    2. Anorectal flexure: 
      • A ventral convex bend representing the transition between the rectum above and the anal canal below
      • Formed by the puborectalis Puborectalis Vagina, Vulva, and Pelvic Floor: Anatomy muscle (part of the levator ani complex) wrapping like a sling from the pubic bones anteriorly around the posterior anorectum
  • Rectal folds (valves of Houston):
    • 3 transverse submucosal folds of tissue protruding into the lumen:
      1. Superior
      2. Intermediate
      3. Inferior
    • To create “shelves” that help hold feces
    • Help separate solid feces from gas
  • Rectal ampulla:
    • Terminal portion of the rectum
    • Present on top of the pelvic diaphragm Pelvic diaphragm Soft tissue formed mainly by the pelvic diaphragm, which is composed of the two levator ani and two coccygeus muscles. The pelvic diaphragm lies just below the pelvic aperture (outlet) and separates the pelvic cavity from the perineum. It extends between the pubic bone anteriorly and the coccyx posteriorly. Vagina, Vulva, and Pelvic Floor: Anatomy
    • Capable of expanding to serve as a reservoir Reservoir Animate or inanimate sources which normally harbor disease-causing organisms and thus serve as potential sources of disease outbreaks. Reservoirs are distinguished from vectors (disease vectors) and carriers, which are agents of disease transmission rather than continuing sources of potential disease outbreaks. Humans may serve both as disease reservoirs and carriers. Escherichia coli for feces

Anal canal

  • A straight tubular structure at the end of the GI tract
  • Connects the rectum to the anus (external opening)
  • Length: approximately 3‒4 cm
  • Completely extraperitoneal (known as “subperitoneal”)
  • Anal columns (also called columns of Morgagni):
    • Approximately 6‒10 vertical folds of the mucosa, submucosa, and circular muscle layer
    • Found in the upper half of the lumen of the anal canal
  • Anal sinuses (rectal sinuses):
    • Furrows in the anal canal that separate the anal columns from each other 
    • End in small valve-like folds (anal valves)
  • Anal valves:
    • Small valve-like folds at the lower ends of the anal sinuses in the rectum
    • Join together at the lower ends of the anal columns
    • Valves and sinuses form the pectinate line.
  • Pectinate line (also known as the dentate line):
    • Divides the anal canal into upper ⅔ and lower ⅓
    • Multiple neurovascular and histologic differences above versus below the line
  • The anal canal has 2 anal sphincters:
    • Internal anal sphincter (IAS):
      • Under involuntary control
      • Surrounds the upper ⅔ of the anal canal
      • Formed from a thickening of the circular smooth muscle in the bowel wall
    • EAS:
      • Under voluntary control
      • Surrounds the lower ⅔ of the anal canal
      • Consists of several parts: subcutaneous, superficial, and deep

Anatomic relations

The rectum is the most posterior visceral organ in the pelvic cavity.

  • Anterior to the rectum and anal canal are:
    • In males:
      • Bladder Bladder A musculomembranous sac along the urinary tract. Urine flows from the kidneys into the bladder via the ureters, and is held there until urination. Pyelonephritis and Perinephric Abscess
      • Seminal vesicles Seminal vesicles A saclike, glandular diverticulum on each ductus deferens in male vertebrates. It is united with the excretory duct and serves for temporary storage of semen.
      • Prostate Prostate The prostate is a gland in the male reproductive system. The gland surrounds the bladder neck and a portion of the urethra. The prostate is an exocrine gland that produces a weakly acidic secretion, which accounts for roughly 20% of the seminal fluid. gland
    • In females:
      • Uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Uterus, Cervix, and Fallopian Tubes: Anatomy
      • Cervix Cervix The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Externally, the cervix is lined by stratified squamous cells; however, the cervical canal is lined by columnar epithelium. Uterus, Cervix, and Fallopian Tubes: Anatomy
      • Vagina Vagina The vagina is the female genital canal, extending from the vulva externally to the cervix uteri internally. The structures have sexual, reproductive, and urinary functions and a rich blood supply, mainly arising from the internal iliac artery. Vagina, Vulva, and Pelvic Floor: Anatomy
  • Posterior to the rectum and anal canal:
    • Lower sacrum Sacrum Five fused vertebrae forming a triangle-shaped structure at the back of the pelvis. It articulates superiorly with the lumbar vertebrae, inferiorly with the coccyx, and anteriorly with the ilium of the pelvis. The sacrum strengthens and stabilizes the pelvis. Vertebral Column: Anatomy: S3 S3 Heart Sounds‒S5
    • Coccyx Coccyx The last bone in the vertebral column in tailless primates considered to be a vestigial tail-bone consisting of three to five fused vertebrae. Vertebral Column: Anatomy (joined to the rectum by the anococcygeal ligament)

Microscopic Anatomy

Similar to other segments of the GI tract, the layers of the anorectal wall (from the inner lumen outward) are mucosa → submucosa → muscular layer → serosa. There are no villi or circular folds Circular folds Small Intestine: Anatomy in the anorectal wall like those in the small intestine Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: Anatomy.

Mucosa

  • Consists of 3 sublayers:
    1. Epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology (innermost lining):
      • Above the pectinate line: columnar epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology
      • Below the pectinate line: nonkeratinized, stratified squamous epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology
    2. Lamina propria Lamina propria Whipple’s Disease (contains small vasculature)
    3. Muscularis mucosa (thin layer of smooth muscle)
  • Intestinal crypts Intestinal crypts Colon, Cecum, and Appendix: Anatomy of Liberkühn/anal mucous glands:
    • Pores that open into tubular glands
    • Similar to crypts in the small intestines, but deeper
    • Contain several mucus-secreting goblet cells Goblet cells A glandular epithelial cell or a unicellular gland. Goblet cells secrete mucus. They are scattered in the epithelial linings of many organs, especially the small intestine and the respiratory tract. Glandular Epithelium: Histology that help lubricate feces and ease the passage of stool
Histological image of the rectal mucosa (longitudinal section)

Histological image of the rectal mucosa (longitudinal section):
Rectal crypts with simple columnar epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium: Histology are visible and the stroma ( lamina propria Lamina propria Whipple’s Disease) is seen wrapping around the crypts.

Image: “Histological image of the rectal mucosa (longitudinal section)” by Y. Gao and D. F. Katz. License: CC BY 4.0

Submucosa

  • Loose connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology
  • Contains larger vessels

Muscular layer

Made up of 2 layers of smooth muscle:

  • Circular layer (inner layer)
  • Longitudinal layer (outer layer): Taeniae coli of the colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy coalesce to form a continuous outer layer.
  • Myenteric (Auerbach’s) plexus:
    • Ganglia of the ANS ANS The ans is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ans consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System: Anatomy that control the muscular layer
    • Located between the 2 layers of smooth muscle

Serosa

  • Made up of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology
  • Joins with the peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy anteriorly in the upper regions of the rectum
  • Becomes a thicker fibrous Fibrous Fibrocystic Change layer in the subperitoneal regions

Neurovasculature

Arterial blood supply

  • Above the pectinate line (including the rectum and upper anal canal): superior rectal artery (branch of the inferior mesenteric artery Inferior mesenteric artery The artery supplying nearly all the left half of the transverse colon, the whole of the descending colon, the sigmoid colon, and the greater part of the rectum. It is smaller than the superior mesenteric artery and arises from the aorta above its bifurcation into the common iliac arteries. Small Intestine: Anatomy)
  • Below the pectinate line:
    • Middle rectal artery (branch of the internal iliac artery)
    • Inferior rectal artery (branch of the internal pudendal artery)

Venous drainage

  • Portosystemic anastomosis exists around the pectinate line.
  • Above the pectinate line:
    • Drains into the portal system
    • Internal hemorrhoidal plexus → superior rectal vein → inferior mesenteric vein ( IMV IMV Invasive mechanical ventilation (IMV) is an advanced airway modality used for individuals with immediate or impending respiratory failure and/or in preparation for surgery. The IMV technique involves positive pressure ventilation delivered to the lungs through an endotracheal tube via a ventilator. Invasive Mechanical Ventilation)
  • Below the pectinate line: 
    • Drains into the systemic veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology ( inferior vena cava Inferior vena cava The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels: Anatomy)
    • External hemorrhoidal plexus → middle and inferior rectal veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology → internal pudendal vein → internal iliac vein

Lymphatic drainage

Innervation

  • Above the pectinate line:
    • Autonomic/visceral innervation via the inferior hypogastric plexus (contains both sympathetic and parasympathetic fibers)
    • Parasympathetic fibers relax the IAS.
    • Sympathetic fibers maintain tonic contraction of the IAS.
    • Under involuntary control
  • Below the pectinate line: somatic innervation
    • Inferior rectal branch of the pudendal nerve Pudendal nerve A nerve which originates in the sacral spinal cord (s2 to s4) and innervates the perineum, the external genitalia, the external anal sphincter and the external urethral sphincter. It has three major branches: the perineal nerve, inferior anal nerves, and the dorsal nerve of penis or clitoris. Gluteal Region: Anatomy
    • Under voluntary control
Differences in anal neurovasculature above and below the pectinate line

Differences in anal neurovasculature above and below the pectinate line:
IMA: inferior mesenteric artery Inferior mesenteric artery The artery supplying nearly all the left half of the transverse colon, the whole of the descending colon, the sigmoid colon, and the greater part of the rectum. It is smaller than the superior mesenteric artery and arises from the aorta above its bifurcation into the common iliac arteries. Small Intestine: Anatomy
IMV IMV Invasive mechanical ventilation (IMV) is an advanced airway modality used for individuals with immediate or impending respiratory failure and/or in preparation for surgery. The IMV technique involves positive pressure ventilation delivered to the lungs through an endotracheal tube via a ventilator. Invasive Mechanical Ventilation: inferior mesenteric vein
LN: lymph Lymph The interstitial fluid that is in the lymphatic system. Secondary Lymphatic Organs node
IVC IVC The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels: Anatomy: inferior vena cava Inferior vena cava The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels: Anatomy

Image by Lecturio.

Functions

General functions

The primary roles of the rectum and anal canal involve controlled defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility.

  • The rectum is able to store feces until defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility is consciously desired.
  • Anal sphincters are in a state of tonic contraction, preventing fecal expulsion.
    • Internal sphincter: smooth muscle under involuntary control
    • External sphincter: skeletal muscle under voluntary control
  • Defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility requires:
    • Peristalsis Peristalsis A movement, caused by sequential muscle contraction, that pushes the contents of the intestines or other tubular organs in one direction. Gastrointestinal Motility of rectal muscles
    • Involuntary relaxation of the IAS
    • Voluntary relaxation of the EAS

Defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility reflex

The rectum contains stretch receptors Stretch receptors Stretch receptors found in the bronchi and bronchioles. Pulmonary stretch receptors are sensors for a reflex which stops inspiration. In humans, the reflex is protective and is probably not activated during normal respiration. Respiratory Regulation that stimulate the defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility reflex when the rectum begins to fill with feces.

  • Stretch receptors Stretch receptors Stretch receptors found in the bronchi and bronchioles. Pulmonary stretch receptors are sensors for a reflex which stops inspiration. In humans, the reflex is protective and is probably not activated during normal respiration. Respiratory Regulation activated in rectum → 
  • Sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology nerve fibers Nerve Fibers Slender processes of neurons, including the axons and their glial envelopes (myelin sheath). Nerve fibers conduct nerve impulses to and from the central nervous system. Nervous System: Histology carry signals to the sacral spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord: Anatomy → 
  • Synapses with parasympathetic motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology fibers → 
  • Sends signals for peristaltic waves to the myenteric nerve plexus in the muscular layers of:
    • Descending colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy
    • Sigmoid colon Sigmoid colon A segment of the colon between the rectum and the descending colon. Colon, Cecum, and Appendix: Anatomy
    • Rectum
    • IAS
  • Peristaltic contractions in the colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: Anatomy and rectum move feces downward.
  • IAS relaxes.
  • Defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility occurs only if the EAS is voluntarily relaxed at the same time due to motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology impulses from the cerebral cortex Cerebral cortex The cerebral cortex is the largest and most developed part of the human brain and CNS. Occupying the upper part of the cranial cavity, the cerebral cortex has 4 lobes and is divided into 2 hemispheres that are joined centrally by the corpus callosum. Cerebral Cortex: Anatomy.
  • If defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility is consciously suppressed, peristaltic contractions cease within a few minutes.
The defecation reflex

Defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility reflex:
1. Feces stretch the rectum and stimulate the stretch receptors Stretch receptors Stretch receptors found in the bronchi and bronchioles. Pulmonary stretch receptors are sensors for a reflex which stops inspiration. In humans, the reflex is protective and is probably not activated during normal respiration. Respiratory Regulation, transmitting the signal to the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord: Anatomy.
2. A spinal reflex sends parasympathetic motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology signals to the myenteric nerve plexus, resulting in contraction of the smooth muscles Smooth muscles Unstriated and unstriped muscle, one of the muscles of the internal organs, blood vessels, hair follicles, etc. Contractile elements are elongated, usually spindle-shaped cells with centrally located nuclei. Smooth muscle fibers are bound together into sheets or bundles by reticular fibers and frequently elastic nets are also abundant. Muscle Tissue: Histology within the rectum, pushing feces downward.
3. The same spinal reflex also sends parasympathetic motor Motor Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology signals to relax the internal anal sphincter.
4. Voluntary impulses from the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification prevent defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility by keeping the external anal sphincter contracted. Defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility will occur if voluntary signals allow the external anal sphincter to relax.

Image by Lecturio.

Clinical Relevance

  • Fecal incontinence Fecal incontinence Failure of voluntary control of the anal sphincters, with involuntary passage of feces and flatus. Pediatric Constipation: involuntary leakage of solid or liquid stool. There are 2 major types of fecal incontinence Fecal incontinence Failure of voluntary control of the anal sphincters, with involuntary passage of feces and flatus. Pediatric Constipation: urge incontinence Urge incontinence Involuntary discharge of urine that is associated with an abrupt and strong desire to void. It is usually related to the involuntary contractions of the detrusor muscle of the bladder (detrusor hyperreflexia or detrusor instability). Urinary Incontinence (the desire to defecate is felt, but stool cannot be voluntarily retained) and passive incontinence (there is no desire to defecate before stool is involuntarily passed). Fecal incontinence Fecal incontinence Failure of voluntary control of the anal sphincters, with involuntary passage of feces and flatus. Pediatric Constipation can result from dysfunction of anal sphincters, abnormal rectal compliance Compliance Distensibility measure of a chamber such as the lungs (lung compliance) or bladder. Compliance is expressed as a change in volume per unit change in pressure. Veins: Histology, decreased rectal sensation, altered stool consistency Consistency Dermatologic Examination, or frequently, a combination of these factors.
  • Hemorrhoids Hemorrhoids Hemorrhoids are normal vascular cushions in the anal canal composed of dilated vascular tissue, smooth muscle, and connective tissue. They do not cause issues unless they are enlarged, inflamed, thrombosed, or prolapsed. Patients often present with rectal bleeding of bright red blood, or they may have pain, perianal pruritus, or a palpable mass. Hemorrhoids: swollen veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology in the hemorrhoidal plexuses surrounding the anus and lower rectum that are similar to varicose veins Varicose veins Enlarged and tortuous veins. Chronic Venous Insufficiency. Hemorrhoids Hemorrhoids Hemorrhoids are normal vascular cushions in the anal canal composed of dilated vascular tissue, smooth muscle, and connective tissue. They do not cause issues unless they are enlarged, inflamed, thrombosed, or prolapsed. Patients often present with rectal bleeding of bright red blood, or they may have pain, perianal pruritus, or a palpable mass. Hemorrhoids can develop inside the rectum (internal hemorrhoids Hemorrhoids Hemorrhoids are normal vascular cushions in the anal canal composed of dilated vascular tissue, smooth muscle, and connective tissue. They do not cause issues unless they are enlarged, inflamed, thrombosed, or prolapsed. Patients often present with rectal bleeding of bright red blood, or they may have pain, perianal pruritus, or a palpable mass. Hemorrhoids) or under the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions around the anus (external hemorrhoids Hemorrhoids Hemorrhoids are normal vascular cushions in the anal canal composed of dilated vascular tissue, smooth muscle, and connective tissue. They do not cause issues unless they are enlarged, inflamed, thrombosed, or prolapsed. Patients often present with rectal bleeding of bright red blood, or they may have pain, perianal pruritus, or a palpable mass. Hemorrhoids). The most important clinical manifestations include hematochezia Hematochezia Gastrointestinal Bleeding, pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways associated with a thrombosed hemorrhoid Thrombosed hemorrhoid Hemorrhoids, and perianal pruritus Pruritus An intense itching sensation that produces the urge to rub or scratch the skin to obtain relief. Atopic Dermatitis (Eczema). Risk factors include advancing age, diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea, pregnancy Pregnancy The status during which female mammals carry their developing young (embryos or fetuses) in utero before birth, beginning from fertilization to birth. Pregnancy: Diagnosis, Physiology, and Care, pelvic tumors, prolonged sitting, straining, and chronic constipation Constipation Constipation is common and may be due to a variety of causes. Constipation is generally defined as bowel movement frequency < 3 times per week. Patients who are constipated often strain to pass hard stools. The condition is classified as primary (also known as idiopathic or functional constipation) or secondary, and as acute or chronic. Constipation.
  • Perianal and perirectal abscesses: pus collection in the enclosed space near the perirectal tissues. Perianal and perirectal abscesses typically originate due to obstruction of the anal crypt glands. Affected individuals present with severe pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways in the anal or rectal area and a fluctuant Fluctuant Dermatologic Examination mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast on physical exam. Management requires prompt surgical incision and drainage Incision And Drainage Chalazion, which may be followed by a course of antibiotics. If untreated, abscesses can lead to the formation of fistulas.
  • Anal fistula Anal fistula Anal fistulas are abnormal communications between the anorectal lumen and another body structure, often to the skin. Anal fistulas often occur due to extension of anal abscesses but are also associated with specific diseases such as Crohn’s disease. Anal Fistula: abnormal communications between the anorectal lumen and the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions or another structure in the body. An anal fistula Anal fistula Anal fistulas are abnormal communications between the anorectal lumen and another body structure, often to the skin. Anal fistulas often occur due to extension of anal abscesses but are also associated with specific diseases such as Crohn’s disease. Anal Fistula often occurs due to the extension Extension Examination of the Upper Limbs of anal abscesses but is also associated with specific conditions such as Crohn disease. Symptoms include pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways or irritation around the anus; abnormal discharge or purulent drainage; and swelling Swelling Inflammation, redness Redness Inflammation, or fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, if an abscess Abscess Accumulation of purulent material in tissues, organs, or circumscribed spaces, usually associated with signs of infection. Chronic Granulomatous Disease is present.
  • Anal fissures Anal fissures A painful linear tear at the margin of the anus. It appears as a crack or slit in the mucous membrane of the anus and is very painful and difficult to heal. Constipation: painful, superficial tears in the epithelial lining (anoderm) of the anal canal that cause anal pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways and bleeding. Anal fissures Anal fissures A painful linear tear at the margin of the anus. It appears as a crack or slit in the mucous membrane of the anus and is very painful and difficult to heal. Constipation are common in infants and middle-aged individuals and are typically secondary to local trauma or irritation that results from constipation Constipation Constipation is common and may be due to a variety of causes. Constipation is generally defined as bowel movement frequency < 3 times per week. Patients who are constipated often strain to pass hard stools. The condition is classified as primary (also known as idiopathic or functional constipation) or secondary, and as acute or chronic. Constipation, diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea, or anal intercourse Anal intercourse Hemorrhoids. Clinical manifestations include severe pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways during bowel movements, bright-red blood in the stool, the presence of blood on toilet paper, a visible crack Crack The purified, alkaloidal, extra-potent form of cocaine. It is smoked (free-based), injected intravenously, and orally ingested. Use of crack results in alterations in function of the cardiovascular system, the autonomic nervous system, the central nervous system, and the gastrointestinal system. The slang term ‘crack’ was derived from the crackling sound made upon igniting of this form of cocaine for smoking. Cocaine Use Disorder in the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions around the anus, and a small lump or skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions tag on the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions near the anal fissure Fissure A crack or split that extends into the dermis Generalized and Localized Rashes.
  • Rectal prolapse Rectal prolapse Rectal prolapse, also known as rectal procidentia, is the protrusion of rectal tissue through the anus. The tissue may include just the mucosa or the full thickness of the rectal wall. Common risk factors include chronic straining, constipation, bowel motility disorders, and weakening of the pelvic floor muscles. Rectal Prolapse: a condition in which the rectum protrudes from the anal orifice either spontaneously or after defecation Defecation The normal process of elimination of fecal material from the rectum. Gastrointestinal Motility. Rectal prolapse Rectal prolapse Rectal prolapse, also known as rectal procidentia, is the protrusion of rectal tissue through the anus. The tissue may include just the mucosa or the full thickness of the rectal wall. Common risk factors include chronic straining, constipation, bowel motility disorders, and weakening of the pelvic floor muscles. Rectal Prolapse may involve all layers of the rectal wall or only the mucosa. Damage to the pelvic floor Pelvic floor Soft tissue formed mainly by the pelvic diaphragm, which is composed of the two levator ani and two coccygeus muscles. The pelvic diaphragm lies just below the pelvic aperture (outlet) and separates the pelvic cavity from the perineum. It extends between the pubic bone anteriorly and the coccyx posteriorly. Vagina, Vulva, and Pelvic Floor: Anatomy and loss of internal rectal support are some of the causes of rectal prolapse Rectal prolapse Rectal prolapse, also known as rectal procidentia, is the protrusion of rectal tissue through the anus. The tissue may include just the mucosa or the full thickness of the rectal wall. Common risk factors include chronic straining, constipation, bowel motility disorders, and weakening of the pelvic floor muscles. Rectal Prolapse. Symptoms of rectal prolapse Rectal prolapse Rectal prolapse, also known as rectal procidentia, is the protrusion of rectal tissue through the anus. The tissue may include just the mucosa or the full thickness of the rectal wall. Common risk factors include chronic straining, constipation, bowel motility disorders, and weakening of the pelvic floor muscles. Rectal Prolapse include the development of an anal mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast, or abdominal pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways, incomplete evacuation, altered bowel habits, and mucus/stool discharge. Rectal prolapse Rectal prolapse Rectal prolapse, also known as rectal procidentia, is the protrusion of rectal tissue through the anus. The tissue may include just the mucosa or the full thickness of the rectal wall. Common risk factors include chronic straining, constipation, bowel motility disorders, and weakening of the pelvic floor muscles. Rectal Prolapse is most common in elderly women.
  • Colorectal cancer Colorectal cancer Colorectal cancer (CRC) is the 2nd leading cause of cancer-related deaths in the United States. Colorectal cancer is a heterogeneous disease that arises from genetic and epigenetic abnormalities, with influence from environmental factors. Colorectal Cancer (CRC): almost all cases of CRC are adenocarcinoma and the majority of lesions arise from the malignant transformation Transformation Change brought about to an organism’s genetic composition by unidirectional transfer (transfection; transduction, genetic; conjugation, genetic, etc.) and incorporation of foreign DNA into prokaryotic or eukaryotic cells by recombination of part or all of that DNA into the cell’s genome. Bacteriology of an adenomatous polyp Adenomatous polyp Benign neoplasms derived from glandular epithelium. Colon Polyps. The most important clinical manifestations of CRC include changes in bowel habits, a rectal mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast that is palpable on digital exam, iron Iron A metallic element with atomic symbol fe, atomic number 26, and atomic weight 55. 85. It is an essential constituent of hemoglobins; cytochromes; and iron-binding proteins. It plays a role in cellular redox reactions and in the transport of oxygen. Trace Elements deficiency anemia Anemia Anemia is a condition in which individuals have low Hb levels, which can arise from various causes. Anemia is accompanied by a reduced number of RBCs and may manifest with fatigue, shortness of breath, pallor, and weakness. Subtypes are classified by the size of RBCs, chronicity, and etiology. Anemia: Overview and Types, and rectal bleeding; however, most individuals are asymptomatic. A rectal exam, colonoscopy Colonoscopy Endoscopic examination, therapy or surgery of the luminal surface of the colon. Colorectal Cancer Screening, and/or stool tests for screening Screening Preoperative Care are generally recommended in individuals ≥ 50 years of age.

References

  1. Nakashima, J., Zulfiqar, H. (2021). Embryology, rectum, and anal Canal. StatPearls. Retrieved August 22, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK551682/
  2. Kapoor, V.K. (2016). Anal Canal Anatomy. https://emedicine.medscape.com/article/1990236-overview
  3. Drake, R., Vogl, A.W., Mitchell, A.W.M. (2020). Abdomen, regional anatomy. In Drake, R., et al. (Ed.), Gray’s Anatomy for Students (4th ed., pp.322). Churchill Livingstone/Elsevier.
  4. Bleday, R., Breen, E. (2021). Hemorrhoids: Clinical manifestations and diagnosis. UpToDate. Retrieved August 22, 2021, from https://www.uptodate.com/contents/hemorrhoids-clinical-manifestations-and-diagnosis
  5. Stewart, D.B. (2021). Anal fissure: Clinical manifestations, diagnosis, prevention. UpToDate. Retrieved August 22, 2021, from https://www.uptodate.com/contents/anal-fissure-clinical-manifestations-diagnosis-prevention
  6. Wang, Y.H., Wiseman, J. (2021). Anatomy, abdomen and pelvis, rectum. StatPearls. Retrieved August 24, 2021, from https://www.statpearls.com/articlelibrary/viewarticle/29712/
  7. Ahmed, A., Qureshi, W. (2021). Anatomy, abdomen and pelvis, anal canal. StatPearls. Retrieved August 24, 2021, from https://www.statpearls.com/articlelibrary/viewarticle/26783/

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