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    Related Topics

    From Digestive System

    External Anal Sphincter
    Voluntary muscle around anus.
    Sublingual Glands
    Salivary glands beneath the tongue.
    Gingiva
    Gums; soft tissue covering the bones of the jaw.
    Stomach
    Muscular sac that begins digestion of protein.
    Duodenal Bulb
    Initial section of duodenum closest to the stomach.
    Lingual Frenulum
    Fold of mucous membrane anchoring the tongue to the floor of the mouth.
    Palatine Tonsils
    Lymphatic tissues on either side of the oropharynx.
    Mesentery
    Fold of peritoneum anchoring intestines.
    Fundus
    Upper curved portion of the stomach.
    Round Ligament of Liver
    Remnant of fetal umbilical vein.
    Peritoneum
    Serous membrane lining the abdominal cavity.
    Uvula
    Dangling soft tissue at the back of the soft palate.
    Internal Anal Sphincter
    Involuntary muscle around anal canal.
    Rectum
    Straight section of the colon leading to anus.
    Tongue
    Muscular organ aiding in taste, speech, and food manipulation.
    Transverse Colon
    Horizontal part of the colon.
    Pyloric Sphincter
    Regulates passage of chyme from stomach to duodenum.
    Pancreas
    Gland with both endocrine and exocrine functions.
    Lesser Omentum
    Connects stomach and liver.
    Laryngopharynx
    Lower part of pharynx leading to esophagus.
    Vermiform Appendix
    Worm-like appendage of the cecum.
    Splenic Flexure
    Bend between transverse and descending colon.
    Common Hepatic Duct
    Carries bile from liver to bile duct.
    Gallbladder
    Stores and concentrates bile.
    Cystic Duct
    Connects gallbladder to common bile duct.

    Anal Canal

    Reviewed by our medical team

    Terminal part of the large intestine.

    Overview

    The anal canal is the terminal segment of the gastrointestinal tract, responsible for the controlled expulsion of feces from the body. It connects the rectum to the external environment and is surrounded by muscular structures that regulate continence. Despite its short length, the anal canal has a complex anatomical and functional design involving specialized mucosa, vascular structures, and both voluntary and involuntary muscles.

    Location

    The anal canal is located in the perineum, specifically within the anal triangle. It begins at the level of the pelvic floor (levator ani muscles) and ends at the anus, the external opening.

    Key anatomical landmarks include:

    • Superiorly: Continuous with the rectum at the anorectal junction (approximately at the level of the puborectalis sling)

    • Inferiorly: Opens externally as the anus

    • Anteriorly: In males, related to the urethra and prostate; in females, related to the vagina

    • Posteriorly: Related to the coccyx and sacrum

    Structure

    The anal canal is approximately 3–5 cm long and is divided into three zones based on epithelial lining and vascular drainage:

    • Upper third: Lined by columnar epithelium, derived from endoderm, containing anal columns and crypts

    • Middle third (transitional zone): Transitional epithelium (columnar to stratified squamous)

    • Lower third: Non-keratinized or keratinized stratified squamous epithelium, similar to skin

    Key anatomical features:

    • Pectinate (dentate) line: Marks the junction between endodermal and ectodermal regions, important for neurovascular and lymphatic differences

    • Internal anal sphincter: Involuntary smooth muscle, a continuation of the rectal muscularis externa

    • External anal sphincter: Voluntary skeletal muscle, part of the pelvic floor

    • Anal columns (of Morgagni): Longitudinal mucosal folds in the upper canal

    • Anal valves and sinuses: Located at the base of anal columns; can be sites of infection or abscess formation

    Function

    The anal canal serves critical functions in:

    • Defecation: Passage and voluntary expulsion of fecal material

    • Continence: Maintains closure of the anal orifice at rest through tonic contraction of the internal anal sphincter and voluntary control of the external sphincter

    Defecation involves coordinated relaxation of both sphincters, abdominal contraction, and straightening of the anorectal angle via the puborectalis muscle.

    Physiological Role(s)

    The anal canal plays several integrated physiological roles:

    • Sensory function: Highly innervated area, especially below the pectinate line, allowing discrimination between gas, liquid, and solid content

    • Venous drainage: Forms part of the portosystemic anastomosis; upper canal drains to the portal system, while lower canal drains to systemic circulation

    • Lymphatic drainage: Above the pectinate line drains to internal iliac nodes; below drains to superficial inguinal nodes

    • Immunological role: Mucosal surfaces and lymphoid tissue contribute to local defense mechanisms

    Clinical Significance

    Several common and significant conditions involve the anal canal:

    • Hemorrhoids: Swollen venous plexuses — internal (above pectinate line, usually painless) and external (below pectinate line, painful)

    • Anal fissures: Painful tears in the mucosa, often occurring posteriorly; associated with constipation and trauma

    • Abscesses and Fistulas: Infection of anal glands can lead to perianal abscesses and chronic fistulous tracts (fistula-in-ano)

    • Anal carcinoma: Squamous cell carcinoma below the pectinate line is linked to HPV infection; adenocarcinoma may occur above the line

    • Incontinence: May result from damage to sphincters or pelvic floor muscles (e.g., childbirth, spinal injury)

    • Congenital anomalies: Conditions such as imperforate anus and anorectal malformations require surgical correction

    Examination of the anal canal includes visual inspection, digital rectal examination, anoscopy, and imaging. Understanding its detailed anatomy is crucial for diagnosing and treating both common and complex colorectal disorders.

    Did you know? The liver produces bile, which helps break down fats in your digestive system.