Medical School · Year 1 · Anatomy Pelvis Head Neck · includes a quiz and discussion video
Lecture 6: Rectum and Anal Canal
Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the anatomy of the rectum including its course, relations, and peritoneal covering
- Describe the anatomy of the anal canal including its epithelial zones and sphincters
- Identify the arterial supply, venous drainage, and lymphatic drainage of the rectum and anal canal
- Explain the innervation and mechanisms of fecal continence
- Describe the anatomy of the ischioanal fossa
- Correlate anatomical features with clinical conditions
Overview of the Rectum and Anal Canal
The rectum and anal canal represent the terminal portions of the gastrointestinal tract, specialized for the storage and controlled elimination of feces. The rectum serves as a temporary reservoir, while the anal canal, under the control of both involuntary and voluntary sphincters, permits defecation at appropriate times.
The rectum begins at the rectosigmoid junction at the level of the third sacral vertebra, where the sigmoid mesocolon ends and the bowel becomes retroperitoneal. It extends approximately 12-15 centimeters to the anorectal junction, where the puborectalis muscle slings around the gut to create a sharp posterior angulation. The anal canal continues from this junction for approximately 3-4 centimeters to the anus, the external opening.
<image>Panel A: Sagittal section of pelvis showing rectosigmoid junction marked at S3 level where mesocolon ends. Panel B: Rectum following sacral curve and dilating as ampulla before anorectal junction. Panel C: Puborectalis sling creating anorectal angle where gut turns posteriorly with anal canal passing through pelvic floor. Panel D: Length measurements (rectum 12-15 cm, anal canal 3-4 cm) with adjacent bladder/prostate or uterus/vagina and sacrum relationships.</image>
The Rectum: Course and Flexures
The rectum follows a curved path dictated by the bony pelvis and pelvic floor musculature. In the sagittal plane, it traces the concavity of the sacrum before bending sharply posteriorly at the anorectal junction—this posterior bend is created by the puborectalis sling and constitutes the anorectal angle, crucial for fecal continence.
In the coronal plane, the rectum describes three lateral flexures as it descends, alternating to the right and left. The upper flexure is convex to the right, the middle (and most prominent) flexure is convex to the left, and the lower flexure is again convex to the right. These flexures correspond internally to the transverse rectal folds, also known as the valves of Houston.
The transverse rectal folds are semilunar shelves of mucosa projecting into the rectal lumen, containing extensions of the circular muscle layer. The most constant and clinically significant is the middle fold, located approximately 7-8 centimeters from the anal verge on the left anterior rectal wall—this serves as a useful landmark during rigid sigmoidoscopy. These folds support the fecal mass during rectal filling and must be negotiated when passing a sigmoidoscope.
Below the lowest transverse fold, the rectum dilates to form the rectal ampulla, the main reservoir for feces immediately proximal to the anorectal junction.
<image>Panel A: Lateral view showing sagittal flexure following sacral curve with sharp posterior bend at anorectal junction and puborectalis sling. Panel B: Anterior view demonstrating three lateral flexures alternating right-left-right. Panel C: Internal view of opened rectum showing three transverse rectal folds (valves of Houston) with middle fold most prominent at 7-8 cm from anal verge. Panel D: Rectal ampulla as dilation below lowest fold serving as fecal reservoir.</image>
Rectal Relations
The rectum maintains important relationships with surrounding pelvic structures that have surgical significance.
Posteriorly, the rectum lies against the sacrum and coccyx, separated by the presacral fascia that covers the middle sacral vessels, piriformis and coccygeus muscles, sacral plexus, and sympathetic trunk. This relatively avascular presacral plane is exploited during rectal mobilization, though injury to the presacral veins can cause troublesome bleeding.
Laterally, the rectum relates to the pararectal fossae (gutters of the peritoneal cavity), the lateral ligaments of the rectum (which contain the middle rectal vessels), and the ureters superiorly. The levator ani muscles embrace the lower rectum.
Anteriorly, the relationships differ by sex. In males, the upper rectum is related to the rectovesical pouch (containing loops of small intestine or sigmoid colon), the middle rectum to the posterior bladder base, seminal vesicles, and ampullae of the vasa deferentia, and the lower rectum to the prostate, separated by the rectovesical fascia (Denonvilliers' fascia). In females, the upper rectum relates to the rectouterine pouch (pouch of Douglas, the most dependent part of the peritoneal cavity), the uterus, and small intestine. The middle rectum lies behind the posterior vaginal fornix, and the lower rectum is separated from the vagina by the rectovaginal fascia.
<image>Panel A: Posterior rectal relations identical in both sexes showing sacrum, coccyx, presacral vessels, and musculature. Panel B: Male anterior relations with rectovesical pouch superiorly and bladder, seminal vesicles, prostate separated by Denonvilliers' fascia. Panel C: Female anterior relations with rectouterine pouch (pouch of Douglas) superiorly and uterus, posterior vaginal fornix, vagina separated by rectovaginal fascia. Panel D: Lateral ligaments containing middle rectal vessels indicated bilaterally in both sexes.</image>
Peritoneal Covering and Fasciae
The peritoneum covers the rectum in a decreasing fashion from proximal to distal. The upper third is covered anteriorly and laterally, with a short mesorectum attaching posteriorly. The middle third is covered only anteriorly, where the peritoneum reflects onto the bladder (forming the rectovesical pouch in males) or onto the uterus and vagina (forming the rectouterine pouch in females). The lower third lies entirely below the peritoneal reflection and is completely extraperitoneal.
The peritoneal reflection lies approximately 7-9 centimeters from the anal verge in males and 5-7 centimeters in females. This distinction has important oncological implications: tumors above the peritoneal reflection may spread intraperitoneally, while those below spread locally or to pelvic lymph nodes.
The mesorectum is the fatty envelope surrounding the rectum, containing the rectal lymph nodes, lymphatic vessels, and branches of the superior rectal vessels. It is enclosed by the mesorectal fascia (fascia propria), a thin but surgically identifiable layer that can be seen on preoperative MRI. Total mesorectal excision (TME), in which the entire mesorectum is removed intact within its fascial envelope, is the standard surgical approach for rectal cancer and has dramatically improved oncological outcomes.
The rectovesical or rectovaginal fascia (Denonvilliers' fascia) separates the rectum from anterior structures, providing an important surgical plane for anterior dissection during rectal surgery while protecting the bladder, prostate, seminal vesicles, or vagina from injury.
<image>Panel A: Sagittal section showing upper third rectum with translucent peritoneum covering anterior and lateral surfaces. Panel B: Middle third with peritoneum on anterior surface only reflecting onto bladder/uterus forming pouches. Panel C: Lower third below peritoneal reflection (7-9 cm in males, 5-7 cm in females) completely extraperitoneal. Panel D: Axial cross-section of mesorectum showing fatty envelope with vessels and lymph nodes surrounded by mesorectal fascia and Denonvilliers' fascia anteriorly.</image>
The Anal Canal: Structure
The anal canal extends from the anorectal junction (at the level of the puborectalis sling) to the anus, a distance of approximately 3-4 centimeters. Its direction is posteroinferior, making an angle with the rectum above. The canal is surrounded by the internal and external anal sphincters, which together control defecation.
The dentate (pectinate) line, located approximately 2 centimeters from the anal verge, is the single most important landmark of the anal canal. This line marks the junction between the endodermally-derived upper canal and the ectodermally-derived lower canal, a distinction with profound implications for epithelium type, blood supply, venous drainage, lymphatic drainage, and sensory innervation.
Above the dentate line, the mucosa is lined by columnar epithelium (similar to the rectum), blood supply derives from the superior rectal artery (from the IMA), venous drainage flows via the superior rectal vein to the portal system, lymphatic drainage goes to internal iliac and inferior mesenteric nodes, and innervation is autonomic (insensitive to sharp pain but responsive to stretch).
Below the dentate line, the mucosa transitions to stratified squamous epithelium (skin-like), blood supply derives from the inferior rectal artery (from the internal pudendal), venous drainage flows via the inferior rectal vein to the systemic circulation (internal iliac), lymphatic drainage goes to superficial inguinal nodes, and innervation is somatic via the pudendal nerve (exquisitely sensitive to pain, touch, and temperature).
<image>Panel A: Longitudinal section of anal canal highlighting dentate line as key dividing landmark approximately 2 cm from anal verge. Panel B: Pink columnar epithelium above dentate line with portal venous drainage, internal iliac lymphatics, and autonomic innervation. Panel C: Differently colored squamous epithelium below dentate line with systemic venous drainage, inguinal lymphatics, and somatic pudendal innervation. Panel D: Summary table of differences between upper and lower anal canal zones.</image>
Internal Features of the Anal Canal
The upper anal canal above the dentate line displays characteristic mucosal features. The anal columns (columns of Morgagni) are 6-10 vertical ridges of mucosa containing terminal branches of the superior rectal vessels. At the inferior ends of these columns, the mucosa forms small semilunar folds called anal valves, which connect adjacent columns. Above each valve is a small pocket called an anal sinus (crypt), into which the anal glands drain.
The anal glands, located in the submucosa and intersphincteric space, secrete mucus into the anal canal through ducts opening at the base of the crypts. Obstruction and infection of these glands initiates the pathogenesis of perianal abscesses and fistulae—the glands become infected, form abscesses that track through anatomical planes, and may eventually form fistulous tracts to the perianal skin.
The dentate line is visible as an irregular line marking the junction of columns and valves with the smooth lower canal. A narrow transition zone of approximately 1 centimeter above and below the dentate line contains a mixture of epithelial types.
Below the dentate line, the mucosa becomes smooth, losing the columnar features of the upper canal. Initially, this zone lacks hair and glands. The intersphincteric groove (white line of Hilton) marks the palpable junction between the internal and external sphincters. The anal verge marks the transition to true perianal skin, which contains hair follicles, sweat glands, and sebaceous glands.
<image>Panel A: Internal anal canal showing anal columns as vertical ridges in upper canal connected by anal valves forming sinuses/crypts. Panel B: Inset showing anal gland opening into crypt with secretory function. Panel C: Dentate line marking junction between columnar upper zone and smooth squamous lower zone with transition zone highlighted. Panel D: Intersphincteric groove (white line) indicated with transition to hairy perianal skin at anal verge.</image>
Anal Sphincters
Two sphincters surround the anal canal, providing complementary mechanisms for fecal continence.
The internal anal sphincter (IAS) is a thickened continuation of the circular smooth muscle layer of the rectum. This involuntary muscle maintains constant tonic contraction, contributing approximately 70-85% of the resting anal pressure that prevents passive fecal leakage. Its extent is approximately 2.5-4 centimeters, ending just above the level of the anal verge. Innervation is autonomic: sympathetic fibers (from L5) cause contraction and maintain tone, while parasympathetic fibers (from S2-S4) cause relaxation during defecation (the rectoanal inhibitory reflex).
The external anal sphincter (EAS) is a cylinder of striated (skeletal) muscle surrounding the anal canal superficial to the internal sphincter. Unlike most skeletal muscles, it maintains continuous tonic contraction even at rest, though this tone can be voluntarily augmented when continence is threatened. The EAS is traditionally described as having three parts: the subcutaneous part, which lies below the level of the internal sphincter surrounding the anal verge; the superficial part, which attaches posteriorly to the coccyx via the anococcygeal ligament and anteriorly to the perineal body; and the deep part, which merges superiorly with the puborectalis muscle to form a continuous muscular sling. Innervation is somatic via the inferior rectal nerve (branch of the pudendal nerve, S2-S4), providing voluntary control.
The intersphincteric space lies between the internal and external sphincters, containing the intersphincteric extensions of the anal glands. This space is important in the pathogenesis of perianal sepsis, providing a potential route for abscess and fistula extension.
<image>Panel A: Coronal section showing internal anal sphincter as pink thickened smooth muscle ring indicating involuntary control. Panel B: External anal sphincter as red striated muscle indicating voluntary control with subcutaneous, superficial, and deep parts labeled. Panel C: Intersphincteric space between sphincters with superficial part connecting to coccyx and perineal body. Panel D: Puborectalis sling creating anorectal angle at junction with rectum with deep EAS merging superiorly.</image>
Blood Supply
Arterial Supply
The rectum and anal canal receive arterial blood from three sources derived from different parent vessels, reflecting the boundary between midgut and hindgut derivatives.
The superior rectal artery, the terminal branch of the inferior mesenteric artery, is the primary supply to the rectum and the anal canal above the dentate line. It descends in the sigmoid mesocolon, then behind the rectum before dividing into right and left branches that run along the rectal wall, giving numerous branches to the mucosa.
The middle rectal arteries, paired branches of the internal iliac arteries, supply the muscular wall of the lower rectum. These arteries are variable in size and may be small or absent; they run in the lateral ligaments of the rectum to reach the lower rectum.
The inferior rectal arteries, branches of the internal pudendal arteries, supply the anal canal below the dentate line and the external sphincter. They cross the ischioanal fossa to reach the anal canal.
Rich anastomoses exist between all three arterial territories, providing collateral circulation that can maintain perfusion if one vessel is compromised.
Venous Drainage
Venous drainage mirrors the arterial supply and has important clinical implications for portal-systemic anastomoses.
The internal rectal (hemorrhoidal) venous plexus lies in the submucosa of the upper anal canal above the dentate line. It drains via the superior rectal vein to the inferior mesenteric vein and thus to the portal system. Dilation of these veins produces internal hemorrhoids.
The external rectal (hemorrhoidal) venous plexus lies subcutaneously below the dentate line. It drains via the middle and inferior rectal veins to the internal iliac veins and thus to the systemic circulation. Thrombosis of these veins produces external hemorrhoids.
The two plexuses communicate freely, creating a portosystemic anastomosis. In portal hypertension, back pressure causes dilation of the upper plexus, contributing to internal hemorrhoids.
<image>Panel A: Superior rectal artery descending from IMA as portal territory supply above dentate line. Panel B: Middle rectal arteries from internal iliac and inferior rectal arteries from internal pudendal as systemic territory. Panel C: Internal hemorrhoidal plexus above dentate line with blue arrows showing portal system drainage. Panel D: External hemorrhoidal plexus below dentate line draining to systemic circulation with portosystemic anastomosis communication highlighted.</image>
Lymphatic Drainage
The lymphatic drainage of the rectum and anal canal follows patterns that are critical for understanding cancer spread.
The rectum drains primarily along the superior rectal vessels to the inferior mesenteric nodes and ultimately to the para-aortic nodes. Some lateral drainage follows the middle rectal vessels to internal iliac nodes.
The upper anal canal (above the dentate line) drains to internal iliac nodes and also follows the superior rectal vessels to inferior mesenteric nodes, paralleling the venous pattern.
The lower anal canal (below the dentate line) drains to the superficial inguinal nodes, reflecting its embryological origin from surface ectoderm. This pattern explains why anal margin (perianal) cancers may present with inguinal lymphadenopathy.
Understanding these drainage patterns is essential for proper staging and treatment planning in anorectal malignancy.
<image>Panel A: Sagittal view showing green rectal lymphatics coursing superiorly along superior rectal vessels to inferior mesenteric and para-aortic nodes. Panel B: Lateral channels following middle rectal vessels to internal iliac nodes. Panel C: Upper anal canal above dentate line with similar upward drainage pattern. Panel D: Lower anal canal below dentate line draining anterolaterally to superficial inguinal nodes with dentate line as watershed marked.</image>
Innervation and Mechanisms of Fecal Continence
Innervation
The rectum receives autonomic innervation through the inferior hypogastric plexus. Sympathetic fibers from the hypogastric nerves (L1-L2) inhibit peristalsis and maintain internal sphincter contraction. Parasympathetic fibers from the pelvic splanchnic nerves (S2-S4) stimulate peristalsis and cause internal sphincter relaxation. Visceral sensation (distension, stretch) travels with these autonomic fibers; pain is poorly localized.
The upper anal canal receives the same autonomic supply, sensing stretch and distension but insensitive to sharp pain—this allows office procedures (such as rubber band ligation of internal hemorrhoids) without anesthesia.
The lower anal canal receives somatic sensory innervation via the inferior rectal nerves (branches of the pudendal nerve, S2-S4). This region is exquisitely sensitive to pain, touch, and temperature, requiring anesthesia for procedures. The ability to discriminate between solid, liquid, and gas at this level is crucial for the sampling reflex.
Mechanisms of Fecal Continence
Multiple factors work together to maintain fecal continence. The anorectal angle (approximately 80-90 degrees), created by the puborectalis sling, acts as a flap valve that occludes the upper anal canal when the muscle is contracted—the angle effectively kinks the gut closed. The internal anal sphincter maintains constant tonic contraction (70-85% of resting pressure), providing involuntary baseline continence. The external anal sphincter adds voluntary squeeze pressure when continence is challenged.
Rectal compliance allows the rectum to accommodate increasing volumes at low pressure through receptive relaxation, preventing the pressure rise that would trigger defecation. The anal transition zone mucosa permits discrimination of rectal contents (solid, liquid, or gas) through the sampling reflex—small amounts of rectal content enter the upper anal canal, where specialized sensory endings identify the nature of the content, allowing selective passage of flatus while retaining feces.
Defecation
When defecation is socially appropriate, a coordinated sequence occurs. Rectal distension triggers the conscious urge to defecate. The puborectalis relaxes, straightening the anorectal angle and opening the flap valve. The internal sphincter relaxes through the rectoanal inhibitory reflex. The external sphincter is voluntarily relaxed. The Valsalva maneuver increases intra-abdominal pressure. Rectal contraction expels the contents.
<image>Panel A: Anorectal angle at rest with puborectalis sling creating 90-degree angle kinking canal closed as flap valve mechanism. Panel B: Sphincter contributions showing IAS providing 70-85% resting pressure with EAS voluntary augmentation. Panel C: Resting state with angle maintained and sphincters contracted versus defecation with angle opening. Panel D: Defecation sequence with puborectalis relaxation, sphincter relaxation, and Valsalva increasing intra-abdominal pressure indicated by arrows.</image>
The Ischioanal Fossa
The ischioanal fossa (previously called the ischiorectal fossa) is a fat-filled wedge-shaped space lateral to the anal canal and below the pelvic floor, one on each side. Its configuration allows the anal canal to expand during defecation.
The boundaries of each fossa include: medially, the external anal sphincter and the sloping inferior surface of the levator ani; laterally, the obturator internus muscle covered by its fascia (superiorly) and the ischial tuberosity (inferiorly); the roof (apex) is formed by the junction of the levator ani with the obturator fascia; the floor (base) is the perianal skin; anteriorly, the posterior edge of the urogenital triangle (perineal membrane); and posteriorly, the sacrotuberous ligament and lower border of gluteus maximus.
The principal content is fat, the ischioanal fat pad, which provides a compressible cushion permitting anal expansion. Traversing the lateral wall is the pudendal canal (Alcock's canal), a fascial tunnel containing the pudendal nerve and internal pudendal vessels. The inferior rectal nerves and vessels cross the fossa from the pudendal canal to reach the anal canal.
Importantly, the two ischioanal fossae communicate posteriorly behind the anal canal through the deep postanal space. This communication allows infection to spread from one side to the other, producing the characteristic horseshoe abscess pattern.
<image>Panel A: Coronal section showing ischioanal fossae as yellow triangular fat-filled spaces bounded medially by external anal sphincter and levator ani. Panel B: Lateral boundaries formed by obturator internus and ischial tuberosity with pudendal canal on lateral wall. Panel C: Inferior rectal vessels and nerves crossing fossa to reach anal canal from pudendal canal. Panel D: Posterior communication via deep postanal space behind anal canal illustrating horseshoe abscess spread potential.</image>
Clinical Correlations
Hemorrhoids
Hemorrhoids represent pathological dilation and downward displacement of the anal cushions—specialized vascular structures that contribute to anal continence. Internal hemorrhoids arise from the internal venous plexus above the dentate line; because this zone lacks somatic innervation, they are typically painless but may present with bright red rectal bleeding or prolapse. External hemorrhoids arise from the external venous plexus below the dentate line; when thrombosed, they present as painful, tender perianal swellings because this zone is somatically innervated.
Anal Fissure
An anal fissure is a longitudinal tear in the mucosa of the distal anal canal, most commonly occurring in the posterior midline (where blood supply is poorest). Patients experience severe pain during and after defecation, often accompanied by bright red bleeding. The pain triggers reflex spasm of the internal sphincter, which perpetuates ischemia and impairs healing.
Perianal Abscess and Fistula
The cryptoglandular theory explains most perianal sepsis: infection originates in an anal gland at the level of the dentate line, spreads into the intersphincteric space, and may track in various directions depending on the anatomical plane followed. An abscess presents as painful perianal swelling requiring surgical drainage. A fistula-in-ano develops when an abscess drains spontaneously or is inadequately drained, creating a chronic tract between the anal canal and perianal skin. The Parks classification describes fistula types based on their relationship to the sphincter complex: intersphincteric (most common, through the intersphincteric space), transsphincteric (through both sphincters), suprasphincteric (over the puborectalis), and extrasphincteric (outside the sphincter complex entirely).
Rectal Cancer
Rectal cancer staging includes assessment of the tumor's relationship to the peritoneal reflection—tumors above this landmark may spread intraperitoneally, while those below spread locally within the pelvis. Total mesorectal excision (TME), removing the rectum with its entire mesorectal envelope intact, is the standard surgical approach. Low tumors may require abdominoperineal resection (APR) with permanent colostomy.
Fecal Incontinence
Fecal incontinence results from disruption of any continence mechanism. Sphincter injury (most commonly obstetric trauma during childbirth) damages the muscular barrier. Neurological injury disrupts the sensory or motor innervation. Rectal prolapse mechanically stretches and damages the sphincter complex. Evaluation includes endoanal ultrasound (visualizing sphincter defects) and anorectal manometry (measuring sphincter pressures).
<image>Panel A: Internal hemorrhoids above dentate line (painless, may bleed or prolapse) compared with external hemorrhoids below (painful when thrombosed). Panel B: Anal fissure as longitudinal tear in posterior midline with associated sphincter spasm perpetuating ischemia. Panel C: Parks classification showing intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric fistula types. Panel D: Rectal cancer specimen with intact mesorectum (TME) and peritoneal reflection relationship for staging.</image>
Summary
The rectum (12-15 cm) extends from S3 to the anorectal junction, following the sacral curve and displaying three lateral flexures with corresponding transverse folds (valves of Houston). The lower third lies below the peritoneal reflection and is completely extraperitoneal. The mesorectum, containing lymph nodes and vessels within its fascial envelope, is excised intact during total mesorectal excision for rectal cancer. The anal canal (3-4 cm) is divided by the dentate line into an upper zone (columnar epithelium, portal drainage, internal iliac lymphatics, autonomic innervation) and a lower zone (squamous epithelium, systemic drainage, inguinal lymphatics, somatic pudendal innervation). The internal anal sphincter (smooth muscle) provides 70-85% of resting tone involuntarily, while the external anal sphincter (striated muscle) provides voluntary control. The puborectalis sling creates the anorectal angle essential for continence. The superior rectal artery (from the IMA) supplies above the dentate line, while the inferior rectal artery (from the internal pudendal) supplies below. The portosystemic anastomosis between the internal and external hemorrhoidal plexuses explains hemorrhoid development in portal hypertension.
Key Terms
Dentate (pectinate) line: The junction approximately 2 cm from the anal verge that divides the upper anal canal (columnar epithelium, portal drainage, autonomic innervation) from the lower canal (squamous epithelium, systemic drainage, somatic innervation).
Anorectal angle: The approximately 80-90 degree angle between the rectum and anal canal created by the puborectalis sling, functioning as a flap valve for fecal continence.
Mesorectum: The fatty envelope surrounding the rectum, containing lymph nodes and vessels, enclosed by the mesorectal fascia; its complete excision (TME) is the standard for rectal cancer surgery.
Internal anal sphincter: The involuntary smooth muscle sphincter formed by thickening of the rectal circular muscle, providing 70-85% of resting anal tone.
Anal columns (of Morgagni): The 6-10 vertical mucosal folds in the upper anal canal containing terminal branches of the superior rectal vessels.
Ischioanal fossa: The fat-filled wedge-shaped space lateral to the anal canal, communicating posteriorly with its counterpart via the deep postanal space and containing the pudendal canal along its lateral wall.
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