Rectum and Anal Canal

Extent:

• Continues from the sigmoid colon at approximately the S3 vertebral level (where the sigmoid mesocolon ends), extending to the anorectal junction (where it is encircled by the puborectalis sling), approximately 12-15 cm in length.

Curves:

• Follows the concavity of the sacrum/coccyx in the sagittal plane (the sacral flexure), then curves sharply forward at the anorectal junction (the perineal (anorectal) flexure, maintained by puborectalis) before continuing as the anal canal.
• Also displays three lateral curves in the coronal plane, corresponding internally to the transverse rectal folds.

Relations:

• Posterior: Sacrum, coccyx, median sacral vessels, inferior hypogastric plexus branches.
• Anterior: In males, rectovesical pouch (upper third), bladder base, seminal vesicles, prostate (separated by Denonvilliers' fascia). In females, rectouterine pouch (upper third), posterior vaginal wall (separated by the rectovaginal septum).
• Lateral: Levator ani, and the lateral rectal (stalk) ligaments: fascial condensations containing the middle rectal vessels, connecting the rectum to the pelvic side wall.

Peritoneal covering:

• Upper third: Covered by peritoneum on its anterior and lateral surfaces.
• Middle third: Covered by peritoneum on its anterior surface only.
• Lower third: Entirely extraperitoneal (subperitoneal), below the level of the peritoneal reflection (rectovesical or rectouterine pouch), this progressive loss of peritoneal covering moving distally is a key organising principle, mirroring but distinct from the transition seen at the pectinate line further down (a different, more fundamental embryological transition).

Transverse rectal folds (valves of Houston):

• Typically three semilunar mucosal folds projecting into the rectal lumen, corresponding internally to the rectum's external lateral curves; thought to help support the weight of faeces and reduce the sensation of urgency, though their precise functional significance is debated, clinically relevant as internal landmarks during rigid/flexible sigmoidoscopy and as a site requiring careful technique to avoid perforation during instrumentation.

Blood supply:

• Superior rectal artery (terminal branch of the IMA), supplies the upper rectum, the dominant supply overall.
• Middle rectal artery (branch of the internal iliac artery, anterior division), supplies the middle/lower rectum, variable in size and sometimes minimal, with the superior and inferior rectal arteries compensating.
• Inferior rectal artery (branch of the internal pudendal artery), supplies the lower rectum/anal canal region, particularly the anal sphincter complex.
• This triple arterial supply spanning three different parent systems (IMA, internal iliac, internal pudendal) reflects the rectum/anal canal's transitional embryological nature (hindgut proximally, ectoderm-derived distally) and provides substantial collateral protection against ischaemia compared to more "single-system" dependent bowel segments.

Venous drainage:

• Mirrors the arterial supply: superior rectal vein drains to the portal system (via the IMV); middle and inferior rectal veins drain to the systemic system (via the internal iliac vein and internal pudendal vein respectively), the rectal venous plexus is therefore a key portosystemic anastomosis site (see Abdomen content, Section 11, and Section 5 of this content).

Lymphatics:

• Upper and middle rectum: Drain superiorly along the superior rectal vessels to the inferior mesenteric nodes (following the hindgut pattern).
• Lower rectum: Drains predominantly to the internal iliac nodes, with some lymphatic communication toward the inguinal nodes possible in advanced disease, though this is a less dominant pathway than the internal iliac route for the rectum proper (contrasted with the anal canal below the pectinate line, where inguinal drainage is the dominant pattern).

Innervation:

• Autonomic: sympathetic (via the hypogastric nerves/inferior hypogastric plexus) and parasympathetic (pelvic splanchnic nerves, S2-S4), the rectum, as a hindgut-derived structure, receives NO vagal parasympathetic supply (consistent with the general principle established in Abdomen content, Section 11).
• The rectum is relatively insensate to pain from cutting (visceral innervation only) but sensitive to distension (the sensory basis of the urge to defecate).

Extent:

• Continues from the rectum at the anorectal junction (marked by the puborectalis sling) to the anal verge (external opening), approximately 3-4 cm in length.

Upper and lower portions:

• Divided by the pectinate (dentate) line, the single most important landmark in the anal canal, marking the junction between the hindgut-derived (endodermal) upper portion and the ectoderm-derived (proctodeum-derived) lower portion: this single embryological boundary underlies essentially every anatomical, vascular, lymphatic, and clinical distinction.

Anal columns (columns of Morgagni):

• Longitudinal mucosal folds in the upper anal canal, each containing a terminal branch of the superior rectal artery and vein, the vascular cushions here contribute to the internal haemorrhoidal plexus.

Anal valves:

• Small mucosal folds connecting the lower ends of adjacent anal columns, together forming the pectinate line itself; the small pockets above each valve are termed anal sinuses, which can trap faecal matter and become a site of infection (the proposed origin of some anal fistulae, via infection of the anal glands that open into these sinuses).

Internal anal sphincter:

• Involuntary smooth muscle, a thickened continuation of the rectum's circular muscle layer, surrounding the upper two-thirds of the anal canal, under autonomic control (sympathetic, contraction/tone maintenance; parasympathetic, relaxation, part of the defecation reflex), providing the majority of resting anal canal pressure/tone.

External anal sphincter:

• Voluntary skeletal muscle, surrounding the entire length of the anal canal (both above and below the pectinate line, though most developed at the lower/mid canal), conventionally described in three parts (subcutaneous, superficial, deep (the deep part blending with puborectalis)) innervated by the inferior rectal nerve (branch of the pudendal nerve), providing voluntary control over defecation, allowing deferral even when the internal sphincter reflexively relaxes in response to rectal distension.

Puborectalis:

• The sling-like portion of levator ani encircling the anorectal junction, maintaining the anorectal angle critical for continence at rest, functionally and anatomically continuous with/closely related to the deep part of the external anal sphincter, together forming an integrated continence mechanism alongside the internal sphincter.

Internal haemorrhoids:

• Dilated internal rectal venous plexus (part of the portosystemic anastomosis network), arising above the pectinate line: classically painless (visceral innervation) even when they bleed or prolapse, though prolapse with strangulation can become secondarily painful if it drags surrounding somatically-innervated tissue.
• Classically described at three constant anatomical positions (3, 7, and 11 o'clock in the lithotomy position, corresponding to the terminal branches of the superior rectal artery), a frequently tested detail.
• Graded by the degree of prolapse (Grade I: no prolapse, to Grade IV, permanently prolapsed).

External haemorrhoids:

• Dilated external rectal venous plexus, arising below the pectinate line, covered by sensate anoderm/perianal skin, characteristically painful, particularly if thrombosed (an acutely painful, palpable perianal lump).

Anal fissure:

• A tear in the anoderm (below the pectinate line), most commonly located in the posterior midline (given this region's relatively poorer blood supply, a true anatomical/vascular watershed zone at the posterior anal canal), causing severe pain on defecation (given the rich somatic sensory innervation here) and often a reflex internal sphincter spasm that perpetuates poor local blood flow and delays healing, the anatomical basis for medical treatments (e.g., topical nitrates, botulinum toxin) aimed at relaxing this sphincter spasm to improve local perfusion.

Rectal prolapse:

• Full-thickness protrusion of the rectal wall through the anus, related to weakness of the rectum's normal supporting structures (lateral ligaments, pelvic floor musculature), more common in older individuals, particularly women with a history of significant pelvic floor trauma/weakness, and associated with chronic straining/constipation.
• Distinct from (though sometimes confused with) prolapsing internal haemorrhoids, distinguished clinically by the pattern of mucosal folds (concentric in true rectal prolapse vs radial in haemorrhoidal prolapse).