RECTUM — LOGIC-BASED, ZERO-OMISSION NOTE

1️⃣ Name, Shape & Curvatures — why “rectum” is misleading

• Rectus = straight (Latin)
• Rectum is straight in monkeys, NOT in humans
• Human rectum:
• Follows posterior concavity of sacrum
• Shows three lateral curves (flexures), best seen when distended
• Upper curve → convex to right
• Middle curve → convex to left
• Lower curve → convex to right
• Result:
• Middle part bulges to the left
• Lowest part slightly dilated → rectal ampulla

🔑 Exam logic: “Rectum not straight → sacral concavity + lateral flexures”

2️⃣ Internal Folds — transverse rectal folds (Houston’s valves)

• Correspond to the three external curves
• Three sickle-shaped transverse rectal folds
• Project into lumen from concave side
• Old name: rectal valves of Houston

Structural importance

• Incorporate circular muscle layer
• NOT just mucosa
• Unlike circular folds of duodenum/jejunum

Middle transverse fold (MOST IMPORTANT)

• Largest
• Projects from right wall
• Located:
• Just above rectal ampulla
• At level where peritoneum reflects forward
• Forms rectovesical pouch (male)
• Forms rectouterine pouch (female)
• Distance:
• ~8 cm from anal orifice
• Clinical use:
• Key landmark in sigmoidoscopy

🔑 Exam pearl: “Middle Houston valve = right wall, 8 cm, peritoneal reflection”

3️⃣ Length, Beginning & End — transition logic

• Length: ~12 cm
• Begins:
• At S3 vertebral level
• Continuous with sigmoid colon
• Transition:
• Gradual, not abrupt

Loss of mesentery

• Sigmoid mesocolon ends
• Rectum → no mesentery

4️⃣ Muscular Changes — why rectum has no sacculations

• Sigmoid colon:
• Has three taeniae coli
• As sigmoid → rectum:
• Taeniae broaden
• Form anterior + posterior longitudinal bands
• These meet laterally → continuous longitudinal muscle
• Result:
• No haustra
• No appendices epiploicae

🔑 Reason: Uniform longitudinal layer → smooth tube

5️⃣ Anorectal Junction — continence & defecation logic

• Rectum becomes anal canal
• ~2–3 cm anterior to tip of coccyx

Puborectalis sling

• U-shaped puborectalis
• Pulls anorectal junction forward
• Merges with:
• Top of external anal sphincter
• Forms:
• Anorectal ring
• Palpable on rectal exam

Angle mechanism

• Creates right-angled bend posteriorly
• During defecation:
• Puborectalis relaxes
• Angle widens
• Faeces enter anal canal

🔑 Core concept: Continence = angle + sling

6️⃣ Mesorectum & Fascia — surgical anatomy backbone

• Rectum has no mesentery
• But surrounding tissue = mesorectum
• Covering fascia = mesorectal fascia

Contents of mesorectum

• Superior rectal artery + branches
• Superior rectal vein + tributaries
• Lymphatics + lymph nodes

Distribution

• Bulkier posteriorly
• Often midline groove posteriorly

7️⃣ Surgical Dissection Plane — TME logic

• Between:
• Mesorectal fascia
• Parietal pelvic fascia
• Tissue:
• Avascular areolar plane
• Used in:
• Total Mesorectal Excision (TME)

Regional variation

• Best developed posteriorly
• Minimal laterally
• Because inferior hypogastric plexus lies tangentially

Structures crossing plane

• Autonomic nerve fibres → rectum
• Occasional small middle rectal vessels

“Lateral ligament” of rectum

• Iatrogenic, surgeon-defined
• Not a true ligament
• Not visible on MRI/CT

8️⃣ Peritoneal Relations — thirds of rectum

Upper third

• Covered anteriorly + laterally

Middle third

• Covered anteriorly only

Lower third

• No peritoneal covering
• Peritoneum reflects forward to form:
• Rectovesical pouch (male)
• Rectouterine pouch / Douglas (female)

Depth from anal margin

• Male: ~7.5 cm
• Female: ~5.5 cm
• Reachable by fingertip on rectal exam

9️⃣ Anterior Relations — male vs female

Male (above pouch)

• Base of bladder (uppermost)
• Tops of seminal vesicles

Male (below pouch)

• Rest of bladder base
• Seminal vesicles
• Prostate
• Distal ureters
• Vas deferens

🔟 Rectogenital Septum — Denonvilliers fascia (male)

• Condensation of fascia between:
• Rectum
• Prostate + seminal vesicles
• Called:
• Rectovesical fascia of Denonvilliers
• Attachments:
• Superior → floor of rectovesical pouch
• Inferior → apex of prostate

Developmental basis

• Fetal rectovesical pouch extends lower
• Fusion of anterior + posterior walls → septum

Surgical importance

• Whitish, distinct
• Lies closer to rectum
• Usually removed in rectal cancer excision

1️⃣1️⃣ Female anterior relations

• Above rectouterine pouch:
• Upper vagina (fornix)
• Cervix projects into fornix
• Below peritoneal reflection:
• Vagina separated by:
• Rectovaginal fascia
• Inferiorly:
• Rectovaginal fascia fuses with perineal body

1️⃣2️⃣ Rectourethralis muscle (male)

• Slips from longitudinal muscle of rectal ampulla
• Pass forward to:
• Perineal body
• Sphincter urethrae
• Must be divided during excision of rectum + anal canal

1️⃣3️⃣ Blood Supply — arterial logic

Main artery

• Superior rectal artery
• Continuation of inferior mesenteric artery
• Changes name at pelvic brim

Course

• Enters sigmoid mesocolon
• Crosses left common iliac vessels
• Medial to ureter
• Descends to S3
• Divides into two branches
• Descend on each side of rectum
• Pierce muscle → supply full wall
• Continue submucosally into anal canal

Anastomosis

• With inferior rectal arteries

Other contributors

• Middle rectal arteries
• Present in ~20%
• Supply muscle only
• Inferior rectal arteries
• Can supply rectum from below up to peritoneal reflection
• Median sacral artery
• Minor supply
• Surgical bleeding risk near anorectal junction

1️⃣4️⃣ Venous Drainage — plexus logic

• Internal rectal venous plexus (submucosal)
• External rectal venous plexus (outside muscle)
• Lower internal plexus → anal cushions

Drainage routes

• Superior rectal vein → inferior mesenteric vein
• Inferior rectal veins → internal pudendal veins

1️⃣5️⃣ Lymphatic Drainage — cancer relevance

Main drainage (upward)

• Mucosal follicles → epicolic nodes
• → pararectal nodes (mesorectum)
• → nodes along inferior mesenteric artery
• → pre-aortic nodes

Minor drainage (less important)

• Lower rectum → internal iliac nodes
• Along middle/inferior rectal arteries
• Along median sacral artery
• Unlikely route unless mesorectal fascia breached

1️⃣6️⃣ Nerve Supply

Sympathetic

• Via inferior mesenteric plexus
• Along inferior mesenteric + superior rectal arteries

Parasympathetic

• Pelvic splanchnic nerves (S2–S4)
• Via inferior hypogastric plexus
• Motor to rectal muscle

Sensation

• Pain → both sympathetic & parasympathetic
• Distension → parasympathetic afferents

1️⃣7️⃣ Rectal Examination — what you can feel

In both sexes

• Coccyx
• Sacrum
• Ischial spines
• Anorectal ring

Male

• Prostate (palpable)
• Seminal vesicles → usually not palpable

Female

• Cervix (through vaginal wall)
• Uterosacral ligaments
• Occasionally ovaries

1️⃣8️⃣ Development — pectinate line logic

• Derived from:
• Anorectal canal (endoderm)
• Proctodeum (ectoderm)
• Anal membrane breaks down
• Remnant indicated by:
• Pectinate line
• Anal valves
• Explains:
• Different blood supply
• Different nerve supply
• Different lymph drainage above vs below line

1️⃣9️⃣ Surgical Management — stepwise logic

Anterior resection / TME

• Remove:
• Rectum
• Mesorectum
• Often sigmoid colon + mesocolon
• Preserve:
• Inferior hypogastric plexuses
• Superior hypogastric plexus
• Inferior mesenteric artery:
• Divided near aortic origin
• Preserves plexus on aorta

Abdominoperineal resection (APR)

• Used when anal canal must be removed
• Adds perineal approach:
• Divide levator ani
• Enter ischioanal fossae
• Excise coccyx
• Divide rectosacral fascia
• Remove:
• Rectum + anal canal
• Collar of levator ani
• Ischioanal fat
• End result:
• Permanent colostomy (left iliac fossa)

RECTUM — COMPLETE LOGIC TABLE (ZERO-OMISSION)

Rectum — Integrated Anatomy, Supply, Innervation & Surgery (Zero-Omission Table)