Anatomy of Lower Gastrointestinal Tract

Thanasil Huanmanop MD, M.ScAnatomy Chula

LGIT = duodenojejunal junction - anus :

Jejunoileum, large intestine & anal canal

Embryology : Foregut, Midgut, Hindgut

Vitelline duct

Small intestine - Jejunoileum :

> duodenojejunal junction - ileocaecal junction ( 20 ft )> proximal 1/3 LUQ , middle 1/3 periumbilical , distal 1/3 pelvis

Ligament of Treitz :

duodenojejunal flexure (L2) - Rt. crus of diaphragm

duodenum

jejunum

Ligamentof

TreitzCeliac trunk

Right crusof diaphragm

Lower GI tract

Ileocaecal junction :

Line ( Rt. ASIS - umbilicus ) X Rt. linear semilunaris , L5

Jejunoileum

Intraperitoneal organ

Anterior : greater omentum ( omental apron )

Posterior : mesentery proper to post. abdominal wall

• double-layered peritoneal fold • root 6-7 inches

Mesentery proper

Root of mesentery

Characteristic jejunum ileum

Length 8 ft (proximal 2/5 ) 12 ft (distal 3/5 )

Diameter 2 – 4 cm 2 – 3 cm

Wall Thick and heavy Thin and light

Color Deeper red Paler pink

Vascularity Greater Less

Vasa recta Long Short

Arterial arcade A few large loop Many short loops

Fat in mesentery Less More

Plicae circulares( circular folds )

Large,tall & closely packed Low & sparse

Villi Large & many Small & discrete

Peyer’s patches Few Many

Jejunum Ileum

Clinical consideration :

• Resect mesentery of jejunoileum -> intestinal mobile • Resection of small bowel : short bowel syndrome (80%)

• Cancer, Inflammation, hemorrhage, obstruction

• Remnant of vitelline duct

• Organ of 2 = 2% , 2 ft from IC valve , 2 inches, 2 special types of mucosa (gastric & pancreatic)

• Bleeding / diverticulitis

MECKEL’S DIVERTICULUM

Large intestine : 5 ft ; ileocaecal junction - anus

Ileocecal junction

Vermiform appendix

Caecum

Ascendingcolon

Hepatic flexureSplenic flexureTransverse colon

Descendingcolon

Sigmoidcolon

RectumAnal canal

General features of large intestine : Small intestine Large intestine

1. Villi 2. Mucosal fold

3. Taenia coli4. Haustra coli (sacculation)5. Appendices epiploicae6. Peritoneal relation

7. Peritoneal fold

+Circular folds

(plicae circulares)---

Intraperitoneal organ(except duodenum)

Mesentery proper(except duodenum)

-Semilunar folds

(plicae semilunares)+(except appendix & rectum)

++ (except caecum & rectum)

Retroperitoneal organ(except caecum, transverse colon,

sigmoid colon & rectum) Transverse mesocolon Sigmoid mesocolon

Caecum : 7.5 x 6 cm., blind intestinal pouch, Rt. iliac fossa (L5)

Ileocaecal orifice : ileocaecal valve (superior & inferior lips), frenulum

Appendicular orifice

Mesenteric support :

- Superior ileocaecal (vascular) fold & fossa

- Inferior ileocaecal fold & fossa

- Rectocaecal fossa

Clinical consideration :

• Fecal reflux at IC valve

• Intussusception

10 cm., blind intestinal diverticulumsearch by following taenia coliIntraperitoneal organ – mesoappendix

Vermiform appendix

RLQ at McBurney’s point : Lat. 1/3 & med. 2/3 of line ( Rt. ASIS - umbilicus )

malrotation or nonrotation, situs inversus

Position : retrocaecal , pelvic

Clinical consideration :

Acute appendicitis

• common abdominal emergency• lymphoid hyperplasia & fecolith• appendiceal mass/abscess• appendectomy• McBurney’s incision…?

Ascending colon : 20 cm., Rt. lumbar region

Rt. colic (hepatic) flexure

Secondarily retroperitoneal organ

Volvulus(L.volvo=to roll)

Transverse colon : 45 cm., umbilical region

intraperitoneal organ

hepatic flexure - Lt. colic (splenic) flexure (phrenococolic or sustenaculum lienis)

Transverse mesocolon

Greater omentum & gastrocolic lig.

Descending colon : 25 cm.,

Lt. hypochondriac, lumbar & iliac regions

splenic flexure to Lt. pelvic brim

Secondarily retroperitoneal organ

Sigmoid colon : 40 cm., S-shape, suprapubic region, intraperitoneal organ

Rectosigmoid junction (15 cm. from anus, S3)

Sigmoid mesocolon (reverse V-shape) & intersigmoid fossa

Clinical consideration :

• Variation

• Sigmoid volvulus

Rectum :

15 cm., extraperitoneal organ,

rectosigmoid junction to anorectal ring (pelvic diaphragm)

S3

3 parts of rectum andPeritoneal reflections : anterior, lateral & posterior

Rectum :

Rectovesical pouchRectouterine pouch

(pouch of Douglas or Cul-de-sac)

Pararectal fossa

Mesorectum

Rectovesical septum or Denonvillier’s fascia Rectovaginal septum

Rectum :

No mesentery, haustra coli, appendices epiploicae, taenia coli spread out and form longitudinal rectal muscle

Rectal ampulla : fecal storage before defecation

Transverse rectal folds (shelves) (Houston’s valves)

Levator ani muscle(pelvic diaphragm)

Rectal ampulla

Taenia coli spread to formLongitudinal rectal muscle

Right middle rectal valve*

Left lower (Infr) rectal valve

Left upper (Supr) rectal valve

* Anterior peritoneal reflection

Clinical consideration :

Colorectal carcinoma • Most at rectum • large bowel obstruction• Colectomy• Colostomy• Barium enema• Colonoscopy, sigmoidoscopy,

proctoscopy

Clinical consideration :

• Digital rectal examination (DRE) or PR(per rectum) ->

prostate gland, rectal mass

• Megacolon (Hirschprung’s disease) - congenital, absent of parasympathetic ganglion cell in myenteric plexus of intestinal wall

• Polyp / Diverticulum

Anorectal ring

Anus, anal verge

Levator ani muscle (pelvic diaphragm)

Anal canal :Terminal part of LGIT, 3-4 cm.

Anorectal ring (pelvic diaphragm) - anus (anal opening, anal verge)

Ant. : perineal body, urogenital diaphragm, bulb of penis or vagina

Post. : anococcygeal raphe’

Upper : pelvic diaphragm

Wall of anal canal :

Internal sphincter ani (involuntary)

External sphincter ani (voluntary)

Pelvic diaphragm(Levator ani muscle)

(rectal sling)

Internal sphincter

Ani (from inner circular muscle of rectum)

Conjoined longitudinal muscle

( from levator ani muscle & outer longitudinal muscle of

rectum)

Corrugator cutisani muscle

Deep external sphincter ani muscle

Superficialexternal

sphincter animuscle

Subcutaneous external sphincter ani muscle

* Anorectal ring : check sphincter tone by DRE, protect fecal incontinence

Mucosa of anal canal :

Anal columns (of Morgagni)

Anal valves

Anal crypts (sinuses)

Opening of anal glands

Dentate (pectinate) line

Transitional (intermediate) Zone or pecten

Hilton white line

Cutaneous zone

Rectal venous plexus

Dentate line Above Below

Development Hindgut (endoderm)

Proctodeum (anoderm)

Vein Portal IVC

Lymphatic drainage

Abdominal Inguinal

Nerve Autonomic(tone) Somatic(pain) (pudendal nerve)

Hemorrhoids Internal(bleed) External(pain)

Hemorrhoids or piles :