Small and Large Intestines Anatomy

Gastrointestinal Tract

• Digestive System components:
  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestines (duodenum, jejunum, ileum)
  • Large intestines (cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal)
• Divisions based on blood supply and development:
  • Foregut: supplied by the celiac trunk
  • Midgut: supplied by the superior mesenteric artery
  • Hindgut: supplied by the inferior mesenteric artery

Abdominal Aorta

• Begins at the T12 vertebra.
• Ends at the L4 vertebra.
• Unpaired Branches:
  • Celiac trunk
  • Superior mesenteric artery
  • Inferior mesenteric artery
• Paired Branches:
  • Inferior phrenic arteries
  • Middle suprarenal arteries
  • Renal arteries
  • Gonadal arteries (testicular or ovarian)
  • Lumbar arteries (4 pairs)
  • Common iliac arteries
• Median sacral artery

Superior Mesenteric Artery

• Supplies the midgut.
• Structures supplied:
  • Jejunum
  • Ileum
  • Cecum
  • Appendix
  • Ascending colon
  • Transverse colon
• Origin: L1 vertebrae.
• Branches:
  • Inferior pancreaticoduodenal artery
  • Middle colic artery: supplies the transverse colon
  • Right colic artery: supplies the ascending colon
  • Ileocolic artery: supplies the cecum
  • Appendicular artery
  • Jejunal arteries
  • Ileal arteries

Inferior Mesenteric Artery

• Supplies the hindgut.
• Structures supplied: Descending and Sigmoid colon
• Origin: Level of L3 vertebra.
• Branches:
  • Left colic artery
  • Sigmoid arteries
  • Superior rectal artery
• Marginal artery:
  • Anastomosis of the superior and inferior mesenteric arteries.
  • No specific origin.

Superior & Inferior Mesenteric Veins

• Veins accompany named arteries.
• Drain into superior or inferior mesenteric vein.
• Inferior mesenteric vein drains into the splenic vein.
• Superior mesenteric vein joins the splenic vein to form the portal vein.

Autonomic Nervous System (ANS)

• Ganglia located by the named artery
• Supplies:
  • Pancreas, duodenum, jejunum, ileum, cecum, appendix, ascending colon, proximal part of the transverse colon
• Posterior vagal trunk: parasympathetic innervation.
• Superior mesenteric plexus: sympathetic innervation (greater and lesser splanchnic nerves).
• Supplies: Distal transverse colon, splenic flexure, descending colon, sigmoid colon, and the upper rectum.
  • Parasympathetic innervation: Pelvic splanchnic nerves (S2-S4) to the inferior hypogastric plexus.
  • Inferior mesenteric plexus: sympathetic innervation (L1-L4 lumbar splanchnic nerves and superior hypogastric plexus).

Small Intestine Anatomy

• Components:
  • Duodenum (covered in Celiac trunk and stomach lecture)
  • Jejunum
  • Ileum

Jejunum & Ileum

• Total length: 6-7 meters.
• Distribution:
  • Jejunum: 40%
  • Ileum: 60%
• Bound by the principle mesentery.
• Root of mesentery.
• Landmarks:
  • Duodenojejunal junction/flexure: left side of L2 vertebra.
  • Ileocolic or ileocecal junction: right side of SI joint.

Jejunum vs. Ileum

• Jejunum

  • Thick wall
  • Vasa recta (long)
  • Peritoneal windows (transparent)
  • 1 or 2 rows of arterial arcades
  • Circular folds (larger and closely set)

• Ileum

  • Thin wall
  • Vasa recta (short)
  • Peritoneal windows (opaque)
  • 4-5 rows of arterial arcades
  • Circular folds (shorter and sparse)
  • Peyer's patches:
    • Solitary nodes

Large Intestine & Colon

• Components:
  • Cecum
  • Ascending colon
  • Transverse colon
  • Descending colon
  • Sigmoid colon
  • Rectum

Large Intestines

• Epiploic (omental) appendices: Small fatty projections
• Teniae coli: Three distinct longitudinal bands of muscle, begin at the base of appendix
• Haustra: Sacculations of colon wall between teniae coli

Cecum

• Blind pouch
• Location: Right lower quadrant, iliac fossa, inferior to the junction of the terminal ileum.
• Ileocecal orifice : Ileocecal valve (fold flaps)
• Vermiform appendix:
  • Blind diverticulum located on the posteromedial aspect of cecum
  • Contains lymphoid tissue
  • Mesoappendix

Ascending & Transverse Colon

• Ascending colon: continuous with cecum on the right side of abdomen
• Ascends to right lobe of liver
• Right colic flexure (hepatic flexure)
• Secondarily retroperitoneal
• Right paracolic gutter
• Transverse colon:
  • Longest and most mobile part of large intestines
  • Right colic flexure to left colic flexure (splenic flexure)
  • Phrenicolic ligament: Attaches splenic flexure to diaphragm
  • Usually hangs down to level of umbilicus but it is variable

Descending & Sigmoid Colon

• Descending colon:
  • Secondarily retroperitoneal
  • Left paracolic gutter
  • Left colic flexure to left iliac fossa
• Sigmoid colon:
  • Sigmoid mesocolon
  • Descending colon to rectum (S3)
• Rectum : True pelvis