Small & Large Intestines Summary

Food Passage & Digestive System

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestines: Duodenum, jejunum, ileum
  • Large intestines: Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal
  • Foregut: Celiac trunk
  • Midgut: Superior mesenteric artery
  • Hindgut: Inferior mesenteric artery

Arteries of the Abdomen

  • Diaphragm
  • Inferior phrenic arteries
  • Celiac trunk
  • Middle suprarenal artery
  • Left renal artery
  • Superior mesenteric artery
  • Testicular or ovarian arteries
  • Lumbar arteries
  • Inferior mesenteric artery
  • Common iliac artery
  • Median sacral artery
  • Psoas major muscle

Superior Mesenteric Artery

  • Supplies midgut: jejunum, ileum, cecum, appendix, ascending colon, and transverse colon
  • Arises from the abdominal aorta at the level of the L1 vertebrae
  • Branches:
    • Inferior pancreaticoduodenal artery
    • Middle colic artery: to transverse colon
    • Right colic artery: to ascending colon
    • Ileocolic artery: to cecum
    • Appendicular artery
    • Jejunal arteries
    • Ileal arteries

Superior & Inferior Mesenteric Veins

  • Veins mirror arteries and drain into superior or inferior mesenteric vein.

Superior Mesenteric Vessels

  • 15-18 branches for jejunum and ileum
  • Unite to form loops called arterial arcades that give rise to vasa recta
  • Superior mesenteric vein drains jejunum and ileum
  • Anterior and right of SMA in root of mesentery
  • Unites with splenic vein (posterior to neck of pancreas) to form portal vein
  • Blood vessels are surrounded by superior mesenteric plexus
  • Sympathetic from T8-T10 (greater and lesser splanchnic nerves)
  • Parasympathetic from Vagus nerve

Jejunum & Ileum (small intestines)

  • 6-7 meters long
  • 40% jejunum
  • 60% ileum
  • Principle mesentery
  • Root of mesentery
  • Duodenojejunal junction/flexure (left side of L2) to ileocolic junction (right side of SI joint)

Small Intestines – Innervation

  • Sympathetic
    • T8-T10 via superior mesenteric plexus
    • Greater (T5-9), lesser (T10-11), & least splanchnic (T12) nerves
    • Synapse on cell bodies of postsynaptic sympathetic neurons of celiac & superior mesenteric ganglia
  • Parasympathetic
    • Posterior vagal trunks
    • Synapse with postsynaptic neurons in intestinal wall

Large Intestines

  • Cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, & anal canal
  • Epiploic (omental) appendices:
    • Small fatty projections
  • Teniae coli
    • Three distinct longitudinal bands of muscle
    • Begin at the base of the appendix
  • Haustra
    • Sacculations of colon wall between teniae coli
  • Toldt fascia

Cecum

  • Blind pouch
  • Right lower quadrant, iliac fossa
  • Inferior to the junction of the terminal ileum & ileocecal orifice/valve
  • 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 is continuous with cecum on the right side of the abdomen
  • Ascends to the right lobe of the 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 the level of the 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

Blood Supply – Superior Mesenteric Artery

  • Cecum
    • Ileocolic artery – terminal branch of SMA
    • Appendicular artery
  • Ascending colon and right colic flexure
    • Right colic artery from SMA
  • Transverse colon
    • Middle colic artery
  • Marginal artery – anastomotic artery between right colic artery, middle colic artery, left colic artery (IMA) and sigmoid arteries (IMA)
    • Parallels and extends the length of the colon
  • Veins of the same name drain into the superior mesenteric vein

Inferior Mesenteric Vessels

  • Branch of Abdominal Aorta
  • Descending and Sigmoid colon = Hindgut
  • Inferior Mesenteric artery (at level of L3)
    • Left colic artery
    • Sigmoid arteries
    • Superior rectal artery
  • Inferior Mesenteric vein
    • Drains into splenic vein then hepatic portal vein

Colon– Innervation

  • Ascending & Transverse Colon
    • Same as small intestines
    • Superior mesenteric nerve plexus provides sympathetic and parasympathetics (vagal)
  • Descending & Sigmoid Colon
    • Sympathetic
      • Lumbar splanchnic nerves to inferior mesenteric plexus & superior hypogastric plexus
    • Parasympathetic
      • Pelvic splanchnic nerves (S2-S4) to inferior hypogastric plexus