Large Intestine

OBJECTIVES

  • Anatomy of the large intestine and the appendix.

  • Mechanism of referred pain.

  • Blood supply and innervation.

  • Clinical cases:

    • Acute appendicitis.

    • Crohn's disease.

    • Hirschsprung's disease.

    • Volvulus.

    • Cancer of the large bowel.

    • Mesenteric ischemia.

    • Blood vessels of the gut: Celiac artery (CA), Superior mesenteric artery (SMA), Inferior mesenteric artery (IMA).

ANATOMY OF THE LARGE INTESTINE

Parts of the Large Intestine

  1. Cecum and Appendix

  2. Colon - which consists of:

    • Ascending colon

    • Transverse colon

    • Descending colon

    • Sigmoid colon

  3. Rectum and Anal Canal

Features of the Large Intestine

  1. Appendices Epiploicae: Fat-filled pouches of peritoneum projecting from the serosal surface of the colon.

  2. Taeniae Coli: Aggregations of longitudinal smooth muscle fibers forming 3 ribbon-like bands on the outer wall of the large intestine, from cecum to sigmoid colon. These bands are:

    • Tenia libera

    • Tenia mesocolica

    • Tenia omentalis

    • These bands merge to form a complete layer around the rectum, serving as a guide during surgical procedures to locate the appendix.

  3. Sacculations (Haustrations): Small sac-like pouches giving the colon a segmented appearance, formed by the shorter teniae coli relative to the underlying circular smooth muscle layer.

  4. Notably, the appendix and rectum do not exhibit taeniae, appendices epiploicae, or haustra.

CECUM

  • The cecum is a blind pouch marking the beginning of the large intestine.

  • Location: Right iliac fossa, above the inguinal ligament.

  • Continuous superiorly with the ascending colon.

  • The ileum opens into the cecum at the ileocecal junction.

  • The appendix opens into the medial posterior wall of the cecum, an inch below the ileocecal valve.

  • The cecum is entirely covered by peritoneum, allowing for significant mobility.

  • Tenia coli on the cecum help in locating the appendix during surgical procedures.

Relations of the Cecum

  • Anteriorly and Medially: Anterior abdominal wall, greater omentum, and coils of small intestine.

  • Medially: Ileocecal junction.

  • Posteriorly: Retrocecal recess of peritoneum containing the vermiform appendix in 64% of subjects.

  • Behind the recess, the cecum is in relation to: iliopsoas muscle, external iliac artery, genitofemoral nerve, femoral nerve, and lateral cutaneous nerve of the thigh.

ILEOCECAL VALVE

  • The terminal part of the ileum forms the ileocecal valve, consisting of two segments: an upper and lower fold joined on each side to form a mucosal ridge known as the frenulum.

  • The circular muscle coat of the ileum is thickened at the valve, creating a one-way muscle valve (ileocecal sphincter) that prevents reflux of colonic contents into the small intestine.

  • Dysfunction of this valve is linked with Crohn’s disease.

Blood Supply of the Cecum

  • Arteries: Anterior and posterior cecal arteries from the ileocolic artery, a branch of the SMA.

  • Veins: Correspond to the arteries and drain into the SMV.

APPENDIX

  • The appendix is a worm-like tube with a thick wall and narrow lumen, average length of 10 cm and average diameter of 5 mm.

  • Contains an abundance of lymphoid tissue, serving an immune function rather than a digestive one.

  • The appendicular structure opens into the cecum, one inch or less below the ileocecal valve.

Position of the Appendix

  • Various possible positions:

    • Retrocecal: Most common (64%), occupying the retrocecal recess behind the cecum.

    • Pelvic: Points toward the pelvis in 32% of subjects; relates to the right ovary and uterine tube in females.

    • Subcecal: Below the cecum, in about 2% of subjects.

    • Preileal: In front of the terminal part of the ileum, in 1% of subjects.

    • Postileal: Behind the terminal part of the ileum in 0.5% of subjects.

Psoas Sign

  • In cases of appendicitis, irritation of the psoas muscle may cause the patient to keep the right hip joint flexed.

Blood Supply of the Appendix

  • Arteries: The appendicular artery (end artery) typically branches from the ileocolic artery or the posterior cecal artery, reaching the tip of the appendix via the mesoappendix.

  • This artery's infection can result in thrombosis at the appendix's tip, leading to gangrene and necrosis.

  • Veins: The appendicular vein drains into the ileocolic vein.

McBURNEY'S POINT

  • The junction between the lateral 1/3 and medial 2/3 of the spino-umbilical line.

  • Surface marking of the base of the appendix; maximal tenderness here indicates acute appendicitis.

  • Tenderness and rebound pain suggest inflammation (appendicitis).

  • Retrocecal position is most common.

ACUTE APPENDICITIS

  • Characterized by sudden onset of abdominal pain.

  • Classic symptom: Pain migrating from the navel (periumbilical) to the right lower quadrant, becoming constant and severe.

  • Prevalence: Most common cause of acute abdominal pain requiring surgery in the US; over 5% of the population experiences appendicitis.

  • Age: Most common in teens and 20s but can occur at any age.

First & Second Pain in Appendicitis

  • First Pain (0-12 hrs): Visceral pain from the appendix, felt as a dull ache in the periumbilical area; caused by lumen distention or muscle spasm. This pain is carried by afferent fibers via the lesser splanchnic nerve, entering at the T10 spinal segment.

  • Second Pain: If the parietal peritoneum becomes involved, the pain shifts to McBurney’s point, becoming precise, severe, and localized due to the mediation by the lower intercostal (T10, T11) and subcostal nerves.

COLON

  • Ascending Colon: Lies retroperitoneally, missing a mesentery; joins with the transverse colon at the right (hepatic) flexure.

  • Transverse Colon: Has a mesentery (transverse mesocolon, intraperitoneal position) and connects to the descending colon at the left (splenic) flexure.

  • Sigmoid Colon: Suspended by the sigmoid mesocolon (intraperitoneal). Ascending and descending colons are secondary retroperitoneal organs.

RECTUM & ANAL CANAL

  • The rectum is a straight, terminal portion of the hindgut; its superior 1/3 is covered by peritoneum while the lower 1/3 is not.

  • The anal canal measures about 1.5 inches long, opening distally at the anus, and makes a 90° bend (anorectal flexure) below the rectum.

SYMPATHETIC INNERVATION

Midgut

  • Origin: T10 – T11 segments of the spinal cord (IML horn). Preganglionic fibers travel through sympathetic trunk and lesser splanchnic nerves to the superior mesenteric ganglion, where they synapse, and long postganglionic fibers target branches of the SMA.

Hindgut

  • Origin: IML horn of L1 - L2 segments → lumbar splanchnic nerves (preganglionic) → inferior mesenteric plexus (where they synapse) → postganglionic fibers reach organs around branches of IMA. This area includes the distal colon, rectum, lower ureter, bladder, internal urethral sphincter, reproductive organs, and lower limb blood vessels & skin.

Diagrams

  • Include diagrams of sympathetic and parasympathetic innervation.

BLOOD VESSELS OF THE GUT

Classification of Blood Supply

  • Foregut: Celiac artery (CA)

  • Midgut: Superior mesenteric artery (SMA)

  • Hindgut: Inferior mesenteric artery (IMA)

Branches of the Abdominal Aorta to the GIT

  • Celiac trunk originates from the aorta at T12.

  • SMA originates from L1 and branches to the organs of the midgut.

  • Renal arteries originate at L2; IMA originates at L3.

  • Two terminal branches are common iliac arteries at L4.

CELIAC ARTERY (TRUNK)

  • Origin: T12.

  • It gives rise to three branches:

    • Left gastric artery

    • Common hepatic artery

    • Splenic artery

Left Gastric Artery

  • Courses upward to the left, reaching the upper end of the lesser curvature of the stomach. It can be subject to erosion by a penetrating ulcer.

  • Branches: Esophageal branches and Gastric branches supply the lesser curvature of the stomach and anastomose with the right gastric artery.

Common Hepatic Artery

  • Passes to the right at the first part of the duodenum, where it bifurcates into:

    • Proper hepatic artery

    • Gastroduodenal artery

Proper Hepatic Artery

  • Supplies the liver, giving off:

    • Right gastric artery

    • Right and left hepatic arteries.

  • The right hepatic artery supplies the gallbladder via the cystic artery.

Gastroduodenal Artery

  • Descends posterior to the first part of the duodenum; branches into:

    • Right gastroepiploic artery

    • Superior pancreaticoduodenal artery.

Splenic Artery

  • Runs horizontally to the left along the upper border of the pancreas, giving branches to the spleen and pancreas.

SUPERIOR MESENTERIC ARTERY

  • Origin: L1, just below the celiac trunk.

  • Travels right of the abdominal aorta; important for understanding SMA syndrome (angle < 25 degrees).

  • Branches include: Inferior pancreaticoduodenal, jejunal, ileal, ileocolic, appendicular, right colic, and middle colic arteries.

INFERIOR MESENTERIC ARTERY

  • Origin: L3, running to the left; branches include:

    • Left colic artery

    • Sigmoid arteries

    • Superior rectal artery

  • The SMA and IMA are interconnected via the marginal artery of Drummond, allowing for collateral circulation.

MESENTERIC ISCHEMIA

  • Commonly caused by atherosclerosis, preventing proper blood flow and resulting in tissue ischemia.

  • Primarily affects the SMA and small intestine; risk factors include age over 60, smoking, and high cholesterol.