Large intestine

Overview

• Terminal part of the gastrointestinal tract
• About 1.5 m (5 ft) long

Function

• Colon: passage of food content and absorption of water and nutrients
• Rectum and anal canal: defecation and fecal continence

Structure and location

Large intestine includes:

• Cecum:
  • Proximal end of the ascending colon; located in the right iliac fossa
  • Includes the vermiform appendix, a tubular structure connected to the cecum 2 cm below the ileocecal junction
• Colon:
  • Ascending: from right iliac fossa to underneath the liver; retroperitoneal
  • Transverse: from underneath the liver to under the spleen; intraperitoneal
  • Descending: from underneath the spleen to the left iliac fossa; retroperitoneal
  • Sigmoid: from left iliac fossa to rectum; intraperitoneal
• Rectum: posterior to the bladder; becomes the anal canal at the coccyx level; partially retroperitoneal
• Anal canal: divided by the dentate (pectinate) line into upper two-thirds and lower third

Related structures

• Greater omentum: layers of peritoneum that are connected to the transverse colon are draped over the bowel.
  • Gastrocolic ligament: part of the omentum; contains the gastroepiploic arteries
  • Gastrosplenic ligament: part of the omentum; contains the short gastric arteries and the left gastroepiploic vessels
• Mesentery: contiguous double layered fold of peritoneum that contains intestinal vessels, lymphatics, and nerves and attaches the bowel to the abdominal wall, which anchors it in place
  • Most of the bowel has a mesentery ;, with the exception of:
    • Cecum
    • Ascending and descending colon, rectum, and anal canal (since they are retroperitoneal)

Vasculature and innervation

		Arteries	Veins	Lymphatics	Innervation
Proximal ⅔ of large intestine Ascending and transverse colon		Branches of superior mesenteric artery (SMA) Ileocolic artery (cecal and appendicular branches) Right colic artery Middle colic artery	Branches of the superior mesenteric vein → portal vein	Superior mesenteric lymph nodes: colon to splenic flexure	Visceral: superior mesenteric plexus
Distal ⅓ of large intestine Descending colon, sigmoid		Branches of inferior mesenteric artery (IMA) Left colic artery Multiple sigmoid arteries	Left colic vein and sigmoid veins that drain into the inferior mesenteric vein → portal vein	Inferior mesenteric lymph nodes: colon from splenic flexure to upper rectum	Visceral: inferior mesenteric plexus
Rectum and anal canal	Above pectinate line	Superior rectal artery, branch of the IMA	Superior rectal vein → inferior mesenteric vein → splenic vein → portal vein	Internal iliac lymph nodes
	Below pectinate line	Internal iliac arteries Middle rectal artery Inferior rectal artery, a branch of the internal pudendal artery	Middle and inferior rectal vein → internal pudendal vein → internal iliac vein → common iliac vein → inferior vena cava	Superficial inguinal lymph nodes	Somatic: inferior rectal branch of pudendal nerve (S2–S4)

• The arc of Riolan: anastomosis between SMA and IMA that provides collateral flow in the setting of arterial occlusion or significant stenosis

The rectum is a major site of porto-systemic anastomoses. In cases of portal hypertension (e.g., in cirrhosis), venous blood is shunted from the portal system through the portosystemic anastomosis between the superior rectal vein (portal system) and the middle and inferior rectal veins into the systemic venous system. Chronic volume overload may result in formation of rectal varices.

The splenic flexure (Griffith's point) and the rectum rectosigmoid region (Sudeck's point) are watershed areas. Limited collateral vascularity means these areas are at highest risk of ischemia if there is hypoperfusion.

References:^[1][2]

Layers of the cecum, colon, and sigmoid

• Mucosa (innermost):
  • Columnar epithelium with lamina propria and mucosa-associated lymphoid tissue (MALT)
• Submucosa:
  • Contains blood vessels and nerves from the Meissner nerve plexus
• Muscularis propria:
  • Inner circular and outer longitudinal muscles
  • Myenteric Auerbach nerve plexus
  • In the colon, the outer longitudinal muscles form the tenia coli and the haustra.
• Serosa (outermost):
  • Consists of visceral peritoneum
  • Contains intestinal glands (or crypts of Lieberkuhn) and a preponderance of goblet cells that secrete mucus

Histology of the rectum and anal canal

• Rectum (above the pectinate line)
  • Columnar epithelium
  • Short crypts of Lieberkuhn
  • Goblet cells
• Anal canal
  • Stratified squamous epithelium that transitions to keratinized stratified squamous epithelium
  • Pain innervation
• Perianal area
  • Keratinized stratified squamous epithelium with pain innervation, hair follicles, and apocrine glands

Whereas the part of the rectum above the pectinate line with columnar epithelium is the site of adenocarcinoma, the lower third of the anal canal is the site of squamous cell carcinoma.

References:^[3]

• Cecum to proximal two-thirds of transverse colon: derived from the midgut
• Distal third of transverse colon to anal canal above the pectinate line: derived from the hindgut; endodermal in origin
• Anal canal below pectinate line: derived from the proctodeum; ectodermal in origin
• The gut is composed of three germ layers:
  • Mesoderm (forms smooth muscle layer)
  • Endoderm (forms epithelial lining)
  • Ectoderm (includes enteric nervous system)

References:^[4]

Colon

• Functional disorders
  • Constipation
  • Irritable bowel syndrome
• Colitis
  • Inflammatory
    • Inflammatory bowel diseases
      • Crohn disease
      • Ulcerative colitis
    • Appendicitis
  • Infectious
    • Necrotizing enterocolitis
  • Ischemic
    • Acute mesenteric ischemia
    • Chronic mesenteric ischemia
    • Colonic ischemia
• Tumors
  • Colonic polyps
  • Polyposis syndromes
    • Familial adenomatous polyposis
    • Juvenile polyposis syndrome
    • Lynch syndrome
    • Peutz-Jeghers syndrome
    • Gardner syndrome
    • Turcot syndrome
  • Colorectal cancer
• Others
  • Intussusception (more common in small intestine)
  • Sigmoid volvulus
  • Angiodysplasia
  • Diverticulosis and diverticulitis
  • Hirschsprung disease

Rectum and anal canal

• Hemorrhoids
• Anal fissures
• Anal atresia
• Cauda equina syndrome (fecal incontinence)

Internal hemorrhoids occur above the pectinate line (superior rectal vein) and are not painful (visceral innervation). External hemorrhoids occur below the pectinate line and are painful because the pudendal nerve provides this area with somatic innervation.