Large intestine

Summary

The large intestine is the terminal portion of the gastrointestinal tract and is derived from the midgut, hindgut, and cloaca. Derivatives of the midgut include the cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon. Derivatives of the hindgut include the descending colon, sigmoid colon, rectum, distal third of the transverse colon, and the portion of the anal canal above the pectinate line. The portion of the anal canal distal to the pectinate line is derived from the cloaca (ectoderm). The superior mesenteric artery supplies the midgut and the inferior mesenteric artery supplies the hindgut. The veins of the midgut and hindgut derivatives drain into the portal vein. The distal anal canal is supplied by the internal pudendal artery and drains into the inferior vena cava. The mesenteric lymph nodes receive lymphatic drainage from the colon. The internal iliac and inguinal lymph nodes drain lymph from the rectum and anal canal. The large intestine is innervated by the autonomic nervous system (mesenteric plexus), except for the distal anal canal, which receives somatic innervation from the pudendal nerve. The mucosa of the large intestine is columnar epithelium, except for the distal anal canal, which is lined with stratified squamous epithelium. The main functions of the large intestine are the absorption of sodium and water, excretion of potassium and bicarbonate, synthesis of vitamin K and group B vitamins (by the colonic bacteria), and the storage and expulsion of feces.

Gross anatomy

Overview

The large intestine is the terminal portion of the gastrointestinal tract and is approx. 1.5 m (5 ft) long. The large intestine is divided into the cecum and appendix, the ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.

Function [1]

  • Absorbs water and electrolytes
  • Absorbs vitamins
  • Eliminates feces

Subdivisions of the large intestine

Cecum

  • Intraperitoneal
  • U-shaped, sac-like structure located in the right iliac fossa
  • The three teniae coli converge at the base of the cecum.
  • Receives chyme through the ileocecal junction

Vermiform appendix [2]

  • Intraperitoneal
  • Blind tubular structure that arises from the base of the cecum
  • Located in the right iliac fossa
  • The position of its free end is variable.
    • The most common position is retrocecal
    • Other positions include paracecal , preileal , postileal , and pelvic

McBurney point represents where the base of the appendix lies in the right iliac fossa. Tenderness at this point is a sign of acute appendicitis.

Colon

The intraperitoneal parts of the large intestine (i.e., the cecum, transverse colon, and sigmoid colon) are susceptible to volvulus.

Rectum

Because the rectovesical (m)/rectouterine (f) pouch is the most caudal aspect of the peritoneal cavity, fluids (e.g., ascitic fluid, blood, pus) tend to collect in this space.

Anal canal

Proximal to the pectinate line Distal to the pectinate line
Embryologic origin
Epithelium
Arterial supply
Venous drainage
  • Portal system
  • Caval system
Lymphatic drainage
Innervation

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.

The pudendal nerve (S2–S4) innervates the external anal sphincter. Injury to this nerve (e.g., during childbirth) can cause fecal incontinence and perianal sensory loss.

Vasculature and innervation of the large intestine [3]

Arteries Veins Lymphatics Innervation

Midgut derivatives

Cecum, appendix, ascending colon, proximal 2/3 of the transverse colon

  • Branches of the superior mesenteric veinportal vein

Hindgut derivatives

Distal 1/3 of the transverse colon, descending colon, and sigmoid colon

Rectum and anal canal Above pectinate line
Below pectinate line
  • Middle and inferior rectal veininternal pudendal veininternal iliac vein → common iliac veininferior vena cava

The rectum is a site of portosystemic anastomosis. Rectal varices develop in portal hypertension due to the shunting of venous blood from the superior rectal vein (portal system) to the middle and inferior rectal veins (systemic or caval circulation).

The watershed areas of the splenic flexure (Griffiths point) and rectosigmoid junction (Sudeck point) are the regions of the colon at highest risk of ischemia secondary to hypoperfusion.

References:[5][6][7]

Microscopic anatomy

Mucosa Submucosa Muscularis propria Serosa
Cecum and colon
Appendix
  • Contains numerous lymphoid follicles that distort the mucosal crypts
  • Contains blood vessels and the Meissner plexus
Rectum and anal canal Above the pectinate line
  • Adventitial tissue; no serosa
Below the pectinate line

Anal cancers are typically adenocarcinomas above the pectinate line and squamous cell carcinomas below the pectinate line.

References:[8]

Function

  • Ascending and transverse colon [1]
    • Absorption of Na+ and Cl-
    • Absorption of water along the osmotic gradient created by Na+ absorption
    • Aldosterone increases the absorption of Na+ and water as well as the excretion of K+ and HCO3- from the colon.
    • Synthesis of vitamin K and B vitamins by colonic bacteria
    • Solidification of chyme into stool
    • Lubrication for the passage of feces
  • Distal colon, rectum, and anal canal: storage, propulsion, and expulsion of feces

Na+ and water are reabsorbed by the colon, while K+ and HCO3- are excreted.

Embryology

References:[9]

Clinical significance

Colon

Rectum and anal canal