Large intestine

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.

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

• Connects the cecum proximally and the rectum distally
• Ascending colon
  • Retroperitoneal
  • Ascends along the right posterolateral abdominal wall from the cecum up to the right subcostal region, where it makes a 90° turn to the left (hepatic flexure)
• Transverse colon
  • Intraperitoneal
  • Extends from the hepatic flexure to the splenic hilum, where it makes a 90° turn caudally (splenic flexure)
• Descending colon
  • Retroperitoneal
  • Descends along the left posterolateral abdominal wall from the splenic flexure to the left iliac fossa
• Sigmoid colon
  • Intraperitoneal
  • The S-shaped terminal portion of the descending colon located in the left iliac fossa
  • Extends up to the S3 vertebra

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

Rectum

• Partially retroperitoneal; does not have a mesentery
• Extends from the S3 vertebra to the anorectal junction
• The peritoneum on the anterior rectal wall reflects anteriorly onto the bladder in males and the uterus in females, forming a blind pouch.
  • Rectovesical pouch (in males): separates the anterior wall of the rectum from the posterior wall of the bladder, the prostate, the seminal vesicles, and vas deferens
  • Rectouterine pouch (in females): separates the anterior wall of the rectum from the posterior wall of the uterus and upper vagina
• Anorectal junction
  • Located at the pelvic diaphragm
  • Anorectal flexure
    • Puborectalis muscle forms a sling around the anorectal junction, forming a posterior curve
    • Contributes to fecal continence

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

• Extraperitoneal
• The terminal part of the gastrointestinal tract
• Approx. 4 cm long
• Extends from the anorectal junction at the pelvic diaphragm to the anal orifice
• Pectinate line (dentate line)
  • A circumferential scalloped line formed by the anal valves at the inferior end of the anal columns
  • Divides the anal canal into proximal and distal thirds

• Internal anal sphincter
  • An involuntary smooth muscle sphincter
  • Innervated by the autonomic nervous system
• External anal sphincter
  • Voluntary skeletal muscle sphincter
  • Innervated by the somatic nervous system (inferior rectal branch of the pudendal nerve)
• Anal glands lie in the intersphincteric groove between the internal and external anal sphincters.

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]

• Watershed areas of the colon: ^[4]
  • The splenic flexure (Griffiths point): junction between the SMA and IMA, located between the left colic artery and the marginal artery of Drummond
  • Rectosigmoid junction (Sudeck point): junction between the IMA and internal iliac artery, located between the last sigmoid branch of the IMA and the superior rectal artery

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]

• The large intestine is composed of the same four histological layers of the alimentary canal
• Teniae coli and haustra
  • Only present in the muscularis propria of the cecum and colon
  • Not present in the sigmoid colon, rectum, or anal canal
  • Converge at the base of the appendix

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

• 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
  • Defecation process
    1. Involuntary relaxation of the internal anal sphincter for anal sampling: prior to defecation, a small amount of rectal content passes into the anal canal to test consistency (gas, liquid, or solid)
    2. Voluntary relaxation of the external anal sphincter and puborectalis muscle and contraction of abdominal muscles or Valsalva maneuver expel stool from the anal canal
    3. Contraction of external anal sphincter to reestablish continence (closing reflex)

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

• Midgut: from the distal duodenum through the proximal two-thirds of the transverse colon
• Hindgut (endoderm): from the distal third of the transverse colon through the anal canal above the pectinate line
• Proctodeum (ectoderm): anal canal below the pectinate line
• See “Embryology” in “Gastrointestinal tract.”

Colon

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

Rectum and anal canal

• Hemorrhoids
• Anal fissures
• Anal atresia
• Cauda equina syndrome (fecal incontinence)
• Anal cancer
• Anal abscess and fistula