Gastrointestinal tract

Gross anatomy

Overview

Peritoneum and retroperitoneum

Layers of peritoneum

Peritoneal cavity

Epiploic foramen (foramen of Winslow)

Organs of the peritoneal cavity

Intraperitoneal organs

Secondary retroperitoneal organs

Retroperitoneal organs
Stomach Duodenum (2nd and 3rd part) Kidneys
Duodenum (1st part) Ascending colon Adrenal glands
Jejunum Descending colon Ureters
Ileum Upper rectum Aorta
Cecum/ appendix Pancreas (head, neck, body) Inferior vena cava
Transverse colon Lower rectum
Sigmoid colon Anal canal
Liver and gallbladder
Spleen
Pancreas (tail)
Bladder
Genital organs

Vasculature

The abdominal aorta is the direct continuation of the descending thoracic aorta and gives rise to all the vessels that supply the organs of the abdominal cavity.

Celiac trunk

Superior mesentery artery (SMA)

  • SMA passes anterior to the third segment of the duodenum
  • Inferior pancreaticoduodenal artery
    • Anastomosis with the superior pancreaticoduodenal artery
    • Supplies the head of the pancreas
  • Intestinal arteries (form vascular arcades and vasa rectae): supply the jejunum and ileum
  • Ileocolic artery: supplies the distal ileum, the cecum, and appendix
  • Right colic artery: supplies the ascending colon
  • Middle colic artery: supplies the proximal two-thirds of the transverse colon

Inferior mesentery artery (IMA)

  • Left colic artery: supplies the distal third of the transverse colon and descending colon
  • Sigmoid artery: supplies the sigmoid colon
  • Superior rectal artery: supplies the superior part of the rectum and anal canal

Colonic anastomosis

  • Marginal artery (of Drummond): anastomosis between the superior and inferior mesentery artery. It is a continuous artery that travels along the mesenteric side of the colon.
  • Central anastomotic mesenteric artery (arc of Riolan): anastomosis between the middle colic artery and the left colic artery
  • Cannon point: area where the innervation of the colon changes from the vagus nerve to the pelvic splanchnic nerves
    • Proximal: innervation mediated by the vagus nerve
    • Distal: innervation mediated by pelvic splanchnic nerves

The splenic flexure and the rectosigmoid junction are watershed areas, which are most vulnerable to bowel ischemia.

Venous drainage of the GI tract

Portocaval anastomoses

Microscopic anatomy

Overview

The gastrointestinal tract is composed of four main layers. Although variations exist between each part of the gut, the general structure remains similar. The 4 layers are as follow:

  • Mucosa
    • Epithelium: contains glands that aid in digestion
    • Lamina propria: attachment site for epithelial cells
    • Muscularis mucosae
  • Submucosa: contains the Meissner plexus (submucous)
    • Provides parasympathetic innervation to glands
    • Responsible for the secretomotor activity of the GIT
  • Muscularis externa: contains the Auerbach plexus (myenteric)
  • Serosa or adventitia

Regional characteristics

Structure

Epithelium

Mucosal cells and glands

Notable features Functions

Oral cavity

  • Lubrication of food
  • Antibacterial protection
  • IgA secretion
Esophagus
  • Skeletal muscle in upper third
  • Smooth muscle in lower third
  • Transmits the bolus of food from the pharynx to the stomach
  • The lower esophageal sphincter prevents regurgitation of food from the stomach back into the esophagus (failure to do so causes GERD).

Stomach

  • Body and fundus: shallow pits, deep glands
  • Pylorus: deep pits, shallow, branched glands

Duodenum

  • Brunner glands
  • Goblet cells
  • Paneth cells
  • Enteroendocrine (EE) cells

Jejunum

  • Secretions
    • Lysozyme
    • Cholecystokinin (CCK)
    • Secretin

Ileum

  • Endocytosis of antigens and transport to lymphoid tissue
Colon
  • Active transport of Na+ and passive reabsorption of water from the intestinal lumen
Rectum
  • Contains inner circular and outer circular muscle layers
  • Voluntary and involuntary control of defecation

Intestinal stem cells are located in the crypts of Lieberkuhn. These divide to replace all of the cells of the intestinal epithelium every 5 days.

Embryology

The gastrointestinal tract (GIT) is derived from the primitive gut tube, which is divided into foregut, midgut, and hindgut. The epithelium arises from endoderm, which also gives rise to the parenchyma of the liver and pancreas. Although the spleen is in the peritoneal cavity, it is derived from mesoderm rather than endoderm.

  • Foregut
    • 4th week
    • Derived from the respiratory diverticulum.
    • The tracheopharyngeal septum divides the trachea and the esophagus.
    • Forms the esophagus, stomach, duodenum (1st and 2nd part), liver, pancreas, biliary apparatus, and gallbladder
    • Blood supply from the celiac artery
  • Midgut
    • 4th week
    • Starts below the 2nd portion of the duodenum (3rd and 4th part)
    • Forms the distal part of the duodenum, jejunum, ileum, cecum, appendix, ascending colon, and proximal transverse colon
    • Blood supply from the superior mesentery artery
    • Rotates 270° counterclockwise around the superior mesenteric artery (SMA) outside of abdominal cavity in the 10th week
  • Hindgut
    • 4th week
    • Connects with the allantois ventrally and the cloaca caudally
    • Forms the distal transverse colon; up to the splenic flexure, descending colon, sigmoid colon, rectum, and the anal canal above the pectinate line
    • Blood supply from the inferior mesentery artery

The midgut herniates through the umbilical ring during the 6th week and returns to the abdominal cavity on the 10th week.

Gastrointestinal function and regulators

Molecule Source Action Regulation
Cholecystokinin
  • I cells
    • Duodenum
    • Jejunum
  • ↑ Contraction of the gallbladder
  • ↑ Secretion of pancreatic enzymes
  • ↑ Relaxation of the sphincter of Oddi
  • ↓ Gastric emptying
Secretin
  • S cells: duodenum

Gastric inhibitory polypeptide (GIP)

  • K cells
    • Duodenum
    • Jejunum
Motilin
  • Small intestine
  • ↑ Intestinal motility (migratory motor complexes)
  • ↑ During fasting
Vasoactive intestinal peptide (VIP)
  • ↑ Secretion of water and electrolytes
  • ↑ Relaxation of sphincters
Nitric oxide (NO)
  • ↑ Relaxation of smooth muscle and sphincters (e.g., LES)

Clinical significance

Oral cavity

Esophagus

Stomach

Small intestine

Large intestine

Liver, gallbladder, and pancreas

last updated 11/26/2018
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