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)

  • Anterior branch of the abdominal aorta that arises at the level of the first lumbar vertebra (L1)
  • 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)

Colonic anastomosis

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

References:[1]

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: composed of the visceral peritoneum

Regional characteristics

Structure

Epithelium

Mucosal cells and glands

Notable features Functions

Oral cavity

  • Lubrication of food
  • Antibacterial protection
  • IgA secretion
Esophagus
  • 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

  • Villi
  • Plicae
  • Crypts of Lieberkühn

Jejunum

  • Secretions
    • Lysozyme
    • Cholecystokinin (CCK)
    • Secretin

Ileum

  • Endocytosis of antigens and transport to lymphoid tissue
Colon
  • No villi
  • Crypts of Lieberkühn
  • Active transport of Na+ and passive reabsorption of water from the intestinal lumen
Rectum and anal canal
  • 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.

Gastrointestinal function and regulators

Molecule Source Action Regulation
Cholecystokinin
  • ↑ Contraction of the gallbladder
  • ↑ Secretion of pancreatic enzymes
  • ↑ Relaxation of the sphincter of Oddi
  • ↓ Gastric emptying
Secretin

Gastric inhibitory polypeptide (GIP)

Motilin
  • ↑ Intestinal motility (migratory motor complexes)
  • ↑ During fasting
Vasoactive intestinal peptide (VIP)
  • ↑ Secretion of water and electrolytes
  • ↑ Relaxation of sphincters
Nitric oxide (NO)

Embryology

Embryology of the gastrointestinal tract

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.

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

Clinical significance

Oral cavity

Esophagus

Stomach

Small intestine

Large intestine

Liver, gallbladder, and pancreas

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last updated 05/09/2019
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