Gastrointestinal tract

The gastrointestinal tract (GIT) is the system of organs that allows for the consumption and digestion of food, absorption of nutrients, and excretion of waste in the form of fecal matter. It includes the oral cavity, esophagus, stomach, small intestine, and large intestine. It is derived from the primitive gut tube and can be divided into the foregut, midgut, and hindgut, each of which is distinct in its embryological development and neurovascular supply. The veins of all three embryological segments drain directly or indirectly into the portal vein, which is connected to the caval venous system via a system of portocaval shunts. The GIT is innervated by the sympathetic nervous system, parasympathetic nervous system, and enteric nervous system, the last of which is unique to the GIT. All organs of the GIT are composed of four histological layers: mucosa, submucosa, muscularis propria, and serosa (intraperitoneal organs) or adventitia (extraperitoneal organs). The secretory and regulatory products of the gastrointestinal tract vary from segment to segment depending on the regional histological characteristics, presence of specialized cells, and function.

This article provides an overview of the gastrointestinal tract as a whole. The oral cavity, stomach, small intestine, and large intestine are covered in detail in their own articles, as are the accessory organs such as the liver, gall bladder, and pancreas. The peritoneum and major blood vessels of the abdominal cavity are covered in the “Abdominal cavity” article.

Organs ^[1]

• Parts of the gastrointestinal tract
  • Oral cavity and pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
• Accessory organs to the gastrointestinal tract
  • Liver
  • Gall bladder
  • Pancreas

Function

• Ingestion of food and water
• Mastication
• Transport of the food bolus or chyme through the gastrointestinal tract
• Digestion (i.e., breaking food down into nutrients)
• Absorption of nutrients and water
• Excretion of waste as feces
• Immunity

Developmental derivatives of the primitive gut tube ^[1]

The organs of the gastrointestinal tract and their associated neurovascular structures can be grouped into three groups based on their development from the primitive gut tube: foregut, midgut, hindgut. (For more info on the embryological development of the gastrointestinal tract, see the “Embryology” section below.)

Foregut derivatives are supplied by the celiac artery, midgut derivatives by the superior mesenteric artery, and hindgut derivatives by the inferior mesenteric artery.

Referred pain from the foregut is felt in the epigastrium, pain from the midgut in the umbilical region, and pain from the hindgut in the hypogastrium.

For more detailed information about the branches, course, and supply by individual vessels, see the corresponding sections in the "Abdominal cavity" article.

Arterial supply

• Most of the GIT is supplied by the anterior (unpaired) branches of the abdominal aorta. In contrast, non-GI organs in the abdominal cavity are supplied by the posterior and/or lateral (paired) branches of the abdominal aorta.
• Exceptions:
  • Esophagus: supplied by the esophageal branches of the inferior thyroid artery, the thoracic aorta, and the left gastric artery
  • Distal anal canal: supplied by the middle rectal artery and the inferior rectal artery
  • Watershed zones
    • Splenic flexure: supplied by the branches of both superior and inferior mesenteric arteries
    • Rectosigmoid junction: supplied by the superior rectal artery and last sigmoid branch of the inferior mesenteric artery

Venous drainage

• The veins of most of the gastrointestinal tract drain directly or indirectly into the portal vein.
• Exceptions:
  • Esophagus: drains into the inferior thyroid vein, the azygos vein, hemiazygos vein, and the left gastric vein
  • Distal anal canal (below the pectinate line): drains into the inferior vena cava
• For more information about the tributaries of each individual vein and description of portocaval anastomoses and cavocaval anastomoses, see the “Abdominal cavity” article.

Innervation of the gastrointestinal tract ^[2]

• The gastrointestinal tract is innervated by the autonomic nervous system.
  • Extrinsic innervation: parasympathetic and sympathetic nervous system
  • Intrinsic innervation: enteric nervous system
    • Embryologically derived from the neural crest
    • Regulated by the autonomic nervous system but can function independently
    • Crucial for gastrointestinal motility and secretion
• For more information about the extrinsic and intrinsic innervation, see “Nerves of the abdominal cavity” and “Enteric nervous system” correspondingly.

Motility and contractile activity of the gastrointestinal tract ^{[3] [4]}

The sympathetic system has an inhibitory effect on the gastrointestinal tract.
The parasympathetic system promotes gastrointestinal secretion and motility.
The enteric nervous system can function independently of the central nervous system.

Layers of the gastrointestinal tract

All organs of the gastrointestinal tract have four histological layers.

Erosive gastritis only affects the mucosa, whereas peptic ulcers also affect the deeper submucosal layer.

The Submucosal layer contains the MeiSSner plexus and produces Secretions.Remember the layers of gut wall from inside to outside with MSMS: Mucosa, Submucosa, Muscularis propria, Serosa.

Regional histological characteristics of the gastrointestinal tract

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.

Brunner glands, which produce alkaline secretions (e.g., bicarbonate) to neutralize stomach acid, are often hyperplastic in the tissue surrounding duodenal ulcers because it is frequently exposed to excess acid.

The secretions of the pancreas are also involved in digestion. For a description of these, see “Exocrine pancreas” and “Endocrine pancreas.”

Autoimmune destruction of parietal cells → ↓ intrinsic factor production → ↓ absorption of B₁₂ in the terminal ileum → pernicious anemia

Elevated serum gastrin levels can be used to support the diagnoses of atrophic gastritis and Zollinger-Ellison syndrome.

Ghrelin is increased in Prader-Willi syndrome and decreased following gastric bypass surgery.

Ghrelin causes greed for food.

The antibiotic erythromycin belongs to the class of drugs called motilin agonists, which can induce peristalsis and increase gastric emptying.

Octreotide, a somatostatin analog, is used to treat bleeding esophageal varices, gastrinomas, severe noninfectious diarrhea (e.g., chemotherapy-induced diarrhea, carcinoid syndrome), glucagonoma, and acromegaly.

VIPoma is a VIP-secreting tumor that can manifest with WDHA syndrome: Watery Diarrhea, Hypokalemia, and Achlorhydria.

In achalasia, degeneration of inhibitory neurons within the myenteric plexuses (Auerbach plexus) → deficient inhibitory neurotransmitters such as nitric oxide and vasoactive intestinal peptide → higher resting pressures of the lower esophageal sphincter.

• Primitive gut tube : Most of the gastrointestinal tract is derived from the primitive gut tube, which is a derivative of the endodermal lining of the yolk sac; ; : following craniocaudal and lateral embryonic folds. (See “Morphogenesis” in the article on embryogenesis.)
  • 4^th week of development: The three regions of the primitive gut tube become distinguishable (i.e., foregut, midgut, and hindgut), and each region has its own neurovascular supply.
  • Most organs of the gastrointestinal tract are derived from these three regions. (See “Foregut, midgut, and hindgut derivatives and corresponding neurovascular structures” above.)
• Non-gut tube derivatives
  • Buccopharyngeal membrane: oral cavity
  • Cloacal membrane: the anal canal distal to the pectinate line
  • Dorsal mesentery: spleen, dorsal pancreatic bud, the gastrosplenic ligament, splenorenal ligament, and the greater omentum
  • Ventral mesentery: liver, ventral pancreatic bud, the falciform ligament, and the lesser omentum
  • Somatic mesoderm: parietal peritoneum
  • Splanchnic mesoderm: visceral peritoneum
• Germ layers of the gastrointestinal tract
  • Endoderm : epithelium and mucosal glands of the gastrointestinal tract and the parenchyma of the liver and pancreas (the splenic parenchyma is derived from mesoderm.)
  • Mesoderm: lamina propria, muscularis mucosa, submucosa, and muscularis propria
  • Ectoderm (neural crest cells): enteric nervous system
• Rotation of the foregut: 90° clockwise around its longitudinal axis
  • The anterior border forms the lesser curvature and the posterior border forms the greater curvature of the stomach.
  • The liver and duodenum move to the right, while the spleen and pancreas move to the left of the abdominal cavity.
  • The left vagus nerve is then anterior to the stomach, while the right vagus is posterior to it.
• Rotation of the midgut: 270° counterclockwise around the superior mesenteric artery (SMA)
  1. 4^th week: midgut elongation in utero
  2. 6^th week: physiological herniation of the midgut out of the umbilical ring; → 90° counter-clockwise rotation
  3. 10^th week: reentry of the midgut into the abdominal cavity → 180° rotation around the superior mesenteric artery inside the abdominal cavity (for a total of 270°) → fixation of the duodenojejunal flexure and cecum to the posterior abdominal wall

Omphalocele is caused by a failure of the midgut to return to the abdominal cavity after its physiological herniation through the umbilical ring. Accordingly, the amniotic membrane and peritoneum form the hernial sac of the omphalocele.

Gastroschisis is caused by a developmental defect of the abdominal wall through which intestinal loops herniate out directly (i.e., the hernial sac is absent).

A congenital umbilical hernia is caused by a protrusion of intra-abdominal contents through a patent umbilical orifice. The peritoneum is the hernial sac of an umbilical hernia.

• See “Clinical significance” of the:
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Liver
  • Pancreas
  • Oral cavity and teeth
  • Gall bladder and bile
• Diseases of the salivary glands
• Spontaneous bacterial peritonitis
• Ascites
• Carcinoid tumor
• Esophageal atresia