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Gastrointestinal tract

Last updated: May 11, 2021

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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]

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

Overview of arterial blood supply of the gastrointestinal tract

Artery

Characteristics Branches Region supplied
Celiac trunk
Superior mesenteric artery
Inferior mesenteric artery

Venous drainage

Overview of venous drainage from the gastrointestinal tract
Vein Characteristics Areas drained Drains into
Portal vein

Superior mesenteric vein

(SMV)

Inferior mesenteric vein

(IMV)

Innervation of the gastrointestinal tract [2]

Overview of the innervation of the gastrointestinal tract
Innervation Foregut Midgut Hindgut Effect
Extrinsic innervation Parasympathetic innervation
  • ↑ Secretion
  • ↑ Motility
  • ↓ Sphincter tone

Sympathetic innervation (prevertebral ganglia)

  • ↓ Secretion
  • ↓ Motility
  • ↑ Sphincter tone
Site of referred pain
  • Epigastric region
  • Umbilical regions
  • Hypogastric region
  • -
Enteric nervous system
(Intrinsic innervation)

Submucosal plexus (Meissner plexus)

  • Regulates local gastrointestinal secretion and nutrient absorption

Myenteric plexus (Auerbach plexus)

  • Regulates inherent myogenic motility of the GIT
  • ICC act as pacemaker cells for the rhythmic phasic contractions of the gut (migratory motor complex) that occur in the fasting or interdigestive state

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

Gut motility

Type of contractions Description
Peristalsis

Segmentation

  • Waves of contraction of the muscularis propria
  • Aid mixing of chyme with digestive enzymes
  • Not propulsive

Migratory motor complex (rhythmic phasic contractions)

Ultrapropulsive contractions

Tonic contractions

  • Most significant at sphincters and intestinal junctions
  • Regulates the passage of food/chyme
  • Partially responsible for fecal continence

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.

Layers of gastrointestinal tract wall
Layers Characteristics and contents
Mucosa
Submucosa
Muscularis propria
Serosa or adventitia

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

Epithelium of gastrointestinal tract
Structure

Epithelium type

Specialized cells and secretory products

Special histological features Function

Oral cavity

Esophagus
  • Upper ⅓ of the muscularis propria layer contains skeletal muscle.
  • Middle and lower ⅔ of the muscularis propria layer contain
    smooth muscle.

Stomach

  • Mucosa contains gastric glands that open onto the surface via gastric pits.
  • Epithelium is covered by a protective mucus layer.

Duodenum

Jejunum

  • Absorption of nutrients and water
  • IgA secretion
  • Primary site of folate absorption

Ileum

Appendix
  • Submucosa contains numerous lymphoid follicles.
  • Vestigial organ; also hypothesized to play a role in gut immunity
Colon
  • Absorption of Na+ and Cl-
  • Absorption of water
  • Synthesis of vitamin K and B vitamins by colonic bacteria
  • Solidification of chyme into stool
  • Lubrication for the passage of feces
Rectum and anal canal Above the pectinate line
  • Voluntary and involuntary control of defecation
Below the pectinate line

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.

Hormones and secretions of gastrointestinal tract
Products Cell type Function Control
Stimulation Inhibition
Gastric secretory products

Gastric acid (hydrochloric acid)

Intrinsic factor (IF; vitamin B12 binding protein)
Pepsin (converted from prohormone pepsinogen)
  • Protein digestion
Bicarbonate
  • None
Gastric regulatory products Gastrin
Ghrelin
Histamine
  • Gastric acid secretion
Small intestinal regulatory products Cholecystokinin
Somatostatin
Secretin
Gastric inhibitory polypeptide (GIP)
  • Insulin secretion (more pronounced with the administration of oral glucose compared to intravenous glucose)
  • ↓ Secretion of gastric acid
Motilin
  • Intake of food
Vasoactive intestinal peptide (VIP)
  • ↑ Secretion of water and electrolytes
  • ↑ Relaxation of intestinal sphincters and smooth muscle
Nitric oxide (NO) [8]
  • -
  • -

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 Achlorydia.

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.

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.

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