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 vary from segment to segment depending on the regional histological characteristics, presence of specialized cells, and function.
Parts of the gastrointestinal tract
- pharynx and
Accessory organs to the gastrointestinal tract
- Ingestion of food and water
- 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
Developmental derivatives of the primitive gut tube 
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, midgut, and hindgut derivatives and corresponding neurovascular structures|
|Vertebral level|| || || |
|Sympathetic|| || || |
For more detailed information about the branches, course, and supply by individual vessels, see the corresponding sections in the "Abdominal cavity" article.
- 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.
- 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
|Overview of arterial blood supply of the gastrointestinal tract|
|Celiac trunk|| |
|Superior mesenteric artery|
|Inferior mesenteric artery|
- The veins of most of the gastrointestinal tract drain directly or indirectly into the portal vein.
- For more information about the tributaries of each individual vein and description of and cavocaval anastomoses, see the “ ” article.
|Overview of venous drainage from the gastrointestinal tract|
|Vein||Characteristics||Areas drained||Drains into|
|Portal vein|| |
Superior mesenteric vein
Inferior mesenteric vein
Innervation and gut motility
Innervation of the gastrointestinal tract 
- The gastrointestinal tract is innervated by the .
- For more information about the extrinsic and intrinsic innervation, see “Nerves of the abdominal cavity” and “Enteric nervous system” correspondingly.
|Overview of the innervation of the gastrointestinal tract|
|Extrinsic innervation||Parasympathetic innervation|| |
Sympathetic innervation ( )
|Site of referred pain|| || || || |
| Enteric nervous system |
Motility and contractile activity of the gastrointestinal tract  
|Type of contractions||Description|
Migratory motor complex (rhythmic phasic contractions)
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|
|Serosa or adventitia|
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|
Specialized cells and secretory products
|Special histological features||Function|
|Appendix|| || || |
|Rectum and anal canal||Above the pectinate line|| || |
|Below the pectinate line|
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|
|Gastric secretory products|| |
|(IF; binding protein)|
|(converted from prohormone pepsinogen)|| || |
| || |
|Gastric regulatory products|| |
| || || |
|Small intestinal regulatory products||Cholecystokinin|
|Gastric inhibitory polypeptide (GIP)|
|Motilin|| || |
|Vasoactive intestinal peptide (VIP)|| || |
|Nitric oxide (NO) || || |
Ghrelin causes greed for food.
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.
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 .)
- 4th 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
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)
- 4th week: midgut elongation in utero
- 6th week: physiological herniation of the midgut out of the umbilical ring → 90° counter-clockwise rotation
- 10th 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).
See “Clinical significance” of the: