The duodenum is the first and shortest section of the small intestine, connecting the stomach to the jejunum. In adults, it is approximately 25–30 centimetres long and forms a C-shaped curve around the head of the pancreas. It receives partially digested food from the stomach together with pancreatic and biliary secretions, making it an important site of digestion. (ncbi.nlm.nih.gov)
Anatomy
The duodenum begins at the pylorus, the stomach’s outlet, and ends at the duodenojejunal junction. Most of it lies behind the peritoneum, against the posterior abdominal wall; its initial portion is more mobile. Four parts are distinguished:
- Superior part: begins immediately beyond the pylorus; its proximal region is commonly called the duodenal bulb.
- Descending part: runs down the right side of the vertebral column and receives bile and pancreatic secretions.
- Horizontal part: crosses the abdomen in front of the aorta and inferior vena cava, behind the superior mesenteric vessels.
- Ascending part: rises toward the jejunum.
The major duodenal papilla, in the descending part, is the principal opening through which the common bile duct and main pancreatic duct deliver their contents. These relationships are important in endoscopic procedures and abdominal surgery. (ncbi.nlm.nih.gov)
Microscopic structure
The duodenal wall has an inner mucosa, a supporting submucosa, muscular layers, and an outer covering. Its mucosal surface bears villi, projections that increase the area available for absorption. The absorptive epithelial cells have microscopic projections called microvilli, collectively forming a brush border. Intestinal glands, or crypts, extend into the mucosa between the villi. Goblet cells within the epithelium produce protective mucus. (histology.leeds.ac.uk)
A characteristic feature is the presence of Brunner glands in the submucosa. These glands secrete mucus and bicarbonate, helping protect the intestinal lining from acidic stomach contents. Their location distinguishes duodenal tissue from other regions of the small intestine in microscopic examination. (ncbi.nlm.nih.gov)
Digestive and regulatory functions
Neutralization and digestion
Food enters the duodenum as chyme, a mixture of partially digested material and gastric secretions. Bicarbonate-containing secretions help neutralize its acidity, providing conditions suitable for intestinal and pancreatic digestive enzymes. (ncbi.nlm.nih.gov)
Pancreatic juice supplies enzymes that break down carbohydrates, proteins, and fats. Bile, produced by the liver and stored in the gallbladder, assists fat digestion by dispersing fat into smaller droplets. The duodenum mixes these secretions with chyme as intestinal muscular movements carry the contents onward. Digestion and absorption continue through the remaining small intestine. (niddk.nih.gov)
Hormonal coordination
Cells in the duodenal lining release hormones that coordinate digestive activity:
- Secretin is released in response to acid entering the upper small intestine and stimulates bicarbonate-rich pancreatic secretion.
- Cholecystokinin, released in response to nutrients including fats and protein digestion products, promotes pancreatic enzyme secretion and gallbladder contraction.
These hormonal responses operate alongside nerve signals to regulate digestive secretions and the delivery of stomach contents to the intestine. (ncbi.nlm.nih.gov)
Clinical significance
Duodenal ulcers
A duodenal ulcer is a form of peptic ulcer involving the duodenal lining. The most common causes of peptic ulcers are infection with Helicobacter pylori and the use of nonsteroidal anti-inflammatory drugs, such as ibuprofen and aspirin. Complications can include bleeding, perforation, penetration into nearby tissues, and obstruction. (niddk.nih.gov)
Celiac disease
The duodenum is an important site for investigating celiac disease. Samples obtained during upper gastrointestinal endoscopy can be examined for characteristic damage to the small-intestinal lining. Diagnostic evaluation commonly combines blood tests with intestinal biopsies, including samples from the duodenal bulb and more distal duodenum. (niddk.nih.gov)
Congenital obstruction
Duodenal atresia is a congenital condition in which the duodenal passage is closed, preventing stomach contents from moving normally into the intestine. It typically presents in newborns with vomiting, which may contain bile. Imaging may show the double-bubble sign, representing an enlarged stomach and proximal duodenum. The condition can occur alongside other congenital abnormalities, including Down syndrome. (medlineplus.gov)
Examination
Upper gastrointestinal endoscopy allows direct inspection of the duodenal lining and collection of tissue samples. It is used to investigate ulcers, bleeding, and disorders affecting the mucosa. The instrument passes through the mouth, esophagus, and stomach before entering the duodenum. (niddk.nih.gov)
References
- Anatomy, Abdomen and Pelvis: Duodenum - StatPearls - NCBI Bookshelfncbi.nlm.nih.gov
- Small Intestine — The Histology Guidehistology.leeds.ac.uk
- Brunner Glands - MeSH - NCBIncbi.nlm.nih.gov
- Anatomy, Abdomen and Pelvis, Small Intestine - StatPearls - NCBI Bookshelfncbi.nlm.nih.gov
- Physiology, Gastrointestinal - StatPearls - NCBI Bookshelfncbi.nlm.nih.gov
- Your Digestive System & How it Works - NIDDKniddk.nih.gov
- Physiology, Gastrointestinal Hormonal Control - StatPearls - NCBI Bookshelfncbi.nlm.nih.gov
- GI Endocrine Functionnaspghan.org
- Peptic Ulcers (Stomach or Duodenal Ulcers) - NIDDKniddk.nih.gov
- Diagnosis of Celiac Disease - NIDDKniddk.nih.gov
- Celiac Disease Tests - NIDDKniddk.nih.gov
- Duodenal atresia: MedlinePlus Medical Encyclopediamedlineplus.gov