The duodenum is the first of the three parts of the small intestine and is directly attached to the pylorus of the stomach. It is about 25 to 30 cm long (“twelve fingers' length”), C-shaped and is located in the upper abdomen at the level of L1-L3. The head of the pancreas lies in the C loop. The duodenum may be subdivided into four sections: superior part, descending part, horizontal part and ascending part.
- The superior part lies intraperitoneally and is enlarged proximally (duodenal bulb). It is connected to the liver by the hepatoduodenal ligament. The superior part ends at the superior duodenal flexure and becomes the descending part.
- The descending part and the rest of the duodenum lie retroperitoneally. The common bile duct and the pancreatic duct unify to a conjoint duct at the hepatopancreatic ampulla (=ampulla of Vater) and empties into the descending part of the duodenum. At the opening there is an elevation of the mucosa, the major duodenal papilla (=papilla of Vater). Many people have an accessory pancreatic duct which empties into an additional papilla, the minor duodenal papilla (=papilla of Santorini). The transition from the descending to the horizontal part of the duodenum takes places at the inferior duodenal flexure.
- The horizontal part runs from right to left ventrally from the abdominal aorta and inferior vena cava.
- The ascending part runs cranially along the left side of the vertebral column. This last part of the duodenum joins the intraperitoneally lying jejunum at the duodenojejunal flexure. Here the duodenum is attached to the back of the abdominal wall through the suspensory ligament of duodenum (=ligament of Treitz). Clinically the ligament of Treitz marks the border between the upper and lower gastrointestinal tract.
The supply of blood to the duodenum is carried by the anterior and posterior superior pancreaticoduodenal arteries (branches of the gastroduodenal artery) and the inferior pancreaticoduodenal artery (branch of the superior mesenteric artery) which form an arterial arcade. The correspondent veins are responsible for the venous drainage. The sympathetic innervation is carried by nerves of the coeliac plexus, the parasympathetic innervation by the vagus nerve (cranial nerve X).
Histologically the duodenum is similar to all the other hollow organs of the gastrointestinal tract: mucosa, submucosa and muscularis. The mucosa consists of simple columnar epithelium (lamina epithelialis), a connective tissue layer (lamina propria) and a smooth muscle layer (lamina muscularis). The intestinal epithelial cells (enterocytes) are overlaid by a layer of glycoproteins and mucin. The submucosa comprises loose connective tissue, numerous blood vessels and the Meissner's plexus. The muscularis consists of an inner circular and an outer longitudinal musculature between which the Auerbach’s plexus lies.
Typical for all sections of the small intestines are microvilli (hairlike structures projecting from the surface), finger-shaped villi and circular folds of the mucosa and submucosa (valves of Kerckring). These structures increase the absorption area of the duodenum up to 1500 times. The duodenum is rich of absorbing enterocytes, mucus producing goblet cells and peptide hormone producing endocrine cells. A characteristic feature of the duodenum is the Brunner’s glands embedded in the submucosa. These produce – amongst others – mucous secret containing bicarbonate which serves to neutralize the gastric acid. Furthermore crypts of Lieberkuhn lie between the villi. Paneth cells are found in the lumen of these crypts. According to today’s knowledge Paneth cells play a role in the antimicrobial defense however their functions are not fully understood yet.
The main tasks of the duodenum are:
- neutralization of the acidic gastric juice through production of alkaline secrets
- mechanical processing and digestion of chyme
- mixing bile and pancreatic enzymes
- absorption of water, electrolytes and nutrients (especially water-soluble substances such as monosaccharides)