Duodenum

The duodenum is the first section of the small intestine, approximately 25 to 30 centimeters in adults. It is largely retroperitoneal, with the exception of its proximal portion, the superior part, which is intraperitoneal. The duodenum receives chyme from the stomach through the pyloric sphincter and mixes it with bile and pancreatic juice via the hepatopancreatic ampulla and the major and minor duodenal papillae, primarily in the descending second portion. It begins the process of digestion and continues the breakdown of nutrients before chyme passes into the jejunum.

Anatomy: It has four parts: the superior (first) part, the descending (second) part containing the major duodenal papilla where bile and pancreatic duct secretions enter, the horizontal (third) part, and the ascending (fourth) part. The second part also hosts the minor papilla for drainage of the accessory pancreatic duct.

Blood supply: The upper portion is supplied by the gastroduodenal artery via the superior pancreaticoduodenal arteries; the lower portion is supplied by the inferior pancreaticoduodenal arteries from the superior mesenteric artery. Venous drainage follows the corresponding veins into the portal system.

Histology and physiology: The mucosa has villi and circular folds; submucosa in the proximal part contains Brunner's glands that secrete bicarbonate-rich mucus to neutralize gastric acid. Enzymes from the pancreas and bile aid digestion; secretin stimulates bicarbonate secretion, while cholecystokinin stimulates pancreatic enzyme release and biliary contraction. The duodenum absorbs iron, calcium, and folate to a modest extent, but most absorption occurs in the jejunum.

Clinical relevance: Common sites of peptic ulcers are in the duodenal bulb; inflammatory conditions include duodenitis; congenital anomalies include duodenal atresia and annular pancreas; neoplasms are rare but recognized.