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In anatomy of the digestive system, the duodenum is a hollow jointed tube connecting the stomach to the jejunum. It is the first and shortest part of the small intestine and it is where most chemical digestion takes place. It begins with the duodenal bulb and ends at the ligament of Treitz. The name duodenum is from the Latin duodenum digitorum, twelve fingers' breadths or inches.
The duodenum is largely responsible for the breakdown of food in the small intestine. Brunner's glands, which secrete mucus, are found in the duodenum. The duodenum wall is composed of a very thin layer of cells that form the muscularis mucosae. The duodenum is almost entirely retroperitoneal. The pH in the duodenum is approximately six.
The duodenum is divided into four sections for the purposes of description. The first three sections form a "C" shape.
The first (superior) part begins as a continuation of the duodenal end of the pylorus. From here it passes laterally (right), superiorly and posteriorly, for approximately 5 cm, before making a sharp curve inferiorly into the superior duodenal flexure (the end of the superior part). It is intraperitoneal.
The second (descending) part of the duodenum begins at the superior duodenal flexure. It passes inferiorly to the lower border of vertebral body L3, before making a sharp turn medially into the inferior duodenal flexure (the end of the descending part).
The pancreatic duct and common bile duct enter the descending duodenum, commonly known together as the hepatopancreatic duct (or pancreatic duct in the United States), through the major duodenal papilla. This part of the duodenum also contains the minor duodenal papilla, the entrance for the accessory pancreatic duct. The junction between the embryological foregut and midgut lies just below the major duodenal papilla.
The third (inferior/horizontal) part of the duodenum begins at the inferior duodenal flexure and passes transversely to the left, crossing the inferior vena cava, aorta and the vertebral column.
The fourth (ascending) part passes superiorly, either anterior to, or to the right of, the aorta, until it reaches the inferior border of the body of the pancreas. Then, it curves anteriorly and terminates at the duodenojejunal flexure where it joins the jejunum. The duodenal flexure is surrounded by a peritoneal fold containing muscle fibres: the ligament of Treitz.
<gallery> Image:Gray533.png|The celiac artery and its branches; the stomach has been raised and the peritoneum removed. Image:Gray1041.png |Superior and inferior duodenal fossæ. Image:Gray1042.png|Duodenojejunal fossa. Image:Gray1050.png |Interior of the stomach. Image:Gray1058.png|Section of duodenum of cat. X 60. Image:Gray1099.png|The pancreas and duodenum from behind. Image:Gray1097.png|Transverse section through the middle of the first lumbar vertebra, showing the relations of the pancreas. Image:Gray1100.png|The pancreatic duct. Image:Small bowel duodenum with amyloid deposition congo red 10X.jpg|Duodenum with amyloid deposition in lamina propria. Image:Illu pancrease.jpg|Region of pancreas Image:Illu stomach2.jpg|Stomach </gallery>