The duodenum is the first of the three parts of the small intestine that receives partially digested food from the stomach and begins with the absorption of nutrients. It is directly attached to the pylorus of the stomach.

"Duodenum" is a Latin word for 12, and this part of the intestines is named for being 12 finger breadths in length.

The duodenum is:

• C-shaped
• Approximately 25 - 30 cm long
• Located at the level of L1 - L3 vertebrae
• Beginning at the pylorus on the right

Its four parts wrap around the head of the pancreas and end at the duodenojejunal flexure at the level of the L2 vertebra, 2 - 3 cm to the left of the midline.

Duodenum has four parts (of which one is intraperitoneal and three - retroperitoneal):

• The superior part lies intraperitoneally and is enlarged proximally. The enlargement is called the 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. This part lies anterolateral to the body of the L1 vertebra.

NOTE: the superior part of the duodenum lies intraperitoneally, while the rest of it is located retroperitoneally.

• The descending part goes inferior to the lower border of the L3 vertebral body. It then makes a sharp turn medially into the inferior duodenal flexure, which is the end of the descending part.

The pancreatic duct and common bile duct enter the descending duodenum through the major duodenal papilla. The descending part of the duodenum also contains the minor duodenal papilla, which is the entrance for the accessory pancreatic duct. 

The junction between the embryological foregut and midgut lies just below the major duodenal papilla.

• The horizontal part crosses the third lumbar vertebra (L3) from the right to the left side, crossing over the aorta and the inferior vena cava.

• The ascending part of the duodenum joins the jejunum at the duodenojejunal flexure, where the duodenum is attached to the back of the abdominal wall through the suspensory ligament of the duodenum (ligament of Treitz). Clinically, the ligament of Treitz marks the border between the upper and lower gastrointestinal tracts.