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 connected 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

The duodenum wraps around the head of the pancreas and ends at the duodenojejunal flexure at the level of the L2 vertebra, 2 - 3 cm to the left of the midline. Further, it is continuous with the next part of the small intestine, which is called the jejunum.

Parts of duodenum

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

• Superior part (duodenal bulb)
• Descending part
• Horizontal part
• Ascending part

The superior part lies intraperitoneally, while the rest of the duodenum is located retroperitoneally. It is the proximal part, closest to the stomach, located anterolateral to the L1 vertebral body. The superior part is enlarged proximally, and because of this it is known as the duodenal bulb. 

The superior part of the duodenum is connected to the liver by the hepatoduodenal ligament. This part ends at the superior duodenal flexure and becomes the descending part. 

The descending part passes inferior to the lower border of the L3 vertebral body. It then makes a sharp turn medially creating 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. (Note: 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 ligament of Treitz, also called the suspensory ligament of the duodenum. 

Clinically, the ligament of Treitz marks the border between the upper and lower parts of the gastrointestinal tract.