The duodenum is the very first part of the small intestine. It receives partially digested food from the stomach and begins absorption of nutrients. The duodenum is located in the abdominal cavity at the level of the first to third lumbar vertebrae (L1 - L3).

The duodenum is a continuation of the pylorus of the stomach. It wraps around the head of the pancreas and surrounds it from three sides. Therefore, it appears C-shaped.

The duodenum terminates at the duodenojejunal flexure, transforming into the jejunum - the middle portion of the small intestine. Overall, the duodenum is about 10 to 12 inches (25 to 30 cm) long.

Most of the duodenum is located retroperitoneally, meaning that the peritoneum covers it only from one side, and that the duodenum is situated behind it. However, the first few centimeters of the duodenum lie intraperitoneally, as they are covered posteriorly and anteriorly by the visceral layer of the peritoneum.

Duodenum parts and flexures

The duodenum has four parts and three flexures. The duodenum parts are as follows:

The flexures of the duodenum include the following:

Superior duodenal flexure
Inferior duodenal flexure
Duodenojejunal flexure

Superior part

The superior part is the proximal portion located anterolateral to the body of the first lumbar vertebra (L1). It is about 2 inches (5 cm) long. The first 0.7 - 1.2 inches (2 - 3 cm) are enlarged, and this enlargement is called the duodenal bulb or duodenal cap.

The superior part extends from the pyloric opening of the stomach to the gallbladder, lying anterior and superior to the head of the pancreas. Posterior to it are the gastroduodenal artery, hepatic portal vein and common bile duct. The superior part ends at the superior duodenal flexure, transitioning into the descending part.

This part of the duodenum also changes its position from entirely intraperitoneal to a retroperitoneal. The first 0.8 inches (2 cm) of the superior part is mobile and attached via the hepatoduodenal ligament to the liver. In contrast, the peritoneum covers only the anterior aspect of the distal 1.2 inches (3 cm) of the superior part.

Descending part

The descending part is the second portion of the duodenum. It begins at the superior duodenal flexure and travels downward in a slightly curved manner. It is about 2.8 - 4 inches (7 - 10 cm) long and ends at the third lumbar vertebra (L3) level, where it forms the inferior duodenal flexure and continues as the horizontal part.

The descending part receives two ducts - the main pancreatic duct and the common bile duct. They enter the descending duodenum through a mucosal swelling known as the major duodenal papilla.

The major duodenal papilla, or the papilla of Vater, is a slight mucosa elevation located within the posteromedial wall of the descending part. It is a common entrance site for the common bile duct and main pancreatic duct. Also, it contains the hepatopancreatic sphincter (sphincter of Oddi) that regulates the flow of bile and pancreatic secretions.

The descending part of the duodenum contains a smaller swelling called the minor duodenal papilla (papilla of Santorini). It is another slight mucosal elevation within the same wall of the descending part, approximately 0.8 inches (2 cm) proximal to the major duodenal papilla. It serves as the entrance site for the accessory pancreatic duct (sometimes absent).

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

Horizontal part

The horizontal part is the third portion of the duodenum. It is the longest part - about 4 inches (10 cm) long. It begins from the inferior duodenal flexure at the third lumbar vertebra (L3) level. The horizontal part goes transversely from the right to the left side, also crossing the abdominal aorta and inferior vena cava. When reaching the left side of the third lumbar vertebra (S3), it further continues as the ascending part of the duodenum.

Ascending part

The ascending part is the final portion, and it is up to 2 inches (5 cm) long. The ascending part begins on the left side of the abdominal aorta and third lumbar vertebra (L3) and ascends laterally to the second lumbar vertebra (L2). Further, the ascending part makes a sharp turn in an antero-inferior direction and forms the duodenojejunal flexure that marks the transition site between the duodenum and jejunum.

The ascending portion of the duodenum becomes more and more invested in the peritoneum towards the jejunum to the point that the duodenojejunal flexure is suspended from the retroperitoneum by a thin and triangular-shaped double fold of the peritoneum called the ligament of Treitz (suspensory ligament of the duodenum).

The ligament of Treitz is a fibromuscular structure, meaning that it also contains muscle fibers, and it is actually composed of two muscular parts joined by the connective tissue. Therefore, it is sometimes referred to as the suspensory muscle of the duodenum.

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

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Duodenum