The descending part is the second part of the duodenum, beginning at the superior duodenal flexure, from which it travels downward in a slightly curved manner. It is about 2.8 - 4 inches (7 - 10 cm) long. It is entirely retroperitoneal, meaning that it is located behind the peritoneum.

The descending part is found on the right side of the inferior vena cava (IVC), it descends along the right side of the L1 - L3 vertebrae, and curves around the head of the pancreas. At the third lumbar vertebra (L3) level, the descending part curves to the left side and forms the inferior duodenal flexure (Read more!), which further continues as the next portion - the horizontal part.

The descending part of the duodenum presents with two notable anatomical landmarks - the major and minor duodenal papillae.

The major duodenal papilla (papilla of Vater) is a slight mucosal elevation that projects on the left side of the descending part. It is found within the posteromedial wall and is the common entrance site for the common bile duct and main pancreatic duct. Also, it contains a smooth muscle sphincter known as the hepatopancreatic sphincter (sphincter of Oddi). It regulates the flow of bile and pancreatic secretions.
The minor duodenal papilla (papilla of Santorini) is another slight mucosal elevation that is found within the same wall of the descending part. It usually presents approximately 0.8 inches (2 cm) superior to the major duodenal papilla. It serves as the entrance site for the accessory pancreatic duct, which is sometimes absent.

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

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Descending part