The descending part is the 2nd part of the duodenum and it is about 7 - 10 cm long. It is entirely retroperitoneal - located behind the peritoneum. It begins to the right of the inferior vena cava (IVC), descends along the right side of the L1 - L3 vertebrae, and curves around the head of the pancreas.

Features of the descending part include the fallowing:

• Hepatopancreatic ampulla (ampulla of Vater) - dilated junction of the common bile duct and main pancreatic duct, located close to the duodenal wall on the left side (medially);
• Hepatopancreatic sphincter (sphincter of Oddi) - smooth muscle sphincter in the duodenal wall that regulates the flow of bile and pancreatic juice into the second part of the duodenum;
• Major duodenal papilla (papilla of Vater) - grossly visible luminal projection from the ampulla of Vater, with its opening into the left side of the second part of the duodenum;
• Minor duodenal papilla (accessory or minor pancreatic duct, papilla of Santorini) - the opening of the accessory pancreatic duct, entering the descending part of the duodenum superior to the major papilla (not always present).