Descending part of duodenum

The descending (second) part of the duodenum (D2) is approximately 7-10 cm long and extends vertically from the superior duodenal flexure at the level of the first lumbar vertebra (L1), near the neck of the gallbladder, to the inferior duodenal flexure at the level of the upper border of the fourth lumbar vertebra (L4). It descends along the right side of the vertebral column and is largely retroperitoneal, with its posterior surface devoid of peritoneal covering. The middle third of the descending duodenum is crossed anteriorly by the transverse colon.

Anatomical relations

• Anteriorly, the descending part of duodenum is related, from above downward, to the duodenal impression of the right lobe of the liver, the transverse colon, and coils of the small intestine.

• Posteriorly, it is related to the hilum of the right kidney, from which it is separated by loose areolar tissue, as well as to the right renal vessels, inferior vena cava, and the right psoas major muscle inferiorly.

• Medially, it is closely applied to the head of the pancreas and the common bile duct,

• Laterally it is related to the right colic (hepatic) flexure.

Openings of biliary and pancreatic ducts

A key anatomical feature of the descending part of duodenum is the opening of the hepatopancreatic ampulla (ampulla of Vater) on its posteromedial wall at the major duodenal papilla, typically located 7-10 cm distal to the pylorus. Here, the common bile duct and main pancreatic duct unite to form the ampulla that enters the duodenal lumen obliquely. The accessory pancreatic duct, when present, opens separately at the minor duodenal papilla, approximately 2 cm above the major papilla. These openings allow the delivery of bile and pancreatic secretions into the intestine and make the descending duodenum an important site in hepato-pancreatobiliary anatomy.

Clinical correlates

The close relationship of second part of duodenum to the head of the pancreas and biliary tract explains its involvement in conditions such as periampullary carcinoma, pancreatic head tumours, choledocholithiasis, and obstructive jaundice.