The duodenum lies in front of the right kidney and renal vessels, the right psoas muscle, the inferior vena cava, and the aorta (Figure 26.1).

The duodenum is approximately 25 cm in length. It is the most fixed part of the small intestine and has no mesentery. It is anatomically divided into four parts:

• The superior or first portion is intraperitoneal along the anterior half of its circumference. Superiorly, the first portion is attached to the hepatoduodenal ligament. The posterior wall is associated with the gastroduodenal artery, common bile duct, and the portal vein.

• The descending or second portion shares a medial border with the head of the pancreas. It is bordered posteriorly by the medial surface of the right kidney, the right renal vessels, and the inferior vena cava. The hepatic flexure and transverse colon cross anteriorly. The common bile duct and main pancreatic duct drain into the medial wall of the descending duodenum.

• The transverse or third portion is also entirely retroperitoneal. Posteriorly, it is bordered by the inferior vena cava and the aorta. The superior mesenteric vessels cross in front of this portion of the duodenum.

• The ascending or fourth portion of the duodenum is approximately 2.5 cm in length and is primarily retroperitoneal, except for the most distal segment. It crosses anterior to and ascends to the left of the aorta to join the jejunum at the ligament of Treitz.

The common bile duct courses laterally within the hepatodudenal ligament and lies posterior to the first portion of the duodenum and pancreatic head, becoming partially invested within the parenchyma of the pancreatic head. The main pancreatic duct then joins the common bile duct to drain into the ampulla of Vater within the second portion of the duodenum. The ampulla of Vater is located approximately 7 cm from the pylorus. The accessory pancreatic duct drains approximately 2 cm proximal to the ampulla of Vater.

The vascular supply to the duodenum is intimately associated with the head of the pancreas. The head of the pancreas and the second portion of the duodenum derive their blood supply from the anterior and posterior pancreaticoduodenal arcades (Figure 26.2). These arcades lie on the surface of the pancreas near the duodenal C loop. Attempts to separate these two organs at this location usually results in ischemia of the duodenum.