The first (superior) part of the duodenum begins as a continuation of the duodenal end of the pylorus. It passes laterally to the right, superiorly and posteriorly, for approximately 5 cm, before making a sharp curve inferiorly into the superior duodenal flexure. It is intraperitoneal for the first 2-3 cm only. Relations ³:

anteriorly: gallbladder, liver
posteriorly: common bile duct, portal vein, gastroduodenal artery
superiorly: epiploic foramen
inferiorly: pancreatic head

Second part (D2)

The second (descending) part of the duodenum begins at the superior duodenal flexure. It passes inferiorly to the lower border of vertebral body L3, before making a sharp turn medially into the inferior duodenal flexure, the end of the descending part. Relations ³:

anteriorly: transverse mesocolon
posteriorly: right kidney, right ureter, right adrenal gland
superiorly: liver, gallbladder (variable)
inferiorly: loops of jejunum
laterally: ascending colon, hepatic flexure, right kidney
medially: pancreatic head

The pancreatic duct and common bile duct enter the descending duodenum through the major duodenal papilla (ampulla of Vater). This part of the duodenum also contains the minor duodenal papilla, the entrance for the accessory pancreatic duct. The junction between the embryological foregut and midgut lies just below the major duodenal papilla.

Third part (D3)

The third (inferior/horizontal) part of the duodenum begins at the inferior duodenal flexure and passes transversely to the left, crossing the vertebral column. Relations ³:

anteriorly: small bowel mesentery root
posteriorly: right psoas muscle, right ureter, gonadal vessels, aorta and IVC
superiorly: pancreatic head
inferiorly: loops of jejunum

Fourth part (D4)

The fourth (ascending) part passes superiorly, either anterior to, or to the left of, the aorta, until it reaches the inferior border of the body of the pancreas. Then, it curves anteriorly and terminates at the duodenojejunal flexure where it joins the jejunum. The duodenojejunal flexure is surrounded by a peritoneal fold containing muscle fibers: the ligament of Treitz. Relations ³:

superiorly: stomach
inferiorly: loops of jejunum
posteriorly: left psoas muscle, aorta

Arterial supply ³

duodenal cap (first 2.5 cm): right gastric artery, right gastroepiploic artery
remaining D1 to mid-D2: superior pancreaticoduodenal artery (branch of gastroduodenal artery)
mid-D2 to ligament of Treitz: inferior pancreaticoduodenal arteries (branch of SMA)

Venous drainage ³

duodenal cap (first 2.5 cm): prepyloric vein (drains to portal vein)
remaining duodenum: superior pancreaticoduodenal vein (drains to portal vein) and inferior pancreaticoduodenal vein (drains to superior mesenteric vein)

Lymphatic drainage

pancreaticoduodenal nodes that drain:
- distally to superior mesenteric nodes
- proximally to celiac nodes ³

Innervation

sympathetic nerve fibers via celiac plexus
parasympathetic nerve fibers via anterior and posterior vagal trunks ²

Variant anatomy

duodenal diverticulum: most commonly occurs in D2 or D3 ⁴
duodenal duplication: most commonly occurs at the medial wall of D2 or D3; appears as a cystic structure that does not communicate with the lumen ⁴