Each part of the duodenum is related to different anatomical structures described below:

• The superior part lies anterolateral to the body of the first lumbar vertebra (L1) (Read more!). Additionally, it lies anterior and superior to the head of the pancreas (Read more!). The gastroduodenal artery, main hepatic portal vein (Read more!), and common bile duct (Read more!) are positioned posterior to it. 
• The anterior surface of the descending duodenum is crossed by the transverse colon. Also, the right lobe of the liver (Read more!), gallbladder (Read more!), and coils of the small intestine lie anterior to it, while the hilum of the right kidney (Read more!) and right ureter are located posterior to the descending part. The head of the pancreas (Read more!), the common bile duct (Read more!), and the main pancreatic duct are positioned medially, while lateral to the descending part of the duodenum is the ascending colon and the right colic flexure.
• The horizontal part is located posterior to the superior mesenteric vessels, coils of the small intestine, and root of the mesentery. It lies anterior to the right ureter, right gonadal (testicular or ovarian) vessels, right psoas major muscle, inferior vena cava (Read more!), and abdominal aorta. The head of the pancreas (Read more!) lies superior to the horizontal portion, while the coils of the jejunum are positioned antero-inferiorly.
• Posterior to the ascending part is the left psoas major muscle, left sympathetic trunk, left renal and gonadal vessels. In contrast, the transverse colon and its mesocolon are located anterior to the ascending part, while the body of the pancreas (Read more!) lies superior to it.