The duodenum has the following four parts:

• Superior part (Read more!) - the most proximal portion that connects with the pylorus of the stomach; part of it is located intraperitoneally;
• Descending part (Read more!) - the second portion that contains two papillae (major and minor duodenal papillae); positioned retroperitoneally;
• Horizontal (inferior) part (Read more!) - the longest portion that crosses the third lumbar vertebra (L3) from the right to the left side; found retroperitoneally; 
• Ascending part (Read more!) - the final portion that continues as the jejunum at the duodenojejunal flexure; situated retroperitoneally.