duodenum duodenum the duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. in mammals it may be the principal site for iron absorption. the duodenum precedes the jejunum and ileum and is the shortest part of the small intestine. duodenumimage of the gastrointestinal tract, with the duodenum highlighted.diagram of the human duodenum with major parts labelleddetailspronunciation/ˌduːəˈdiːnəm, duˈɒdɪ-/precursorforegut (1st and 2nd parts), midgut (3rd and 4th part)part ofsmall intestinesystemdigestive systemarteryinferior pancreaticoduodenal artery, superior pancreaticoduodenal arteryveinpancreaticoduodenal veinsnerveceliac ganglia, vagusidentifierslatinintestinum duodenummeshd004386ta98a05.6.02.001ta22944fma7206anatomical terminology major parts of thegastrointestinal tract upper gastrointestinal tract mouth esophagus stomach duodenum lower gastrointestinal tract small intestine jejunum ileum large intestine sigmoid colon rectum anus