Superior, descending, horizontal and ascending, 2, 8, 10 and 5 centimetres
Mnemonic
“2, 5, 10, 2.5” centimetres, or more usefully the parts by their relations:
- First (superior), at L1, the duodenal cap. The only intraperitoneal part, and the site of most peptic ulcers. A posterior ulcer erodes the gastroduodenal artery, causing brisk haemorrhage
- Second (descending), at L2, receives the ampulla of Vater at its posteromedial wall, halfway down
- Third (horizontal), at L3, crossed anteriorly by the superior mesenteric vessels, which is the site of superior mesenteric artery syndrome
- Fourth (ascending), at L2, ends at the duodenojejunal flexure, suspended by the ligament of Treitz
The ampulla marks the foregut to midgut transition, which is why the blood supply changes there from coeliac to superior mesenteric, and why pain refers from epigastrium to umbilicus across that point.
Expansion
The duodenum is about 25 cm long and forms a C-shape around the head of the pancreas:
- First (superior) part - the duodenal cap; the commonest site of peptic ulceration. Only its proximal 2 cm is intraperitoneal
- Second (descending) part - receives the ampulla of Vater at its posteromedial wall; marks the foregut-midgut junction
- Third (horizontal) part - crossed anteriorly by the superior mesenteric vessels
- Fourth (ascending) part - ends at the duodenojejunal flexure, suspended by the ligament of Treitz
Two vascular relations matter clinically. A posterior first-part ulcer erodes the gastroduodenal artery, causing major haemorrhage, while an anterior ulcer perforates into the peritoneal cavity. And compression of the third part between the aorta and the superior mesenteric artery causes superior mesenteric artery syndrome.