The duodenum, into which the stomach opens, is about 25 cm long, C-shaped and begins at the pyloric sphincter. It is almost entirely retroperitoneal and is the most fixed part of the small intestine.
    The duodenum is described as having four parts:
  1. Part one, superior part (SD)
  2. Part two, descending part (DD)
  3. Part three, horizontal part (HD)
  4. part four, ascending part (AD)
The fourth part of the duodenum terminates at the duodenojejunal flexure DJF with the jejunum.

The ligament of Treitz is a musculofibrous band that extends from the upper aspect of the ascending part of the duodenum to the right crus of the diaphragm and tissue around the celiac artery.

As you can see, the head of the pancreas sits in the C-shaped duodenum, so as long as we are here, we may as well point out the structures here:

  • head of the pancreas PH
  • uncinate process of the head of the pancreas PUP
  • neck of the pancreas PN where the superior mesenteric artery and vein pass behind the pancreas
  • body of the pancreas PB
  • tail of the pancreas PT. This part is within peritoneum and abuts the spleen
The other structures in the area are the:
  • inferior vena cava IVC
  • portal vein PV
  • aorta aorta
  • celiac trunk C
  • kidneys

Blood Supply of the Duodenum