The duodenum is the first part of the small intestine, extending from the valve at the bottom of the stomach that regulates stomach emptying (pylorus valve) to the second part of the small intestine (jejunum).
However, gastric bypass relocates the stomach attachment to either the ileum or jejunum portion of the small intestine, which is much farther along the length of the intestine.
The malrotation, stenosis, or membranous bands are corrected surgically so that the duodenum has a normal opening and connects properly to the stomach and jejunum.
Duodenal volvulus occurs when the duodenum, the portion of small intestine that connects the stomach and jejunum, is twisted.
Instead, food moves from the small pouch directly to the jejunum, the second section of the small intestine.
The duodenum is about 10 in (25 cm) long and connects the stomach and the jejunum.
The alimentary,or intestinal, canal varies greatly in relative length and capacity in different mammals, and also offers manifold peculiarities of form, being sometimes a simple cylindrical tube of nearly uniform calibre throughout, but more often subject to alterations of form and capacity in different portions of its course - the most characteristic and constant being the division into an upper and narrower and a lower and wider portion, called respectively the small and the large intestine; the former being arbitrarily divided into duodenum, jejunum and ileum, and the latter into colon and rectum.