<P> The pancreatic duct and common bile duct enter the descending duodenum, through the major duodenal papilla . The second part of the duodenum also contains the minor duodenal papilla, the entrance for the accessory pancreatic duct . The junction between the embryological foregut and midgut lies just below the major duodenal papilla . </P> <P> The third part, or horizontal part or inferior part of the duodenum is 10 ~ 12cm in length . It begins at the inferior duodenal flexure and passes transversely to the left, passing in front of the inferior vena cava, abdominal aorta and the vertebral column . The superior mesenteric artery and vein are anterior to the third part of duodenum . This part may be compressed between the aorta and SMA causing superior mesenteric artery syndrome . </P> <P> The fourth part, or ascending part, of the duodenum passes upward, joining with the jejunum at the duodenojejunal flexure . The fourth part of the duodenum is at the vertebral level L3, and may pass directly on top of, or slightly left to, the aorta . </P> <P> The duodenum receives arterial blood from two different sources . The transition between these sources is important as it demarcates the foregut from the midgut . Proximal to the 2nd part of the duodenum (approximately at the major duodenal papilla--where the bile duct enters) the arterial supply is from the gastroduodenal artery and its branch the superior pancreaticoduodenal artery . Distal to this point (the midgut) the arterial supply is from the superior mesenteric artery (SMA), and its branch the inferior pancreaticoduodenal artery supplies the 3rd and 4th sections . The superior and inferior pancreaticoduodenal arteries (from the gastroduodenal artery and SMA respectively) form an anastomotic loop between the celiac trunk and the SMA; so there is potential for collateral circulation here . </P>

Where does the duodenum end and the jejunum start
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