The duodenum is the initial C-shaped segment of the small intestine and is a continuation of the pylorus. Distally, it is in continuation with the jejunum and ileum, with the proximal segment being the shortest and widest. Positioned inferiorly to the stomach, the duodenum is approximately 25 to 30 cm long. Interestingly enough, this portion of the small intestine got its name due to its length. In Latin, the term "duodenum" means 12 fingers, which is roughly the length of the duodenum. The 4 segments of the duodenum include the following:
The duodenal bulb, which connects to the undersurface of the liver via the hepatoduodenal ligament, which contains the portal vein, the hepatic artery, and common bile duct.
The second or descending segment is just above the inferior vena cava and right kidney, with the head of the pancreas lying in a C-shaped concavity.
The third segment runs from right to left in front of the aorta and inferior vena cava, with the superior mesenteric vessels in front of it.
The fourth segment continues as the jejunum.
The walls of the duodenum are made up of 4 layers of tissue that are identical to the other layers of the gastrointestinal (GI) tract. From innermost to the outermost layer, these are the mucosa, submucosa, muscularis, and serosa layers. The mucosal layer lines the inner surface of the duodenum and is made of simple columnar cells with microvilli and numerous mucous glands. The submucosal layer is mostly a layer of connective tissue where blood vessels and nerves travel through. The muscularis layer contains the smooth muscle of the duodenum and allow mixing and forward peristaltic movement of chyme. The serosal layer is characterized by squamous epithelium that acts as a barrier for the duodenum from other organs within the human body.
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