the first part of the small intestine, which receives partially digested food from the stomach and begins the absorption of nutrients. The duodenum is the shortest segment of the intestine and is about 23 to 28 cm (9 to 11 inches) long. It is roughly horseshoe-shaped, with the open end up and to the left, and it lies behind the liver. On anatomic and functional grounds, it can be divided into four segments: the superior, descending, horizontal, and ascending duodenum.

A liquid mixture of food and gastric secretions enters the superior duodenum from the pylorus of the stomach, triggering the release of pancreas-stimulating hormones from glands in the duodenal wall. Ducts from the pancreas and gallbladder enter at the descending duodenum, bringing bicarbonate to neutralize the acid in the gastric secretions, pancreatic enzymes to further digestion, and bile salts to emulsify fat. The mucous lining of the last two segments of the duodenum begins the absorption of nutrients, in particular iron and calcium, before the food contents enter the next part of the small intestine, the jejunum.

Its exposure to unneutralized stomach acid makes the duodenum, in particular the superior segment, especially susceptible to peptic ulcers, which are the most common health problem affecting this part of the intestine. The horizontal duodenum, because of its location between the liver, pancreas, and major blood vessels, can become compressed by these structures in people who are severely thin, requiring surgical release to eliminate painful duodenal dilatation, nausea, and vomiting.