The lamina propria of the duodenum possesses finger-like extensions known as villi, which further elongate and branch out into even finer structures called microvilli. The villi and microvilli project into the lumen of the duodenum. They contain a large number of surface absorptive cells, as well as goblet cells. Surface absorptive cells are used for taking up small particles of food and other energy molecules that neither the stomach nor the pancreas have digested. Goblet cells secrete mucus, which protects the mucosal membrane and facilitates passage of food down the gut. In the case of this tissue type (the mucosa), the structure of the villi (long, finger-like projections into the lumen) facilitates the function of absorption and secretion because their repetitive, branching nature increases the total surface area of the upper layer of the duodenum, allowing for more absorption per unit of the organ's surface area.
The submucosa lies just underneath of the mucosa. It contains various ducts and unique structures called Glands of Brunner. The Glands of Brunner are responsible for producing an alkaline compound to help neutralize the low pH of chyme making its way into the small intestine from the stomach. These glands usually deposit their materials into ducts, which run upward, through the mucosa and into the lumen of the duodenum. The submucosa also has a thin layer of muscularis mucosae that separates it from the upper mucosa. In this way, the structure contributes to the function of the submucosa, because the muscularis mucosae creates pressure that helps squeeze material through the ducts upward into the lumen of the duodenum.
Finally, the next tissue, the muscularis externa, lies beneath the submucosa. It consists of two layered sheaths of smooth muscle—an upper band of inner circular muscle and a lower band of outer longitudinal muscle. This particular structure facilitates the function of the muscularis externa. As the bands of smooth muscle contract antagonistically across one another, they facilitate peristalsis in the gut—the rhythmic undulation of this smooth muscle in order to move food further down the alimentary canal.