Duodenum is the first and smallest section of the small intestines, with a length of 10-12 inches. It plays a vital role in the complete digestion of the partially digested food (chyme) from the stomach. The secretions present in the duodenum ensure that the macromolecules in the chyme are broken down into their basic subunits, ready to be absorbed into the bloodstream in the later part of the intestine. Duodenum is split into four parts; anterior, posterior, superior and inferior. At the second part of the duodenum is a conical structure called Ampulla of Vater which secretes bile from liver, gallbladder and pancreatic juices from the pancreas (containing enzymes such as lipases and proteases) into the duodenum.
Walls of duodenum are made up of four layers, each vital for its function.
- The innermost layer is called mucosa. It is made up of simple columnar epithelial tissues with microvilli present on them. The microvilli increase the surface area for absorption. Moreover, mucosa consists of Brunner's glands that secrete alkaline mucus containing a high amount of bicarbonate ions that help to neutralize the acidic components in chyme from the stomach and help protect the innermost layer form the damage by a low pH.
- The second layer is called submucosa. Its made up of connective tissues that provide structural strength to other layers too. It is covered with many blood vessels and nerves that ensure good blood circulation in the duodenum, important for digestion and absorption.
- The third layer is called Muscularis. This layer ensures the movement of chyme along the duodenum and the rest of the intestine. It consists of smooth muscle tissues that are responsible for producing peristaltic contractions that mix up the chyme and pushes it through the duodenum.
- Lastly, the outermost layer of the duodenum; serosa, is made up of simple squamous epithelium which secretes serous fluid, providing it a smooth surface that helps prevent friction between the duodenum and the surrounding organs.