The duodenum is a part of the small intestine that connects the stomach to the jejunum. It is mainly concerned with the digestion of food, and absorption of nutrients. It has been observed that more food and nutrients are absorbed in the duodenum than the stomach. The duodenum is a short tube-like structure that is considered as the first and the shortest segment of the small intestine. In humans, the duodenum is about 25 to 30 cm in length.
Structure and Location
The duodenum is located just below the stomach, and its first part is quite close to the liver and the pancreas. It is located between the stomach and the jejunum. Anatomically, this small organ is divided into four parts or segments, which are known as the superior, descending, horizontal, and ascending duodenum.
It begins with the duodenal bulb, which is located next to the stomach. The first or superior part of the duodenum is nothing but a continuation of the duodenal end of the pylorus. The second or the descending part is the portion, where the duodenum begins to curve or descend. This is where the common bile duct and the pancreatic duct enter the duodenum.
The third part of the duodenum is the horizontal portion. The fourth or the ascending part, connects to the diaphragm by the ligament of Treitz, and then leads to the jejunum. The duodenum ends at the duodenojejunal flexure, the point where it meets the jejunum.
Functions of the Duodenum
Its main function is to receive partially-digested food from the stomach, and then complete the process of digestion. The first part of the duodenum is more susceptible to peptic ulcers, mainly due to its exposure to the chyme that contains unneutralized stomach acids. As one of the most important parts of the digestive system, it is concerned with both digestion of food and absorption of nutrients. In fact, most chemical digestion takes place in the duodenum.
Importance in Digestion
~ In the stomach, food is mixed with stomach acids, and then digested partially. This mixture of semi-digested food and stomach acids is known as chyme. The duodenum receives the acidic chyme from the stomach, and then starts breaking it down with the help of enzymes and intestinal juice. There is a small circular opening between the stomach and the duodenum, known as pylorus, which releases the chyme into the duodenum.
~ The cells present in the duodenal epithelium release two hormones, known as secretin and cholecystokinin. Secretin is released in response to the presence of excess acids in the duodenum, while the secretion of cholecystokinin is stimulated by the presence of amino acids and fatty acids.
~ The hormone secretin stimulates the gallbladder to release alkaline bile, while cholecystokinin induces the pancreas to secrete bicarbonates into the duodenum. The pancreas also releases several enzymes (lipase, amylase, and trypsin) into the duodenum. These secretions increase the pH of acidic chyme, and play a crucial role in digestion.
~ The inner lining of the duodenum contains crypts. These crypts are responsible for increasing the surface area of the intestinal membrane, and thereby ensure better digestion. The duodenum also contains smooth muscles, which facilitate the movement of waste materials down the large intestine.
~ The duodenum is responsible for regulating the rate of gastric emptying, and triggering the hunger signals. Gastric emptying refers to the emptying of the stomach, i.e., the process of food leaving the stomach and entering the duodenum. These functions are facilitated by the hormones secreted by the duodenal epithelium.
~ Absorption of many essential nutrients takes place in the duodenum. It houses the Brunner’s glands, which are responsible for producing a mucus-rich alkaline secretion that serves several important purposes. It protects the duodenum from the acidic chyme and releases urogastrone, which prevents the secretion of acid and other digestive enzymes by the parietal and chief cells of the stomach.
~ The duodenum is the site of iron and calcium absorption. Other nutrients that are absorbed in the duodenum are Vitamin A and B1, amino acids, fatty acids, monoglycerides, phosphorus, and mono and disaccharides.
~ The base of the duodenum, jejunum, and ileum contains the paneth cells, that secrete antibacterial enzymes to protect the intestine from microbes. These cells help prevent the excessive growth of intestinal flora.
To sum up, the duodenum is concerned with the digestion of food, absorption of nutrients, regulation of the rate of gastric emptying and hunger, and the movement of food through the intestinal tract. The gastric bypass surgery (used for treating morbid obesity) often involves the duodenum, due to its immense importance in the absorption of nutrients, and regulation of hunger and movement of food.