Duodenum
Anatomy of the Duodenum
The duodenum is the first section of the small intestine in most higher vertebrates, including humans. It plays a crucial role in the digestive system by receiving partially digested food from the stomach and initiating further digestion. The duodenum is approximately 25 to 30 centimeters long and is shaped like a C, wrapping around the head of the pancreas. It is located in the upper abdomen, just below the stomach, and is divided into four parts: the superior, descending, horizontal, and ascending parts.
Superior Part
The superior part of the duodenum is the first segment and is approximately 5 centimeters long. It begins at the pylorus of the stomach and extends to the neck of the gallbladder. This section is intraperitoneal, meaning it is covered by the peritoneum, a serous membrane lining the abdominal cavity. The superior part is also known as the duodenal bulb or cap and is the most mobile section of the duodenum.
Descending Part
The descending part of the duodenum is about 7 to 10 centimeters long and descends along the right side of the vertebral column. It is retroperitoneal, meaning it is located behind the peritoneum. This section is significant because it receives the bile duct and pancreatic duct through the ampulla of Vater, which is controlled by the sphincter of Oddi. The entry of bile and pancreatic juices is essential for the emulsification of fats and the digestion of proteins and carbohydrates.
Horizontal Part
The horizontal part, also known as the inferior part, is approximately 6 to 8 centimeters long. It crosses horizontally from right to left, passing in front of the inferior vena cava, the aorta, and the vertebral column. This section is also retroperitoneal and is fixed in position, unlike the more mobile superior part.
Ascending Part
The ascending part is the final segment of the duodenum and is about 5 centimeters long. It ascends on the left side of the aorta to the level of the second lumbar vertebra, where it joins the jejunum at the duodenojejunal flexure. This junction is supported by the ligament of Treitz, which is a muscular band that helps maintain the angle between the duodenum and jejunum.
Histology
The duodenum's histological structure is specialized to optimize its digestive functions. The inner lining is composed of a mucosa with villi and microvilli, which increase the surface area for nutrient absorption. The mucosa also contains Brunner's glands, which secrete an alkaline mucus to neutralize gastric acid and provide an optimal pH for enzyme activity. The submucosa, muscularis externa, and serosa or adventitia layers support the mucosa and facilitate peristalsis.
Function
The primary function of the duodenum is to continue the process of digestion that begins in the stomach. It achieves this by mixing chyme with bile and pancreatic juices. The presence of chyme in the duodenum stimulates the release of hormones such as secretin and cholecystokinin, which regulate the secretion of digestive enzymes and bile. The duodenum also plays a role in regulating gastric emptying through feedback mechanisms that signal the stomach to slow down or speed up the release of its contents.
Clinical Significance
The duodenum is involved in several clinical conditions that can affect its function. Duodenal ulcers are a common condition caused by the erosion of the mucosal lining, often due to infection with Helicobacter pylori or the use of nonsteroidal anti-inflammatory drugs (NSAIDs). Other conditions include celiac disease, which affects nutrient absorption, and duodenal atresia, a congenital condition characterized by a blockage in the duodenum.