Interestingly, this portion of the small intestine got its name due to its length.
The word duodenum is a Latin corruption of the Greek dodekadaktulus that expresses 12 fingers wide, approximately the duodenum length.
Herophilus coined it in 300 BC to describe the length of the duodenum of animals, and it is presumed that of large domestic animals.
The duodenum is an organ that is anatomically located between the stomach and the jejunum, just below the stomach and its first part is quite close to the liver and pancreas.
The duodenum is the initial C-shaped segment of the small intestine and is a continuation of the pylorus.
The duodenum begins at the pylorus and ends at the ligament of Treitz, and is approximately 25–30 cm in length.
Distally, it is in continuation with the jejunum and ileum, one of its characteristics being the shortest and widest proximal segment.
During the embryological development of the intestine, the duodenum manifests itself in close association with other processes of intestinal organogenesis.
The craniocaudal and lateral folding causes the opening of the intestinal tube to the yolk sac to close, forming a pocket towards the upper end of the embryo, which will eventually become the foregut.
The duodenum arises from the most caudal part of the foregut. However, it is presumed that the duodenum does not undergo any rotation but takes on its C-shaped appearance due to the extension of the stomach to the left and relative fixation by the growth of the liver and pancreas.
Due to this process, the lower portion of the duodenum lies below the superior mesenteric artery.
At the end of development, the loop of the duodenum lies to the right of the abdominal wall, and its peritoneal layers are incorporated into the peritoneal layer that lines the abdominal cavity.
This process is why the duodenum is sometimes described as secondarily retroperitoneal.
Structure of the duodenum
The duodenum walls are made up of 4 layers of tissue that are identical to the other layers of the gastrointestinal tract. From the innermost to the outermost layer, these are the mucosal layers, the submucosal layers, the muscle layers, and the severe layers.
The mucous layer lines the inner surface of the duodenum and is made up of single columnar cells and multiple glands.
The mucosa is arranged as a series of shallow folds approximately 1 mm in height and 5 mm in width that extends transversely to the long axis of the intestine.
The mucosa projects towards the lumen of the duodenum as villi; it is also observed that the inner lining of the duodenum contains crypts to considerably increase the surface area of the intestinal membrane and thus ensure better digestion.
The submucosal layer is primarily a layer of connective tissue through which blood vessels and nerves travel.
The muscular layer of the duodenum includes the circular and longitudinal muscles.
Through the coordination of the contraction of these muscles, peristalsis is allowed to occur throughout the entire gastrointestinal tract, including the duodenum, facilitating the mixing and movement of chyme and waste materials through the large intestine.
The serous layer is characterized by a squamous epithelium that acts as a barrier to the duodenum from other organs within the human body.
The duodenum is mainly retroperitoneal and has an intimate anatomical relationship with the pancreas. This small organ is divided into four segments that include the following:
The first section, or bulb, is located next to the stomach. This first or upper part of the duodenum is nothing more than a continuation of the outer part of the duodenum: the pylorus.
The duodenum begins at the pylorus, bounded by the prepyloric vein, and is approximately 5 cm in length. The posterior wall of this portion of the duodenum is in direct contact with the gastroduodenal artery, the common bile duct, and the portal vein.
The superior border of the first segment of the duodenum is attached to the porta hepatis by the hepatoduodenal ligament, which surrounds the portal triad. This portion of the duodenum begins around the head of the pancreas.
The second or descending segment is just above the inferior vena cava and the right kidney, with the head of the pancreas in a C-shaped concavity.
This is the portion where the duodenum begins to curve or descend; in this area, the common bile duct and the pancreatic duct enter the duodenum.
This second descending section is retroperitoneal and approximately 10 cm long. This segment is anterior to the right kidney and ureter and the lateral border of the inferior vena cava.
The central pancreatic duct, Wirsung duct, and common bile duct unite and empty into the posteromedial wall of the mid-descending duodenum. This opening is known as Vater’s ampulla.
The lesser pancreatic duct, the duct of Santorini, can also empty into the duodenum as the more deficient papilla.
The third segment is a horizontal portion, which runs from right to left. This third cross-section is also retroperitoneal and is limited by the uncinate process of the pancreas superiorly and the hepatic flexure of the anterior colon.
The superior mesenteric vessels run anterior to the transverse duodenum. The right ureter, right gonadal vessels, inferior vena cava, and aorta are posterior to the transverse duodenum.
The fourth segment of the duodenum extends cephalad to the left of the aorta and inferior to the neck of the pancreas.
The ligament of Treitz marks the end of the fourth portion. The ligament serves as an attachment point during bowel rotation and extends from the right fundus of the diaphragm, and attaches to the bowel wall at duodenal jejunal flexion.
This ascending part is connected to the diaphragm by the 3x ligament and leads to the jejunum.
Blood and lymphatic supply
The blood supply of the C-shaped duodenum is shared with the head of the pancreas. The proximal segment of the duodenum is supplied by the gastroduodenal artery and its branches, which include the superior pancreaticoduodenal artery.
The distal segment of the duodenum is supplied by the superior mesenteric artery and the inferior pancreaticoduodenal artery. Venous drainage follows the streets and ultimately drains into the portal system.
The duodenum also has lymphatic vessels that drain into the pancreaticoduodenal lymph nodes located along the pancreaticoduodenal vessels and the superior mesenteric lymph nodes.
The nerves of the duodenum travel through the submucosal layer of the duodenum. The duodenum is richly innervated by the parasympathetic nervous system, including branches of the anterior and posterior vagus trunks.
These parasympathetic nerves pass through the celiac plexuses and follow the celiac trunk into the duodenum. The nerves then synapse on ganglia in the intestinal nets in the duodenum and reach their final targets via short postsynaptic fibers.
The sympathetic nerves are branches of the celiac plexus that originate from T5 to T9. These sympathetic nerves pass through the sympathetic chain and travel through the greater splanchnic nerve and synapse in the celiac ganglia. Sympathetic postsynaptic follow the branches of the celiac trunk into the duodenum.
Most of the chemical digestion occurs in the stomach; the food is mixed with stomach acids and then partially digested; this mixture of semi-digested food and stomach acid is known as chyme.
The duodenum is crucial to the role of the small intestine in the digestive system as a whole because it is where chyme is processed.
The function of the duodenum is thus a continuation of the digestion process that initially began in the stomach by receiving partially digested food in the stomach and then completing the digestion process.
The first part of the duodenum is more susceptible to peptic ulcers, mainly due to its exposure while containing unused stomach acids as one of the essential parts of the digestive system.
The duodenum receives chyme generated by the stomach through a controlled valve between the stomach and the duodenum called the pylorus.
Digestion within the duodenum is facilitated by digestive enzymes and intestinal juices secreted by the intestinal wall and fluids from the gallbladder, liver, and pancreas.
This is received in the duodenum by the significant and minor papilla in the second part of the duodenum. The duodenal papilla is surrounded by a superior semicircular fold and the sphincter of Oddi, which is the muscle that prevents the backflow of duodenal secretions into the bile and pancreatic ducts.
When chyme enters the duodenum, the functions of this organ are facilitated by hormones secreted by the duodenal epithelium, where many essential nutrients are absorbed.
There are housed in the Brahmas glands, which are responsible for producing a mucus-rich alkaline secretion that serves several vital purposes and protects the duodenum from acid chyme by neutralizing stomach acid, preventing it from damaging the most sensitive parts of the intestinal tract.
The duodenum has the unique ability to regulate its environment with hormones released from the duodenal epithelium.
One of these hormones is secretin, which is released when the pH of the duodenum falls below a desirable level. This hormone acts to neutralize the pH of the duodenum by stimulating the secretion of water and bicarbonate in the duodenum.
This helps in the digestion process as pancreatic amylase and lipase require a specific pH to function optimally.
Another hormone that is released by the duodenal epithelium is cholecystokinin.
Cholecystokinin is released in the presence of fatty acids and amino acids within the duodenum. It acts to inhibit gastric emptying and stimulate gallbladder contraction while causing relaxation of the sphincter of Oddi to allow the supply of bile to the duodenum to help digestion and absorption of nutrients.
These bodily fluids process the chyme, breaking down the chemical compounds present so that the nutrients can be more easily extracted as they pass through the intestines.
On the way from the processed chyme to the jejunum, the duodenum absorbs some nutrients, the most important of which is iron.
Gastric bypass surgery used to treat morbid obesity often involves the duodenum due to its immense importance in absorbing nutrients. This requires that those undergoing this duodenal bypass surgery take iron supplements to stay healthy.
Other nutrients absorbed by the duodenum are vitamin A and vitamin B1, calcium, amino acids, fatty acids, monoglycerides, phosphorus, monosaccharides, and disaccharides.
The duodenum is responsible for triggering hunger signals and food movement through the intestinal tract.
Among the diseases that affect the function of the duodenum are:
Duodenal peptic ulcer
A duodenal peptic ulcer is a very common ulcer that causes an imbalance between gastric acid production, epithelial protective factors, and bicarbonate production.
This type of ulcer is associated with Helicobacter pylori bacteria and gastric metaplasia. However, it is not associated with malignancy.
They are aggressive tumors; in the case of G cell tumors (gastrin), they are associated with Zollinger-Ellison syndrome and MEN1 syndrome. In D cell tumors (somatostatin), they are associated with neurofibromatosis.
These tumors are associated with type 1 neurofibromatosis; multiple tumors often appear and have a long period of recurrence or metastasis.
The average age of onset is 56 years; it appears in 10 to 88 years.
Benign tumors such as smooth muscle tumors have also been observed from the muscular propria of the small intestine or the muscular mucosa.
Leiomyomas of the muscular mucosa are incidental, 1 to 2 mm in size, and fuse with the muscular mucosa. Those not of the muscular mucosa are also usually incidental but are more extensive, 2 to 4 cm.