Duodenum: anatomy, location and function


The duodenum, the first and shortest section of the small intestine, is a key organ of the digestive system. The most important function of the small intestine is to digest nutrients and transfer them to the blood vessels located in the intestinal wall for the absorption of nutrients into the bloodstream.

Together, the duodenum and other organs of the digestive tract (the pathway by which food enters the body and excretes solid waste) make up the body's digestive system .

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The duodenum is described as a C-shaped or horseshoe-shaped segment of the small intestine. It is located below the abdomen. This part of the small intestine gets its name from its size; In Latin, duodenum means 12 fingers, which roughly corresponds to the length of the organ. The duodenum can be divided into four segments. Each segment has a different anatomy (shape) and a different primary function. The lining of the duodenum is made up of four layers, each of which has a specific function.


The length of the duodenum is about 20 to 25 centimeters (about 8 to 10 inches) (compared to the jejunum, which is about 2.5 meters or 8 feet long) .

The C-shape of the duodenum surrounds the pancreas, where it receives pancreatic enzymes for digestion. The duodenum is also connected to the liver through a structure called the hepato-duodenal ligament. This is where the duodenum receives bile to mix with the chyme, an important part of the chemical digestion process, which is described in more detail below.

Duodenum segments

The four segments of the duodenum include :

  1. The first segment of the duodenum , the upper part of the duodenum (called the duodenal bulb), connects to the liver through the hepato-duodenal ligament. This compound allows the transport of nutrients from the small intestine to the liver; it also allows the duodenum to receive bile from the liver.
  2. The second segment of the duodenum , the descending (descending) part of the duodenum, is located above the right kidney; it connects to the pancreas through a small tube called the pancreatic duct. The pancreatic duct is the pathway by which pancreatic enzymes enter the duodenum. These enzymes help break down food for proper absorption as food travels further through the small intestine (into the jejunum). The common bile duct, which carries bile from the liver, also enters the second part of the duodenum. If a stone blocks the flow of bile into the duodenum, it can cause jaundice.
  3. The third segment of the duodenum : the transverse part (which passes through the abdominal cavity horizontally) of the duodenum lies in front of the aorta and runs from right to left behind the network of blood vessels.
  4. The fourth segment of the duodenum , the ascending (ascending) part of the duodenum, passes over or slightly to the left of the aorta and eventually becomes the jejunum. The genuno is the middle part of the small intestine, located between the duodenum and the ileum.

Layers of the duodenum

The walls of the duodenum are made up of four layers:

  1. The mucous layer that It is the innermost layer of mucous glands and microvilli (specialized finger-shaped projections that absorb nutrients).
  2. The submucosal layer, which Composed primarily of connective tissue, it has a rich network of blood vessels and nerves that run throughout the duodenum. This submucosal layer also contains glands called Brunner's glands. Brunner's glands secrete mucus (so food can easily move through the duodenum) and a chemical called bicarbonate. The bicarbonate neutralizes the acid in the chyme and prepares it for further digestion.
  3. The outer layer of Muscularis , which is made up of smooth muscle tissue, is responsible for contractions in the gastrointestinal tract. The muscles strike the chyme, mixing it with digestive enzymes and forcing food to travel through the gastrointestinal tract to the jejunum. This muscle movement is called peristalsis .
  4. Serous layer, which It is the outermost layer of the duodenum, made up of squamous epithelium (a layer of squamous cells) that provides a barrier to other organs.


The small intestine is located below the stomach. The small intestine is made up of the duodenum, jejunum, and ileum. The duodenum connects to the stomach with its proximal end (closest to the beginning). It is connected to the middle part of the small intestine, called the jejunum, at its distal (distal) end.

Together, in addition to the esophagus , stomach, large intestine, and accessory organs (such as the liver and pancreas), as well as the duodenum and two other portions of the small intestine, they make up what is commonly called the gastrointestinal tract or gastrointestinal tract. tract.

Anatomical variations

Duodenal atresia (also called duodenal stenosis) is a rare congenital (present at birth) condition of the duodenum. Duodenal atresia involves the complete closure of a portion of the lumen (tubular opening) within the duodenum. Signs and symptoms of duodenal atresia in the fetus include a buildup of amniotic fluid during pregnancy called polyhydramnios. Duodenal atresia also causes intestinal obstruction in newborns.


The main function of the small intestine is to promote the breakdown and absorption of nutrients that the body needs. The duodenum begins this process by preparing the chyme for further breakdown, so that nutrients can be easily absorbed. The process of breaking down food and absorbing nutrients is known as digestion .

What is digestion?

Food that is swallowed travels from the esophagus (a muscular tube lined with a mucous membrane that connects the throat to the stomach) and then enters the stomach through a valve called the pyloric sphincter. The main task of the pyloric sphincter is to open and close to selectively pass only very small particles into the duodenum.

Chemical digestion involves enzymes and other chemicals in the digestive system intended to prepare food / nutrients for absorption into the bloodstream. Chemical digestion begins in the mouth when saliva begins to break down ingested food. This initial digestion process (called chemical digestion) continues in the stomach with stomach acid and continues in the duodenum using enzymes and other chemicals (such as bile from the liver).

Digestion in the duodenum

Undigested food from the stomach, called chyme, enters the duodenum and mixes with digestive juices and enzymes (from the intestinal walls and pancreas) and bile from the gallbladder. This mixing process, called chemical digestion, prepares the stomach's contents to break down food and absorb vitamins, minerals, and other nutrients.

The chemical digestion process begins in the stomach. Chemical digestion continues in the duodenum as pancreatic enzymes and bile mix with the chyme. Nutrient absorption begins in the duodenum and continues in all organs of the small intestine. Nutrient absorption occurs mainly in the second part of the small intestine (called the jejunum ), but some nutrients are absorbed in the duodenum.

The duodenum is considered the mixer of the small intestine because of the agitation process that takes place there: it mixes chyme with enzymes to break down food; adds bicarbonate to neutralize acids, preparing chyme to break down fats and proteins in the jejunum; and includes bile from the liver to aid in the breakdown and absorption of fats.

Other functions

Specific functions of the duodenum include:

  • Mixing and breaking down (breaking into small pieces) food from the stomach through the pylorus (the area between the stomach and the duodenum that contains the pyloric sphincter).
  • It neutralizes the acidity (also called the pH level) of the chyme by mixing it with alkaline digestive juices from the pancreas and liver.
  • Continuation of the digestion process using liver bile, digestive enzymes from the pancreas and intestinal juices, which are secreted by the walls of the duodenum and other organs of the digestive system.
  • Prepare the chyme for further digestion, which takes place in the lower part of the small intestine (including the jejunum and ileum) by mixing it with the bile from the gallbladder to break down fats.
  • Absorption of certain nutrients (such as folic acid, iron, and vitamin D3). According to the Institute of Iron Diseases, "the part of the small intestine called the duodenum is the main area where iron is absorbed. "

Hormonal function

In addition to the function of enzymes, intestinal juices, and bile, certain hormones also play a role in digestion. This includes:

  • Secretin , which is released when a duodenal pH adjustment is required (certain pH levels are required for proper digestion of fats and proteins).
  • Cholecystokinin , which is released to aid in the digestion and absorption of nutrients (such as fats and proteins).

Immune support function

Another important function of the duodenum is to support immunity. The duodenum acts as a barrier to prevent harmful microbes from entering the body. Friendly bacteria in the duodenum (and other parts of the small intestine) take up space and compete for food within the duodenum. As a result, pathogens (pathogenic microbes) can hardly multiply there .

Related conditions

Diseases of the duodenum are common in people of any age. Duodenal diseases are a common source of abdominal discomfort for many people. In fact, symptoms of indigestion, heartburn , and upper abdominal pain can affect approximately 25% of the population .

Due to the complex connection between the duodenum and accessory digestive organs (such as the liver and pancreas), malignant neoplasms (cancer cells) are often seen simultaneously in the duodenum and pancreas, as well as in the bile duct of the liver .
Other common diseases of the duodenum include:

  • Inflammatory bowel disease (IBD), which can cause inflammation of the duodenum or stomach. There are two types of inflammatory bowel disease: Crohn's disease and ulcerative colitis. Only Crohn's disease affects the duodenum. Ulcerative colitis does not affect the duodenum.
  • Celiac disease , a disease that especially affects the duodenum (as a result of side effects when a person ingests gluten or wheat products) .
  • Excessive alcohol use, which can cause inflammation of the duodenum (called duodenitis).
  • Duodenal ulcers (similar to stomach ulcers) that form in the lining of the duodenum.

Duodenitis is an inflammation of the duodenal mucosa. This can be due to a number of different reasons, including :

  • Helicobacter pylori infection (a type of bacteria that commonly causes ulcers and inflammation in the stomach and duodenum)
  • Other types of bacterial infections.
  • Celiac Disease
  • Viral infections
  • NSAIDs (non-steroidal anti-inflammatory drugs), a class of pain relievers that reduce inflammation and include ibuprofen, naproxen, and others. Long-term use of NSAIDs is associated with duodenitis; however, this condition generally does not occur with short-term use of NSAIDs.
  • Autoimmune diseases (such as Crohn's disease)
  • Duodenal lymphocytosis (a condition associated with increased numbers of intraepithelial lymphocytes, a form of small white blood cells, in the duodenal mucosa, detected by biopsy)
  • Smoking tobacco (heavy use)
  • Accidental injury or surgery that negatively affects the duodenum.
  • Chemotherapy or radiation therapy
  • Idiopathic (cause unknown)


Common duodenal conditions, such as duodenitis, can be acute (short-term and severe) or chronic (long-term). The condition may not cause any symptoms; It can be diagnosed when a person is screened for another type of digestive disorder. In other cases, symptoms such as discomfort or a burning sensation in the abdomen may occur.

Other symptoms can include:

  • Bloated feeling after eating (even in small amounts).
  • Nausea and vomiting
  • Indigestion
  • Lower abdominal pain (or in some cases, lower back pain is felt)
  • Black tarry stools (can occur with intestinal bleeding). Be aware that this symptom can be a medical emergency; A person with internal bleeding should seek emergency medical attention immediately.


Various tests are commonly used to diagnose conditions in the duodenum, including duodenitis. This includes:

  • Blood or stool samples (to check for H. pylori ).
  • A urea breath test to check for H. pylori before and after a person drinks the solution.
  • Upper endoscopy, or EGD, is a test used to diagnose the cause of prolonged abdominal pain or heartburn, nausea, vomiting, or blood in the stool. EGD allows a doctor to examine the lining of the duodenum for ulcers or other symptoms such as inflammation or bleeding.
  • Biopsy to look for cancer cells or to diagnose duodenal lymphocytosis.
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