The Anatomy of the Duodenum

The first segment of the small intestine

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

Together, the duodenum and other organs of the alimentary canal (the pathway by which food enters the body and solid wastes are expelled) form the digestive system of the body.

the duodenum

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The duodenum has been described as a C-shaped or horseshoe-shaped segment of the small intestine. It is located below the stomach. This portion of the small intestine received its name due to its size; in Latin, duodenum translates to "12 fingers," which is the approximate length of the organ. 

The duodenum can be separated into four segments. Each segment has a different anatomy (shape) and performs a different function. The lining of the duodenum is composed of four layers—each with its own specialized function.


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

The duodenum’s “C” shape surrounds the pancreas, where it receives pancreatic enzymes for digestion. The duodenum also connects to the liver via a structure called the hepatoduodenal ligament. This junction is where the duodenum receives bile to mix with chyme, an important part of the chemical digestive process described in more detail below.

Segments of the Duodenum

The four segments of the duodenum are:

  1. The first segment of the duodenum: The superior part of the duodenum (called the duodenal bulb) is connected to the liver via the hepatoduodenal ligament. This connection allows for transportation 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 (extending downward) portion of the duodenum is located above the right kidney; it is connected to the pancreas via a small tube called the pancreatic duct. The pancreatic duct is the mode by which pancreatic enzymes travel into the duodenum. These enzymes help to break down food for proper absorption, as the food travels farther through the small intestine (into the jejunum). The common bile duct carrying 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 (extending across the abdomen horizontally) part of the duodenum is located in front of the aorta and travels from right to left, behind a network of blood vessels.
  4. The fourth segment of the duodenum: The ascending (extending upward) part of the duodenum passes on top or slightly to the left of the aorta, and eventually becomes the jejunum. The jejunum is the middle portion of the small intestine, located between the duodenum and the ilium.

Layers of the Duodenum

The walls of the duodenum are composed of four layers:

  1. The mucosa layer, which is the innermost layer, is made up of mucous glands and microvilli (specialized fingerlike projections that function to absorb nutrients). 
  2. The submucosa layer, which is primarily composed of connective tissue, has a rich network of blood vessels and nerves traveling through the length of the duodenum. This submucosal layer also contains glands called Brunner’s glands. Brunner's glands function to secrete mucus (to help enable the food to easily move through the duodenum) and a chemical called bicarbonate. Bicarbonate serves to neutralize the acid content in the chyme, getting it ready for further digestion.
  3. The muscularis externa layer, which is composed of smooth muscle tissue, is responsible for contractions in the gastrointestinal (GI) tract. The muscles churn the chyme, mixing it with digestive enzymes, and cause the food to move along the GI tract into the jejunum. This muscle movement is called peristalsis.
  4. The serosal layer, which is the outermost layer of the duodenum, is composed of squamous epithelium (a single layer of flat cells) that provides a barrier to other organs. 


The small intestine is located below the stomach. The small intestine is composed of the duodenum, the jejunum, and the ileum. The duodenum is connected to the stomach at its proximal (toward the beginning) end. It is connected to the middle section of the small intestine, called the jejunum, at its distal (located away from a specific area) end.

Collectively—in addition to the esophagus—the stomach, large intestine, and accessory organs (such as the liver and pancreas), along with the duodenum and the other two sections of the small intestine, form what is commonly referred to as the gastrointestinal tract or GI tract.

Anatomical Variations

Duodenal atresia (also called duodenal stenosis) is a rare congenital (present at birth) disorder of the duodenum. Duodenal atresia involves complete closure of a portion of the lumen (tube-like opening) inside 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 primary function of the small intestine is to facilitate the breakdown and absorption of nutrients needed by the body. The duodenum begins this process by preparing the chyme to be further broken down so that nutrients can be absorbed easily. The process of breaking down food and absorbing nutrients is known as digestion.

What Is Digestion?

The food that is swallowed moves from the esophagus (the muscular tube lined with mucous membrane that connects the throat with the stomach), then travels into the stomach through a valve called the pyloric sphincter. The pyloric sphincter's primary job is to open and shut in order to selectively allow only very small particles into the duodenum. 

Chemical digestion involves enzymes and other chemicals in the digestive system, which aim to get the food/nutrients prepared to be absorbed in the blood. Chemical digestion begins in the mouth, as saliva starts to break down the food that is ingested. This initial process of chemical digestion continues in the stomach via gastric (stomach) acid, and then in the duodenum by the use of enzymes and other chemicals (such as bile from the liver).

Digestion in the Duodenum

The duodenum receives undigested food from the stomach—called chyme—and mixes it with digestive juices and enzymes (from the intestinal wall and pancreas) as well as with bile from the gallbladder. This mixing process, called chemical digestion, prepares the stomach contents for the breakdown of food and the absorption of vitamins, minerals, and other nutrients.

Digestion continues in the duodenum as pancreatic enzymes and bile are mixed with the chyme. Absorption of nutrients begins in the duodenum and continues throughout the organs of the small intestine. Nutrient absorption primarily occurs in the second portion of the small intestine (called the jejunum), but some nutrients are absorbed in the duodenum.

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

Other Functions

Specific functions of the duodenum include:

  • Receiving the food that has been mixed and churned (broken down into small pieces) from the stomach, through the pylorus (the section between the stomach and duodenum that contains the pyloric sphincter)
  • Neutralizing the acidity (also referred to as the pH level) in the chyme, by mixing it with alkaline digestive juices from the pancreas and liver
  • Continuing the digestive process with the use of bile from the liver, digestive enzymes from the pancreas, and intestinal juices, which are secreted by the walls of the duodenum and other organs of the digestive system
  • Preparing the chyme for further digestion, which takes place in the lower part of the small intestine (including the jejunum and ilium) by mixing in bile from the gallbladder to help break down fats
  • Absorbing certain nutrients (such as folate, iron, and vitamin D3). According to the Iron Disorders Institute, “the portion of the small intestine called the duodenum is the chief area where iron absorption takes place.”

Hormone Function

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

  • Secretin, which is released when the pH of the duodenum needs adjusting (specific pH levels are needed 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 immune support. The duodenum acts as a barrier to prevent harmful microbes from entering the body. The friendly bacteria in the duodenum (and other parts of the small intestine) take up space and compete for food inside the duodenum. As a result, pathogens (disease-causing germs) have a difficult time multiplying there.

Associated Conditions

Conditions of the duodenum are prevalent in people of any age. Maladies of the duodenum are a common source of abdominal discomfort for many people. In fact, symptoms of indigestion, heartburn, and upper abdominal pain may affect approximately 25% of the population.

Due to a complex connection between the duodenum and the accessory organs of digestion (such as the liver and pancreas), malignancies (cancerous cells) are often seen concurrently in the duodenum and pancreas as well as the bile duct of the liver.

Other common disorders of the duodenum include:

  • Inflammatory bowel disease (IBD), which may cause inflammation in the duodenum or the stomach. Inflammatory bowel disease has two types: Crohn's disease and ulcerative colitis. Only Crohn's disease affects the duodenum. Ulcerative colitis does not affect the duodenum.
  • Celiac disease, a condition that particularly impacts the duodenum (as a result of adverse effects when a person eats gluten or wheat products)
  • Excessive alcohol consumption, which can cause inflammation of the duodenum (called duodenitis)
  • Duodenal ulcers (similar to stomach ulcers), which are lesions that form in the lining of the duodenum

Duodenitis is an inflammation of the lining of the duodenum. This can have several different causes, including:

  • Helicobacter pylori infection (a type of bacterium that commonly causes ulcers and inflammation in the stomach and duodenum)
  • Other types of bacterial infections
  • Celiac disease
  • Viral infections
  • NSAIDs (nonsteroidal anti-inflammatory drugs), a class of pain medications that reduce inflammation; they include ibuprofen, naproxen, and others. Long-term use of NSAIDs is associated with duodenitis; however, the condition does not typically occur with short-term use of NSAIDs.
  • Autoimmune diseases (such as Crohn's disease)
  • Duodenal lymphocytosis (a condition involving an increased number of intraepithelial lymphocytes—a form of small white blood cells—in the lining of the duodenum, discovered via a biopsy)
  • Smoking tobacco (heavy use)
  • Accidental injury or surgery that negatively impacts the duodenum
  • Chemotherapy or radiation therapy
  • Idiopathic (unknown causes)


Common conditions of the duodenum, such as duodenitis, may be acute (short-term and severe) or chronic (long-term). The condition may not result in any symptoms at all; it may be diagnosed when a person is being examined for another type of digestive disorder. In other instances, symptoms such as discomfort or a burning sensation in the abdominal region may be present.

Other symptoms may include:

  • Feeling bloated after eating (even small amounts)
  • Nausea and vomiting
  • Indigestion
  • Pain in the lower abdomen (or in some cases, pain felt in the lower back)
  • Black tarry stools (can occur if there is intestinal bleeding). Note that this symptom may constitute a medical emergency; a person with internal bleeding should seek emergency medical care right away.


Several tests are commonly used to diagnose conditions of the duodenum, including duodenitis. These include:

  • Blood or stool samples (to test for H. pylori)
  • A urea breath test, conducted to test for H. pylori before and after a person drinks a solution
  • Upper endoscopy, or EGD, a test used to diagnose the cause of abdominal pain or prolonged heartburn, nausea, vomiting, or blood in the stool. The EGD allows the healthcare provider to view the lining of the duodenum to check for the presence of ulcers or other symptoms such as inflammation or bleeding.
  • A biopsy to check for cancer cells or to diagnose duodenal lymphocytosis
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7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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