The Anatomy of the Jejunum

The jejunum is the second segment of the small intestine. It is located between the first part, the duodenum, and the last part, the ileum. Most of the nutrients in food are absorbed in the small intestine. While it is only one part of the small intestine, most of this absorption takes place in the jejunum.

Anatomy

The small intestine is a long, hollow tube, with an empty space inside called the lumen. It is located in the digestive system between the stomach and the large intestine. After food is chewed in the mouth and swallowed, it travels down the esophagus, into the stomach, and then on into the lumen inside the small intestine.

The small intestine is made up of three sections: the duodenum, the jejunum, and the ileum. It can be anywhere from about 22 to 25 feet (around 6.5 to 7.5 meters) long. In most adults, the second section, the jejunum, is about 8 feet (2.5 meters) long.

The small intestine contains several layers. The outer layer is called the serosa and contains the mesothelium and epithelium.

The next layer is called the muscularis, and it consists of two layers of muscle. These muscles work together to move food through the intestine. The thin outer layer of muscle contracts in a way that shortens the length of intestine and the thicker inner layer constricts into the lumen.

The next layer is connective tissue, called the submucosa, which contains nerves and blood and lymphatic vessels. The innermost layer, the mucosa, is covered with a multitude of finger-like structures called villi.

Function

The purpose of the small intestine is to break down and absorb nutrients and minerals from food, which is a process that occurs throughout the small intestine. A large amount of surface area is needed to absorb the small molecules from digested food as it moves through the lumen and over the cells of the small intestine.

This is achieved through the numerous villi that cover the inside of the intestine walls. The many cells of the villi in the mucosal layer of the small intestine facilitate the uptake of nutrients. 

The jejunum has a specialized role in digestion. In the duodenum, complex proteins called enzymes begin to break down food. Small nutrient molecules are extracted. The process continues as the food moves through the duodenum and into the jejunum.

The small nutrients, including sugars, amino acids, and fatty acids, can then be absorbed by the cells in the jejunum. The food continues on through the jejunum and into the next (and last) section of the small intestine, called the ileum. The ileum is where remaining nutrients, such as vitamin B12, are absorbed.

Associated Conditions

Crohn's disease is a form of inflammatory bowel disease that can affect any part of the digestive tract, including the jejunum. When Crohn's disease affects the jejunum, it is called jejunoileitis. This form of Crohn's disease is less common.

Inflammation in the jejunum can mean that people with this condition don't absorb as many nutrients from their food. That may lead to malnutrition and other complications.

There are some digestive conditions that make eating and digesting food difficult. There are several different ways that people who are experiencing these problems can receive nutrients.

One way is through a tube that is placed through the wall of the abdomen and into the jejunum. This is called a feeding jejunostomy. A feeding jejunostomy is used in select patients for certain conditions and is often a life-saving procedure.

A feeding jejunostomy might be created if there is a blockage higher up in the digestive tract, and food cannot move through to reach the small intestine. This can be caused by what's called a gastric outlet obstruction. A gastric outlet obstruction might be from a tumor, a peptic ulcer, a fistula, or an impacted gallstone.

A jejunostomy might also be done for a condition called gastroparesis. In gastroparesis, there isn't a physical obstruction in the digestive tract. Instead, the muscles aren't working as they should to move food through. This presents feeding problems for patients with this condition, and a feeding jejunostomy helps to deliver nutrients.

Another reason for a feeding jejunostomy is to deliver medications. This might be used for those who live with Parkinson's disease, as it allows for the steady delivery of the drugs that help in improving motor function.

Short bowel syndrome (SBS) is a rare condition that occurs when a large section of the small intestine is missing. This can occur either at birth (as a congenital defect) or after having surgery to remove parts of the small bowel. When there is less than about 6 feet (2 meters) of small bowel left, it's considered SBS.

The jejunum is important in absorbing nutrients from food, with the first 3 feet (1 meter ) doing most of the work. For those with SBS, the type of surgery they've had and how much of their jejunum and their large intestine remains is an important part of understanding what treatments might be needed.

SBS often causes the inability to absorb enough fluid and nutrients from food. SBS is treated with dietary support to ensure patients are receiving enough fluids and vitamins and minerals. Medications might also be used that improve absorption, decrease acid, and control diarrhea.

Jejunal atresia is a rare birth defect that affects the mesentery. The mesentery is a membrane that connects the small intestine to the abdominal wall. If part or all of this membrane is missing, the jejunum might become twisted around an artery that brings blood to the colon. Jejunal atresia is usually treated with surgery.

Tests

Because of its location, the jejunum can be difficult to access. There are, however, several tests that might be used to assess any issues that are happening in the middle of the small intestine.

Capsule endoscopy: During this test, a small camera that's shaped like a pill is swallowed. As the camera travels through the digestive tract, it takes photos. The photos can then be used to see the inside of the digestive tract, including the small intestine, and locate any inflammation or other problems.

Computed tomography (CT) enterography: This type of CT scan is an X-ray that can provide images of the abdomen. Patients will be given contrast to drink before the scan in order to help the structures in the abdomen to show up better.

Magnetic resonance enterography: This test, which is a type of scan that's done using magnets, creates images of the small bowel. Patients will drink a contrast solution before the test in order for the intestine to show up better on the images. In addition, contrast might also be given through an IV.

Push endoscopy: During a push endoscopy, a very thin, flexible tube with a light on the end is passed through the mouth and down into even the lower parts of the digestive tract.

During this test, it might be possible to take small pieces of tissues (biopsies) from parts of the small intestine. Biopsies can be tested to understand if there are any diseases or conditions present affecting the small intestine.