What Is the Small Intestine’s Function?
Sherry Christiansen www.facebook.com
The small intestine is the largest organ of the digestive system, linking the stomach to the large intestine. It digests food and absorbs nutrients.
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Verywell Health
The small intestine is part of the digestive system. Its main function is to break down and absorb ingested nutrients while mixing and moving the intestinal contents—consisting of gastric juices and partly digested food—along the digestive tract into the colon, or large intestine.

The small intestine, or small bowel, is a tubular organ that is about 20 to 25 feet in length and is the longest portion of the digestive system.

It is referred to as the “small” intestine because its lumen, or opening, is smaller in diameter (at approximately 2.5 centimeters or 0.98 inches) than the large intestine.

This article explains the anatomy and function of the small intestine. It also discusses associated conditions and treatments.

What Is the Small Intestine?

The small intestine is a long, narrow, convoluted tube that is folded or coiled and extends from the stomach to the colon. It is contained in the central and lower abdominal cavity.

What Does the Small Intestine Do?

The small intestine has many functions including mixing ingested food, breaking it down, moving it into the colon, and absorbing nutrients.

Digestion

The functions of the small intestine related to digestion and moving contents along the digestive tract include:

Churning and mixing ingested food, making it into chyme (partially digested food)

Moving the food along its entire length and into the colon

Mixing ingested food with mucus, which makes it easier to move

Receiving digesting enzymes from the pancreas and liver via the pancreatic and common bile ducts

Breaking down food with digestive enzymes, making it more digestible

Helping fight disease-causing bacteria ingested with food by mobilizing the body’s immune system cells

Absorption

A key role of the small intestine is to absorb nutrients including fats, carbohydrates, proteins, vitamins, and minerals into the bloodstream. It also helps maintain the balance of fluids (absorbs much of the body’s ingested water) and electrolytes (such as sodium).

Although the small intestine is composed of a surface area that should be approximately 10.7 square feet, the absorptive surface of the small intestine is nearly 2,690 square feet.

Parts of the Small Intestine

The small intestine is made up of three main sections:

The duodenum

The jejunum

The ileum

On its proximal, or near end, the small intestine—beginning with the duodenum—connects to the stomach. On its distal, or far end, the ileum—the last segment of the small intestine—connects to the large intestine. The jejunum lies between the duodenum and the ileum.

Each segment of the small intestine has a different function.

Duodenum

The duodenum is the smallest segment of the small intestine, measuring only 10 to 15 inches in length.

It begins where the stomach ends at the pylorus—the valve that opens and closes, allowing food to pass from the stomach into the small intestine.

Next, the duodenum curves around the pancreas and ends in the area of the upper left quadrant of the abdomen, where it connects with the jejunum.

The ampulla of Vater serves as the site where the bile duct and the pancreatic duct empty their digestive juices (containing enzymes that help to break down ingested food) into the duodenum.

The duodenum receives chyme through the pylorus and also receives digestive enzymes from the pancreas and liver to continue to break it down.

In addition, iron is absorbed in the duodenum. Bicarbonate (a biochemical that plays a role in the body’s buffering system) is released from the pancreas to begin to neutralize the stomach acid before the chyme reaches the jejunum.

The duodenum also helps to control how well the stomach empties and the rate of bile duct juices that empty into the small intestine.

Jejunum

The jejunum is the upper portion of the small intestine which connects to the duodenum at one end (at the duodenojejunal flexure) and to the ileum at the other end. The jejunum comprises approximately 40% of the small bowel in humans.

The jejunum receives the undigested food from the duodenum and absorbs nutrients—such as sugar, amino acids, and fatty acids—via finger-like projections called villi. Over 95% of the body’s carbohydrate and protein absorption takes place in the jejunum.

Ileum

The ileum is the distal end of the small intestine that opens into the large intestine. At the junction between the ileum and the first part of the large intestine lies the ileocecal valve (ileal ostium). The ileum comprises approximately 60% of the small bowel in humans.

The ileum receives the food from the jejunum and empties into the large intestine. It continues the process of absorption via the intestinal wall villi, absorbing any products of digestion that were not absorbed by the jejunum. This includes vitamin B12, bile salts, and more.

Mucosa

When it comes to digestion, the lining of the small intestine, or mucosa, is highly specialized to enable the maximum level of nutrient absorption.

Three key features of the small intestine enable it to account for its enormous absorptive surface area, including:

Mucosal folds: The inside surface of the small intestine is not flat, but rather is made up of circular folds that increase the surface area.

Intestinal villi: The mucous folds in the small intestine are lined with multitudes of tiny finger-like projections that protrude into the opening of the small intestine. These villi are covered with absorptive epithelial cells that take up nutrients from the lumen and transport nutrients into the blood.

Microvilli: Densely packed microscopic protrusions, located on top of the villi, further increase the surface area of the small intestine.

Intestinal mucosa is composed of villi, as well as cells that produce chemicals that help digestion and produce hormones that help to control the digestive process of the small intestine, pancreas, and gallbladder.

Many conditions of the small intestine can impact the villi, resulting in malabsorption of nutrients.

Related Systems

There are several systems and related structures that play a role in the functioning of the small intestine.

Biliary System

The pancreatic and bile ducts form an important system called the biliary system. This attaches to the liver, pancreas, and the duodenum.

Pancreatic juice and bile (made in the liver and stored in the gallbladder) help with the digestive process by breaking down nutrients so they can be easily absorbed in the small intestine.

Mesentery

The mesentery is a contiguous structure—one that touches and shares a common border. It attaches the small intestine and the large intestine to the back of the abdominal wall. It is a thin, vascular layer, composed of a double fold of peritoneum.

One purpose of the mesentery is to supply blood to the intestines.

The jejunum and ileum are intraperitoneal, meaning they are located inside the peritoneum. The peritoneum is a thin, vascular (composed of many small blood vessels) membrane that lines the walls of the abdominal cavity.

The duodenum, by contrast, is only covered by peritoneum on its front-facing surfaces, and therefore it is considered a “retroperitoneal” (behind the peritoneum) organ.

Lymphatic System

The lymphatic system is an organ system that comprises a large network of vessels, as well as lymphatic organs and tissues. The function of the vessels is to carry lymph fluid (composed of fluid and cells) from the tissues toward the heart.

In the small intestine:

Lymphatic drainage begins at the mucosa lining.

It next drains into the lymph nodes located near the small intestine, into the mesentery.

Eventually, the lymph fluid drains into the venous system.

The small intestine serves as a major transportation system for lymph fluid. This is one explanation for how cancer cells originating in various areas of the body (such as the small intestine) can circulate, spreading to other areas of the body (such as in the lymph nodes).

Enteric Nervous System

The term “enteric” means related to the intestines. One small intestine function is to coordinate many of its activities, including its movements.

It does this because the small intestine has a highly integrated nervous system, called the enteric nervous system. This is what keeps the intestinal contents moving along the intestinal tract for proper digestion and absorption of nutrients.

Associated Disorders

Common conditions associated with the small intestine include:

Celiac disease, an autoimmune disease in which eating gluten triggers white blood cells to attack the lining of the small intestine

Crohn’s disease, a long-term condition that causes painful inflammation along the digestive tract

Irritable bowel syndrome (IBS), a common condition related to a signaling issue between the brain and the gut that leads to gastrointestinal symptoms

Small bowel bacterial overgrowth (SIBO), or excessive gut bacteria in the small intestine

Peptic ulcers, which are open sores located along the stomach or duodenum

Intestinal infections that may be caused by viruses or bacteria

Intestinal cancer, such as duodenal cancer

Intestinal bleeding which can occur with ulcers, inflammation, or cancer

Intestinal obstructions, or blockages, such as small bowel obstructions

Small bowel diverticula, which are small sac-like protrusions of the wall of the large or small intestine

Medications that cause injury or impairment of the small intestine, such as NSAIDs, antimicrobials, proton pump inhibitors, or chemotherapy drugs

Anatomical Variations

There are also some congenital (present at birth) conditions that can occur anywhere along the small intestine:

Small bowel atresia (also known as intestinal atresia) is a blockage of the small intestine. It may be due to undeveloped digestive organs or digestive organs that are not positioned correctly. This can cause blockages, ineffective muscle movement, or abnormal nerve transmission (required for normal bowel motility).

Defects of the abdominal wall can lead to disorders that involve an opening in the abdomen (gastroschisis and omphalocele) in which the small intestine and other digestive system organs protrude.

Hirschsprung disease is a condition involving the nerves of the intestines that fail to develop normally, resulting in an intestinal blockage.

Duodenal atresia is reportedly the most common type of congenital bowel obstruction that is diagnosed prenatally. It is usually diagnosed by X-rays and other tests.

The treatment for congenital conditions along the small intestine often involves a surgical procedure to correct the problem. The type of operation depends on the issue and where it is located.

Congenital defects can occur anywhere along the small intestine, as well as other areas of the digestive tract, such as the large intestine, stomach, esophagus, and more.

How to Keep the Small Intestine Healthy

The various treatment modalities for disorders of the small intestine include:

Special diets, such as a gluten-free diet for celiac disease or a low FODMAP diet for IBS

Medications, including corticosteroids such as prednisone and budesonide for conditions such as Crohn’s disease that cause inflammation

Antibiotics, such as ciprofloxacin or piperacillin/tazobactam, that may be used in cases of bacterial infections

Surgical treatment, for conditions such as bowel obstructions or cancer

Intestine transplant, an infrequently performed procedure for acute (severe, short-term) cases of intestinal failure resulting from loss of blood flow to the intestines caused by a blockage or clot in a major artery that supplies blood to the intestines

Small bowel resection, a type of surgical procedure to treat a blockage, cancer, ulcers, infection, bleeding, inflammation of the small intestine from Crohn’s disease, congenital deformities of the small intestine, and more

Tests

Many common tests are used to diagnose conditions of the small intestine. These include:

Bacterial culture: This may be done on stool to look for infectious organisms.

Abdominal X-ray: This looks at the diameter of the small intestine to see if it is dilated. Also, fluid levels in the small intestine can be viewed to make sure there is no obstruction.

Esophagogastroduodenoscopy (EGD): This procedure involves a scope that is used to examine the small intestine, get a fluid sample for culture, or obtain a biopsy. It can also pick up a bleeding ulcer, as well as treat it by injecting drugs to stop the bleeding.

Fecal occult blood test (FOBT): This test involves taking a stool sample to test for blood that cannot be seen by the naked eye.

Ova and parasite test: This is a microscopic examination of stool to test for the presence of parasites or eggs, a common cause of diarrhea.

Endoscopy: This involves a scope with a camera that allows surgeons to view the inside of the small intestine through a very small incision.

Upper gastrointestinal X-ray: This X-ray examination of the upper GI tract (which involves the esophagus, stomach, and duodenum) after the ingestion of a contrast medium such as barium allows for a clear view of the small intestine and other structures.

Intestinal ultrasound: This tests for symptoms of conditions such as inflammatory bowel disease.

Computed tomography (CT) or magnetic resonance imaging (MRI) scans: These check for conditions such as cancer.

Summary

The small intestine extends from the stomach to the colon. It is made up of three sections called the duodenum, the jejunum, and the ileum.

The functions of the small intestine include the digestion of food and absorption of nutrients.

Certain conditions are associated with the small intestine and many can impact how well nutrients are absorbed. Treatment for small intestine-related issues can vary and may include medications, diet changes, and surgery.

By Sherry Christiansen

Christiansen is a medical writer with a healthcare background. She has worked in the hospital setting and collaborated on Alzheimer's research.