Table of ContentsView AllTable of ContentsWhat Is the Small Intestine?FunctionPartsRelated SystemsAssociated DisordersKeeping the Small Intestine HealthyTests
Table of ContentsView All
View All
Table of Contents
What Is the Small Intestine?
Function
Parts
Related Systems
Associated Disorders
Keeping the Small Intestine Healthy
Tests
The small intestine is part of thedigestive 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 thecolon, 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.
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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:
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) andelectrolytes(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:
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.
Theduodenum 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.
Thejejunum receives 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.
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:
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.
Conditions That Cause Villous Atrophy
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 thebiliary system. This attaches to the liver, pancreas, and the duodenum.
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 areintraperitoneal, 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.
Lymphatic System
Thelymphatic systemis 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:
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 theenteric nervous system. This is what keeps the intestinal contents moving along the intestinal tract for proper digestion and absorption of nutrients.
Intestines: Anatomy and Function
Common conditionsassociated with the small intestine include:
What to Know About Short Bowel Syndrome
Anatomical Variations
There are also some congenital (present at birth) conditions that can occur anywhere along the small intestine:
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:
Many common tests are used to diagnose conditions of the small intestine. These include:
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.
10 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.MUSC Health.Small intestine.Colorado State University.Gross and microscopic anatomy of the small intestine.Britannica.Ileum.Walton KD, Freddo AM, Wang S, Gumucio DL.Generation of intestinal surface: an absorbing tale.Development (Cambridge, England). 2016;143(13):2261. doi:10.1242/dev.135400University of California San Francisco Vascular & Endovascular Surgery.Mesenteric ischemia.Collins JT, Badireddy M.Anatomy, abdomen and pelvis, small intestine. StatPearls.National Institute of Diabetes and Digestive and Kidney Diseases.Symptoms and causes of GI bleeding.Scarpignato C, Bjarnason I.Drug-induced small bowel injury: a challenging and often forgotten clinical condition.Curr Gastroenterol Rep. 2019;21(11):55. doi:10.1007/s11894-019-0726-1Children’s Hospital of Philadelphia.Small bowel atresia, intestinal atresia.MedlinePlus.Small intestine disorders.
10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.MUSC Health.Small intestine.Colorado State University.Gross and microscopic anatomy of the small intestine.Britannica.Ileum.Walton KD, Freddo AM, Wang S, Gumucio DL.Generation of intestinal surface: an absorbing tale.Development (Cambridge, England). 2016;143(13):2261. doi:10.1242/dev.135400University of California San Francisco Vascular & Endovascular Surgery.Mesenteric ischemia.Collins JT, Badireddy M.Anatomy, abdomen and pelvis, small intestine. StatPearls.National Institute of Diabetes and Digestive and Kidney Diseases.Symptoms and causes of GI bleeding.Scarpignato C, Bjarnason I.Drug-induced small bowel injury: a challenging and often forgotten clinical condition.Curr Gastroenterol Rep. 2019;21(11):55. doi:10.1007/s11894-019-0726-1Children’s Hospital of Philadelphia.Small bowel atresia, intestinal atresia.MedlinePlus.Small intestine disorders.
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
MUSC Health.Small intestine.Colorado State University.Gross and microscopic anatomy of the small intestine.Britannica.Ileum.Walton KD, Freddo AM, Wang S, Gumucio DL.Generation of intestinal surface: an absorbing tale.Development (Cambridge, England). 2016;143(13):2261. doi:10.1242/dev.135400University of California San Francisco Vascular & Endovascular Surgery.Mesenteric ischemia.Collins JT, Badireddy M.Anatomy, abdomen and pelvis, small intestine. StatPearls.National Institute of Diabetes and Digestive and Kidney Diseases.Symptoms and causes of GI bleeding.Scarpignato C, Bjarnason I.Drug-induced small bowel injury: a challenging and often forgotten clinical condition.Curr Gastroenterol Rep. 2019;21(11):55. doi:10.1007/s11894-019-0726-1Children’s Hospital of Philadelphia.Small bowel atresia, intestinal atresia.MedlinePlus.Small intestine disorders.
MUSC Health.Small intestine.
Colorado State University.Gross and microscopic anatomy of the small intestine.
Britannica.Ileum.
Walton KD, Freddo AM, Wang S, Gumucio DL.Generation of intestinal surface: an absorbing tale.Development (Cambridge, England). 2016;143(13):2261. doi:10.1242/dev.135400
University of California San Francisco Vascular & Endovascular Surgery.Mesenteric ischemia.
Collins JT, Badireddy M.Anatomy, abdomen and pelvis, small intestine. StatPearls.
National Institute of Diabetes and Digestive and Kidney Diseases.Symptoms and causes of GI bleeding.
Scarpignato C, Bjarnason I.Drug-induced small bowel injury: a challenging and often forgotten clinical condition.Curr Gastroenterol Rep. 2019;21(11):55. doi:10.1007/s11894-019-0726-1
Children’s Hospital of Philadelphia.Small bowel atresia, intestinal atresia.
MedlinePlus.Small intestine disorders.
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