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. Show The small intestine, or small bowel, is a tubular structure/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. magicmine / Getty Images This article explains the anatomy and function of the small intestine. It also discusses associated conditions and treatment options. AnatomyThe small intestine is made up of three 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. The duodenum is the smallest segment of the small intestine, measuring only 10 to 15 inches in length.
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. 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 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 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. The MesenteryThe 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 Lymph System and the Small IntestineThe 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:
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). LocationThe 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. Anatomical VariationsSmall bowel atresia (also known as intestinal atresia) is a blockage of the small intestine that is present at birth. 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 cause of small intestinal atresia may be due to undeveloped digestive organs, or it may be due to digestive organs which are not positioned correctly. This can cause blockages, ineffective muscle movement, or abnormal nerve transmission (required for normal bowel motility). Common symptoms of small bowel atresia include:
The treatment of intestinal atresia involves a surgical procedure to correct the problem. The type of operation depends on where the obstruction is located. Birth defects that affect the small intestine include:
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. Small Intestine FunctionOverall, the function of the small intestine is to:
Small Intestine Function by SegmentEach segment of the small intestine has a different function:
Absorption of NutrientsAlthough 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. Three key features of the small intestine enable it to account for its enormous absorptive surface area, including:
The Lining of the Small IntestineWhen it comes to digestion, the lining of the small intestine, or mucosa, is highly specialized to enable the maximum level of nutrient absorption. 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. The Enteric Nervous SystemThe term “enteric” means related to the intestines. One small intestine function is to coordinate many of its activities, including peristalsis. 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 ConditionsCommon conditions associated with the small intestine include:
Many conditions of the small intestine can impact the villi, resulting in malabsorption of nutrients. TreatmentThe various treatment modalities for disorders of the small intestine include:
TestsMany common tests are used to diagnose conditions of the small intestine. These include:
SummaryThe 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 small intestine has many functions including mixing ingested food, breaking it down, moving it into the colon, and absorbing 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. |