The Anatomy of the Small Intestine

The largest digestive organ breaks down food and absorbs nutrients

In This Article

The small intestine (commonly referred to as the small bowel) is a tubular structure/organ that is part of the digestive system. In fact, it is the longest portion of the digestive system, approximately 20 to 25 feet in length. The reason it is referred to as the “small” intestine, is because its lumen (opening) is smaller in diameter (at approximately 2.5 centimeters or 0.98 inches) than the large intestine (colon).

The primary function of the small intestine 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.

Anatomy

The small intestine is made up of thee sections, including the duodenum, the jejunum and the ileum. On its proximal (near) end, the small intestine—beginning with the duodenum—connects to the stomach. On its distal (far) end, the ileum—the last segment of the small intestine—connects to the large intestine (colon). The jejunum lies between the duodenum and the ileum. The anatomy of the three segments of the small intestine includes:

The duodenum is the smallest segment of the small intestine, measuring only 10 to 15 inches in length. The duodenum 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 is an important landmark that serves as the site where the bile duct and the pancreatic duct empty their digestive juices (containing enzymes that help to breakdown ingested food) into the duodenum.

The pancreatic and bile ducts form a system called the biliary system (also called the biliary tract) which forms a vital part of the digestive system and 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 (such as fats, proteins, and carbohydrates) 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 (colon) lies the ileocecal valve (ileal ostium). The ileum comprises approximately 60% of the small bowel in humans.

The jejunum and ileum are intraperitoneal (located inside the peritoneum). The peritoneum is a thin, vascular (comprised of many small blood vessels) membrane that lines the walls of the abdominal cavity. Organs such as the stomach, the jejunum and ileum are wrapped in the peritoneum. The duodenum, by contrast, is only covered by peritoneum on its anterior (front-facing) surfaces and therefore, it is considered a “retroperitoneal” (behind the peritoneum) organ.

The Mesentery

The mesentery is a contiguous structure (one that touches and shares a common border) that attaches the small intestine (as well as the large intestine) to the posterior (back portion) of the abdominal wall. It is a thin, vascular layer, comprised of a double fold of peritoneum. The purpose of the mesentery is to supply blood to the intestines (and more).

The Lymph System and the Small Intestine

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

In the small intestine, lymphatic drainage begins at the mucosa lining of the small intestine. Next, it 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 (containing absorbed fats, and immune cells). 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).

Location

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.

Anatomical Variations

Small 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 (present at birth) 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:

  • Abdominal cramps
  • Pain
  • Distension (swelling) of the abdomen
  • Vomiting bile soon after birth
  • The inability to pass the initial stool (in an infant this first stool is called meconium).

The treatment of intestinal atresia involves a surgical procedure to correct the problem. The type of operation depends on where the obstruction is located.

Note, 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). Birth defects that affect the small intestine include:

  • Defects of the abdominal wall (including gastroschisis and omphalocele), which are congenital disorders that involve an opening in the abdomen in which the small intestine (and other digestive system organs) protrude.
  • Hirschsprung disease is a condition involving the nerves of the intestines, which fail to develop normally. This results in an intestinal blockage because normal transmission of the nerves in the intestines does not occur, preventing peristalsis (the contraction of the muscles in the small intestine that move the food along the digestive tract).

Function

Overall, the function of the small intestine is to:

  • Churn and mix ingested food, making it into chyme
  • Move the food along its entire length (into the colon)
  • Mix ingested food with mucus (making it easier to move)
  • Receive digesting enzymes from the pancreas and liver (via the pancreatic and common bile ducts).
  • Break down food with digestive enzymes, making it more digestible
  • Absorb nutrients (including fats, carbohydrates, proteins, vitamins, and minerals) into the bloodstream
  • Help to maintain the balance of fluids (absorbs much of the body’s ingested water) and electrolytes (such as sodium)
  • Move the food along into the colon

Each segment of the small intestine has a different function, including:

The duodenum receives partially digested food (called chyme) through the pylorus (from the stomach), receives digestive enzymes from the pancreas and liver to continue to break down ingested food. In addition, iron is absorbed in the duodenum. Bicarbonate (a crucial 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. In addition, there is some limited absorption of nutrients that takes place in the duodenum, such as the absorption of iron.

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.

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.  

Absorption of Nutrients

Although the small intestine is comprised 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. How is this possible? There are three key features of the small intestines that 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, comprised 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, which even further increase the surface area of the small intestine.

The Lining of the Small Intestine

When it comes to digestion, the lining of the small intestine (called the mucosa) is highly specialized to enable the maximum level of nutrient absorption. Intestinal mucosa is comprised of villi as well as cells that produce chemicals which help digestion and produce hormones that help to control the digestive process of the small intestine, pancreas, and gallbladder.

The Enteric Nervous System

The term “enteric” means related to the intestines. One function of the small intestine 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 Conditions

Common conditions associated with the small intestine include:

  • Celiac disease
  • Crohn’s disease
  • Inflammatory bowel disease
  • Irritable bowel syndrome (IBS)
  • Small bowel bacterial overgrowth (SIBO)
  • Peptic ulcers (which involve the stomach and duodenum)
  • Intestinal infections
  • Intestinal bleeding
  • Intestinal cancer (such as duodenal cancer)
  • Intestinal obstructions (such as small bowel obstructions)
  • Small bowel diverticula (small sac-like protrusions of the wall of the large or small intestine)
  • Effects of certain medications

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

Treatment

There are various treatment modalities for disorders of the small intestine, they may include:

  • 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)
  • Meckel's diverticulectomy (surgical treatment for small bowel diverticula)
  • Small bowel resection (a type of surgical procedure for many reasons,
    Including a blockage, cancer, ulcers, infection, bleeding, inflammation of the small intestine from Crohn’s disease, congenital deformities of the small intestine, and more)
  • Special diets (such as a gluten-free diet for celiac disease or a low FODMAP diet for IBS)
  • Medications (corticosteroids such as prednisone and budesonide for conditions such as Crohn’s disease that cause inflammation, and more)

Tests

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

  • Bacterial culture may be done on stool to look for infectious organisms.
  • Esophagogastroduodenoscopy (EGD): A procedure that involves a scope that is used to examine the small intestine, get a fluid sample for culture, or to obtain a biopsy.
  • Fecal occult blood test (FOBT): A test that involves taking a stool sample to test for blood that cannot be seen by the naked eye
  • Ova and parasite test: A microscopic examination of stool to test for the presence of parasites or eggs a common cause of diarrhea
  • Endoscopy (involves a scope with a camera that allows surgeons to view the inside of the small intestine through a very small incision).
  • Upper gastrointestinal: An X-ray examination of the upper GI tract (which involves the esophagus, stomach, and the duodenum) after the ingestion of a contrast medium such as barium that will allow for a clear view of the small intestine and other structures
  • Intestinal ultrasound: To test for symptoms of conditions such as inflammatory bowel disease
  • Computed tomography (CT) or magnetic resonance imaging (MRI) scans: To check for conditions such as cancer)
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Article Sources

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