Published May 16, 2023 by

Small Intestine Anatomy : Duodenum, Jejunum, Ileum and their function

General Discussion  

The small intestine is a tightly folded, sinuous tube about 20 feet (6 m) long in adults.  It connects to the stomach at the top and to the large intestine (colon) at the bottom.  Most of the food a person eats is digested and absorbed in the small intestine. The small intestine's lining is made up of tiny, finger-like tissues called "villi." These projecting tissues extraordinarily increment the surface region of the digestive system, which ingests calories and supplements from food.

Portions of Small digestive system

The small digestive system contains three sections. The stomach is connected to the first part, which is called the duodenum. The jejunum is in the middle. The third part, called the ileum, associates with the colon.


The stomach empties food into the duodenum, which is the first section of the small intestine. 

Food that the small intestine can break down reaches the duodenum through the pyloric sphincter. The duodenum tells the stomach to stop emptying when it gets full.

In addition to bile from the liver and gallbladder, the pancreas supplies the duodenum with pancreatic chemicals. Entering the duodenum through an opening called the ampulla of Vater and containing the sphincter of Oddi, these liquids are significant in helping assimilation and retention.

Waves of rhythmic muscle contractions (called peristalsis) also aid digestion and absorption by stirring food and mixing it with intestinal secretions.


The jejunum is the second of the three parts that make up the small intestine, located between the duodenum and the ileum.  The jejunal arteries supply it with irrigation, while the celiac and superior mesenteric plexuses, along with the vagus nerve, supply it with innervation.  The jejunum makes about 40% of the entire small intestine, it is crucial for digesting.  Its duties include nutrition and water absorption.

According to histology, its mucosa is lined by simple columnar epithelium and contains the typical intestinal crypts and villi.


Located between the duodenum and the ileum area of the small intestine

Arterial arcades of the superior mesenteric artery
 superior mesenteric vein
Celiac plexus, superior mesenteric plexus, vagus nerve (cranial nerve X)


Mucosa - simple columnar epithelium, contains intestinal or Lieberkuhn crypts and intestinal villi

Submucosa - loose connective tissue containing vessels and nerves
Muscular -two layers of smooth muscles, one inner and one outside:
Serosa - simple squamous epithelium

Digestion of nutrients
Absorption of lipophilic nutrients
Water absorption

Last of the three parts of the small intestine, which lies between the jejunum and the large intestine.  Its distal end opens into the large intestine.
According to histology, the ileal mucosa is made up of enterocytes and goblet cells in a straightforward columnar epithelium.
The ileum helps continue the digestion of food coming from the stomach and other parts of the small intestine.  It absorbs nutrients (vitamins, minerals, carbohydrates, fats, proteins) and water from food so that it can be used by the body.

Functions of the small intestine
The main function of the small intestine is the absorption of most of the nutrients that reach our body through ingestion.  This occurs after the food has been digested both in the mouth and in the stomach.  In addition, thanks to the liver and pancreatic juices that are secreted in the duodenum, food reaches the small intestine in an almost completely liquid form, which facilitates the absorption of nutrients.
Nutrient absorption occurs when nutrients come into contact with the gut wall, which explains why the organ is so lengthy and has so many folds.
In fact, the internal surface of the small intestine is riddled with irregularities in the form of intestinal villi that, in addition to allowing the absorption of nutrients, also allow the surface area through which said absorption is carried out to be enlarged, making this organ one of the most efficient of the human body.