What is the Small Intestine? - Anatomy and Functions

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  • 0:05 Small Intestine
  • 1:13 Duodenum
  • 1:41 Jejunum
  • 2:27 Ileum and Peyer's Patches
  • 4:10 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

In this lesson, you will learn about the three divisions of the small intestine - the duodenum, jejunum, and ileum. You will also gain an understanding of the basic functions of each of these sections, including their role in removing bacteria.

Small Intestine

We previously learned that food you ingest begins to break down in the mouth. Once food is mixed with saliva, it is passed down through the pharynx and esophagus to the stomach where it is broken down further. Even though the food has now been in your digestive system for a few hours and has gone through a fair amount of processing, only a small portion of total digestion has taken place and even less absorption of nutrients. This fact is about to change as we take a look at the anatomy and functions of the small intestines.

The first and middle sections of the small intestine
Duodenum Jejunum Diagram

The small intestine is the section of your digestive tract where the majority of food digestion and nutrient absorption takes place. To understand how digestion and absorption take place in the small intestine, it is important to first understand the structure or anatomy of the organ. Your small intestine follows a twisty path and takes up much of the space within your abdomen. You can see from this picture that it appears to be framed by the large intestine. It is by far the longest section of your alimentary canal, measuring approximately twenty feet, and it is divided into three sections.


The duodenum is the first part of the small intestine immediately beyond the stomach. The duodenum is the shortest of the three divisions of your small intestine, but its small size does not mean that it's not important. In fact, it is the duodenum that receives many of the digestive juices needed to break down foods into nutrients that are small enough to be absorbed by your body. This makes the duodenum the section of the small intestine where the majority of the chemical digestion of food takes place.


The middle section of the small intestine is called the jejunum. We saw that the duodenum was the section of the small intestine where the majority of the chemical breakdown of nutrients took place. Now that these nutrients have been broken down into their smallest units, they can be absorbed through the wall of the jejunum and dropped into the blood stream to be carried to the cells of your body. Therefore, we see the jejunum as a prominent site for nutrient absorption.

Peyers patches on the ileum
Peyers Patches Diagram

To help you remember the order of these first two sections, you might want to remember that their names, duodenum and jejunum, start with the letters D and J. Since a DJ is the one at the party that gets the action started, you can think of the first two sections of the small intestine as the part of your digestive tract that gets things started.

Ileum and Peyer's Patches

The terminal section of the small intestine is called the ileum. The word ileum is Latin for groin, and because the ileum is the lowest section of the small intestine and found near the groin, this can help you remember the term. The ileum follows the jejunum, and it provides additional area for the absorption of nutrients. The ileum also provides a place for the absorption of vitamin B12 and bile salts.

One interesting modification that we see as we move into the ileum is the presence of local collections of lymphatic tissues called Peyer's patches. Peyer's patches are named after the person who first discovered them, a Swiss anatomist named Johann Conrad Peyer. Because our lymphatic system helps us fight bacteria and other foreign invaders, we see that the presence of lymphatic structures here in the ileum reflects the fact that the remaining food that has passed this far through the digestive tract contains a high number of bacteria. These Peyer's Patches act to prevent bacteria from entering the bloodstream.

Undigested food moves through the ileocecal valve to the large intestine
Ileocecal Valve Diagram

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