Medical Specialties: Family practice, Gastroenterology, Internal medicine

Clinical Definition

The small intestine is a 22-foot-long tube that breaks down food and consists of three segments, including the jejunum. The jejunum lies between the duodenum (i.e., the upper part of the small intestine) and the final section, known as the ileum. Along with the ileum, the jejunum is mainly responsible for nutrients being absorbed into the blood stream.

In Our Own Words

The small intestine consists of three sections: the first part, the duodenum; the second part, the jejunum; and the last part, the ileum, which leads to the large intestine.


The jejunum, about 9 feet long, is in the middle of the small intestine. Having many more circular folds (i.e., plicae circularis) than the ileum, the jejunum is specialized to absorb nutrients with its increased surface area. The small intestine is also the site of immune cells (i.e., lymphoid tissue). These immune cells may be present in the jejunum but become more prominent in the ileum as “Peyer’s patches.”


Contents start out semi-solid in the small intestine and end up in liquid form when they finish their passage through the small intestine. Nutrients are then absorbed into the blood stream, and other substances move on to the large intestine, or colon.

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  • Harvard Health Publications. "Medical Dictionary of Health Terms." Accessed August 2013.
  • The Cleveland Clinic. "The Structure and Function of the Digestive System." Accessed August 2013.
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