Small Intestine Diagram

The small intestine is a part of the GI (gastrointestinal) tract that extends from the pyloric sphincter to the ileocecal valve. Its length is approximately 6 m and it opens into the large intestine. Actually, it is longer than the large intestine which is about 1.5 m. It is termed small intestine due to its smaller diameter when compared to the large intestine. The length of the small intestine is slightly greater in males than in females. Also, it is greater in cadavers due to the loss of tone.

Let’s learn more about the anatomy of the small intestine with a well-labelled diagram.

Labelled Diagram of Small Intestine

Small Intestine – Description

• The small intestine can be distinguished into the duodenum (proximal part), jejunum (middle part) and the ileum (distal part).
• They can also be broadly classified into the upper fixed part and the lower mobile part. The upper fixed portion constitutes the duodenum which is approximately 25 cm in length.
• In the lower mobile part, the upper 2/5th portion is the jejunum and the lower 3/5th is the ileum.
• The duodenum is a short and wide C-shaped structure that is curved around the upper pancreas.
• The jejunum occupies the left and upper part of the small intestine and the ileum occupies the right and lower part. The jejunum is thicker and is about 2.5 m long. The ileum is thinner and is about 3 m long.
• The two main functions of the small intestine are absorption and digestion. Absorption is achieved by its large length and by the presence of mucous membrane, microvilli and villi.
• The mucous membrane of the small intestine has circular folds that start from the second part of the duodenum. These folds are closely set and large in the duodenal part and continue likewise till the proximal half of the jejunum. Their size and number progressively decrease in the distal region.
• The mucous membrane also has lymphatic follicles. The solitary follicles are distributed throughout the intestine and the Peyer’s patch or aggregate follicles are concentrated in the ileum. The Peyer’s patch is fewer and smaller in the distal jejunum.
• The mucous membrane is covered by tiny projections known as villi. These villi are about 1 mm in height and less than 1 mm in diameter. They are lined by special columnar cells called enterocytes.
• Each enterocyte gives rise to another hair-like projection known as the microvilli. These villi and microvilli together increase the surface area of the mucous membrane.
• Moreover, each villus also has a central channel known as the lacteal that opens into the lymphatic vessels. It also contains blood vessels.
• These intestinal villi are numerous and large in jejunum and duodenum but fewer and smaller in the ileum.

Intestinal Gland

The intestinal glands are tubular structures that open into the lumen of the intestine between the villi. They are lined by columnar epithelial cells and secrete mucus and digestive enzymes. In addition to this, the initial part of the duodenum also has some mucous gland called Brunner’s gland. It opens into the intestinal lumen directly. It secretes mucus and also traces of enzymes.

The secretion from the small intestine is called succus entericus (intestinal juice). Enzymes of succus entericus act on the partially digested food and convert them into final digestive products. They are produced and released by the enterocytes of the villi.

• Proteolytic enzymes like peptidases convert peptides into amino acids.
• Amylolytic enzymes like sucrase, lactase and maltase convert the disaccharides into molecules of monosaccharides. Dextrinase converts maltose, maltotriose and dextrin into glucose.
• The intestinal lipase acts on triglycerides and converts them into fatty acids

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