The small intestine is a long, hollow, convoluted tube-like structure that begins at the pylorus of the stomach and ends at the ileocecal junction, continuing with the cecum (Read more!). Therefore, it extends between the stomach and the large intestine.

The small intestine is the longest part of the gastrointestinal tract, occupying the pelvic cavity and the lower aspect of the abdominal cavity. It can measure up to 20 - 23 feet (6 - 7 meters). 

The small intestine is composed of the following three parts:

• Duodenum (Read more!) - first C-shaped part that curves around the head of the pancreas and links the stomach with the rest of the small intestine; it is short and has a characteristic shape;
• Jejunum (Read more!) - middle part that forms a little bit less than half of the remaining length; it can be recognized by its wider diameter and deep red color;
• Ileum (Read more!) - final and longest portion of the small intestine.

Although there are no clear separation marks between all three parts of the small intestine, they can be distinguished by their characteristics. The diameter and morphology of the small intestine gradually change towards the large intestine. The duodenum is mostly located retroperitoneally (except its superior portion), but the jejunum and ileum lie intraperitoneally.

The small intestine is attached to the posterior abdominal wall by a membranous, fan-like double fold of the peritoneum called the mesentery. This structure allows the small intestine to be relatively mobile and change its location and position depending on the intestinal contents, body position, and changes in the volume of adjacent structures and organs. 

Also, the jejunum and ileum are covered by the greater omentum - an apron-like structure. The omentum is a double fold of the peritoneum that extends between the stomach and intestines, attaching them to one another.

Overall, the small intestine provides completion of chemical food processing and nutrient breakdown, nutrient and water absorption, secretion of hormones and intestinal juice, immune response and protection against antigens.

Characteristic features of small intestine

The wall of the small intestine presents several distinct characteristic features that increase its absorptive surface, thus providing the digestion and absorption of nutrients more efficiently. 

• Circular folds (valves of Kerckring) - mucosal folds that encircle the lumen of the small intestine, projecting into it; they are more prominent in the distal duodenum and proximal jejunum but become progressively less pronounced distally and may even be absent in the terminal ileum.
• Intestinal villi - finger-like extensions of the intestinal mucosa that project into the lumen of the small intestine; between the villi are found the intestinal glands (intestinal crypts or crypts of Lieberkühn) that produce intestinal juice; the intestinal villi are more numerous and larger in the duodenum and jejunum, while the ileum has fewer and shorter villi. 
• Microvilli - small hair-like projections found on the apical end of each enterocyte (intestinal epithelial cell).

Some other unique anatomical features include the following:

• Peyer's patches - gastrointestinal associated lymphoid tissue (GALT) that are mainly found in the ileum; they present as large aggregated lymphoid nodules;
• Brunner's glands - small tubular glands located within the submucosa of the duodenum; they are more prominent and numerous near the pylorus of the stomach and in the proximal half of the descending duodenum; these glands produce mucus that protects the duodenum from the destructive effects of gastric acid.

Note: The small intestine is not normally palpable, and because of its mobility, any mass within it must become rather large before it is palpable. When the small intestine is filled with gas, the abdomen may become very protuberant and sound like a drum during percussion (tympanic sound).