The small intestine (or small bowel) is a long, hollow and convoluted tube-like structure that begins at the pylorus of the stomach and ends at the ileocecal valve. Therefore, it extends between the stomach and the large intestine, continuing as the cecum - the first portion of the large intestine. Overall, the small intestine occupies the lower abdomen and pelvis. 

The small intestine is composed of three parts:

• Duodenum - 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 - 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 - final and longest portion of the small intestine.

Although there are no 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 toward 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 double fold of the peritoneum called the mesentery. This structure allows the small intestine to be relatively mobile and change its location depending on the position of the body. 

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

The small intestine is the longest part of the gastrointestinal tract and can measure up to 20 - 23 feet (6 - 7 meters). The main functions of the small intestine include the following:

• Completion of chemical food processing and nutrient breakdown
• Nutrient and water absorption
• Secretion of hormones and intestinal juice
• Immune response providence and protection against antigens

Note: In this article, we do not cover the initial part of the small intestine - the duodenum. It has an entire study unit dedicated to it.