Anatomy of the abdominal viscera: Small intestine

The small intestine is a part of the digestive tract specialized in absorbing nutrients and minerals from the food we eat.

It’s located in the abdominopelvic cavity, and it begins at the pylorus of the stomach and it ends at the ileocecal junction, where it continues with the large intestine.

It has three major components: the duodenum, which can be divided into four parts, the jejunum and the ileum.

Now, when talking about structures of the abdomen we often encounter the terms ‘retroperitoneal’ or ‘intraperitoneal’.

Retroperitoneal is a description for abdominal structures that are only partially covered by peritoneum and lie posterior or behind the abdominal peritoneum, where intraperitoneal means that these structures have invaginated and are completely covered by the visceral peritoneum.

So, of these components, the proximal portion of the first part of the duodenum, in addition to the jejunum and ileum are intraperitoneal, where the distal portion of the first part of the duodenum, in addition to the second, third, and fourth parts of the duodenum are retroperitoneal, and are found posteriorly in the retroperitoneal cavity.

That being said, let’s have a closer look at the duodenum and its four parts.

Overall, the duodenum is shaped like the letter C, curving around the head of the pancreas, and consists of the first, or superior part; second, or descending part; third, or inferior part; and fourth, or ascending part.

The first part lies in the transverse plane and begins anterolaterally to the right of the L1 vertebral body, continuing the pylorus.

The proximal portion of the first part has a segment of the lesser omentum called the hepatoduodenal ligament attached to its superior surface, and the greater omentum attached to the inferior surface.

The proximal 2cm of the first part is also more dilated and freely mobile as it is intraperitoneal with its anterior and posterior surface covered in peritoneum, and is referred to as the ampulla or the duodenal cap.

This is in contrast to the distal 3cm which is retroperitoneal, and therefore not mobile.