The jejunum is the second part of the small intestine. It is a continuation of the duodenum. The jejunum is positioned mainly in the left upper quadrant of the abdomen. It begins at the duodenojejunal flexure, which projects at the second lumbar vertebra (L2) level.

The jejunum represents the proximal two-fifths of the total length of the small intestine (except the first 10 - 12 inches (25 - 30 cm) that correspond to the duodenum), and it has a wider lumen and thicker walls than the ileum. Additionally, the jejunum has more prominent transverse mucosal folds. They are larger than the ones in the ileum and are found more closely to each other.

Another significant characteristic of the jejunum is its more vascular wall. As the jejunum is better perfused than the ileum, it appears more red than pink. In addition, the jejunum has fewer but longer arterial arcades. They are simpler and form only 1 - 2 rows.

And finally, the mesentery of the jejunum contains less fat. It appears semitranslucent or slightly transparent between the vasa recta. That is why it is called the peritoneal window. The ileum does not have a peritoneal window because its mesentery appears opaque.

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Jejunum