The jejunum is the second or middle part of the small intestine, extending between the duodenum and the ileum. The jejunum is a continuation of the duodenum. It is positioned mainly in the left upper quadrant of the abdomen.

The jejunum begins at the duodenojejunal flexure, which projects at the second lumbar vertebra (L2) level. The transition zone between the ileum and jejunum is not very well demarcated. However, the jejunum and ileum do not look the same, and knowing their differences makes it easier to distinguish both parts.

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 circular folds. They are larger than the ones in the ileum and are also found more closely to each other. Thus, the jejunum has more pronounced and numerous valves of Kerckring.

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 larger arterial arcades. They are simpler and form only 1 - 2 rows. Also, the jejunum has longer and fewer vasa recta.

And finally, the mesentery of the jejunum contains less fat. It appears semitranslucent or slightly transparent. 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