The ileum is the last and longest part of the small intestine. It begins as the continuation of the jejunum. The ileum is located mainly in the right lower quadrant of the abdomen, but its distal end can also extend into the pelvic cavity. Around three-fifths of the distal small intestine defines the ileum.

The most distal portion of the ileum is known as the terminal ileum. It ends at the ileocecal junction with the ileocecal opening and valve. The ileocecal valve is composed of two flaps or lips that surround the ileocecal orifice. The muscle cells of the ileal wall extend into each flap and form a sphincter. Therefore, the ileocecal valve is a functional valve that ensures the intestinal content passage from the ileum to the cecum and prevents its backflow.

Further, the ileum continues with the first part of the large intestine - the cecum. The ileocecal junction is positioned in the right iliac fossa, marking the transition between the small and large intestines.

The ileum has a relatively narrow lumen. In fact, it has the smallest lumen among all three parts of the small intestine. Compared to the jejunum, the ileum also has thinner walls, less prominent and fewer mucosal folds, and shorter vasa recta. The transverse mucosal folds become fewer and lower towards the distal ileum and almost disappear at the terminal ileum. Therefore, it may appear flat.

A characteristic feature of the ileum is Peyer's patches, which are gastrointestinal-associated lymphoid tissue (GALT). They are mainly found in the ileum.

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Ileum