Some key facts about the anatomy of the jejunum and ileum:

• Total length: 6-7 meters together.
• Plays an essential role in nutrient absorption.
• There is no clear division between the jejunum and ileum. Changes between both are gradual.
• If one draws a line from the right upper corner of the abdomen to its left lower corner, the jejunum generally lies above the line, but the ileum below it.
• Both jejunum and ileum contain permanent circular folds (also called plicae circulares or valvular conniventes) projecting into the intestinal lumen.

Jejunum

• Mostly located in the left upper quadrant, makes ~2/5 of the total length of the small intestine.
• Has a thicker wall than the ileum due to the higher circular folds.

Ileum

• Mostly located in the right lower quadrant and connects to the cecum at the ileocecal junction.
• Has a thinner wall in comparison to the jejunum.
• Further in its mesentery (“creeps” onto the tube). 
• The folds of the ileum are sparse and low, but it has distinctive patches of lymphoid tissue (Peyer’s patches) that are often several centimeters long.