What portion of the small intestine does the ileum correspond to?

The ileum is the final and longest segment of the small intestine, typically accounting for about the lower three-fifths of its length. It follows the duodenum and jejunum, completing the process of digestion and absorption of nutrients. The ileum is specifically responsible for the absorption of vitamin B12 and bile salts, as well as any remaining nutrients that were not absorbed by the earlier sections of the small intestine. Understanding the anatomy of the small intestine is crucial, as each section plays a unique role in digestion and is distinct in its structure, function, and the types of nutrients it absorbs. Therefore, identifying the ileum as the lower three-fifths aligns with the biological and anatomical knowledge of human digestive physiology.