Duodenum – the proximal part of the small intestine, forming a ‘C’ shape that wraps around the head of the pancreas. It runs from the pylorus of the stomach to the duodenojejunal junction.

The jejunum is a distal part of the small intestine that starts at the duodenojejunal flexure and ends at the ileum. There is no clear border between the jejunum and the ileum. The jejunum differs from the rest of the small intestine by the absence of Brunner’s glands and Peyer’s patches.

The ileum is the longest part of the small intestine. It is thicker and more vascular than the jejunum, and the circular folds are less dense and more separated. Peyer’s patches are found lying in its mucosa, which is an essential part of gut-associated lymphoid tissue.

The duodenum accomplishes a good deal of chemical digestion, as well as a small amount of nutrient absorption; the primary function of the jejunum and ileum is to finish chemical digestion (enzymatic cleavage of nutrients) and absorb these nutrients along with water and vitamins.

Source:
https://www.nursingtimes.net/clinical-archive/gastroenterology/gastrointestinal-tract-4-anatomy-role-jejunum-ileum-19-08-2019/