The duodenum is the upper (first) part of the small intestine. Its length ranges from about 23 to 28 cm. It resembles a horse shoe in shape. The bile and the pancreatic ducts open in the duodenum. It is divided into four segments: the superior, descending, horizontal and ascending duodenum. The food spends a lot of time in the duodenum as maximum digestion and absorption takes place in this part of the small intestine.
The second part of the small intestine is the jejunum. The chyme moves to the jejunum, after being digested and partly absorbed in the duodenum. The inside walls in the jejunum absorb the nutrients in the chyme. The presence of the number of villi increases the surface area of the jejunum manifold which, in turn, absorbs nutrients present in the chyme. The muscular movement of the jejunum is rapid and vigourous to facilitate absorption.
The ileum is the last and longest part of the small intestine, located in the lower abdomen. The muscular movements in this part of intestine are slow. Structurally, this part is narrower and thinner than the previous two parts of the small intestine. It absorbs bile acids and vitamin B12. Bile acids are then returned to the liver for the formation of bile, and vitamin B12 is used by the body for making nerve cells and red blood cells.