The jejunum is the longest part of the small intestine and is highly coiled. It has both digestive and absorptive functions. The jejunum is in the abdominal cavity filling those parts that are not occupied  by other viscera. It is suspended by the mesentery (mesojejunum) which converges to its’ root. It is the last two segments of the duodenum that begins absorption of nutrients.

Imparticular iron and calcium absorption starts in the duodenum before the food contents enter the next part of the small intestine, the jejunum. Juices from the liver, gallbladder, and the pancreas neutralize the stomach acids entering the duodenum and help digest protein, carbohydrates, and fats.

The jejunum is the second part of the small intestine and is a central section of the abdomen. The color of the jejunum is deep red because of its’ rich blood supply and the peristalsis movements are rapid and vigorous. There is little fat in the mesentery that supports this region.

The mesentery is the fold of membrane that attaches the small intestine to the back of the abdominal wall and carries its’ blood supply. It is also called the mesenterium. The mucous membrane lining of the intestinal wall of the small intestine has throughout it folds which are running transversely across it called plicae circulares.

In humans fingerlike projections called villi project into the cavity. The prevalence of these structures increase the area of the secreting and absorbing surface. The walls of the small intestine have numerous microscopic glands.

Secretions from Brunner glands, in the submucosa of the duodenum, function primarily to protect the intestinal wall of the small intestine from gastric juices. Lieberkuhn glands which occupy the mucous membrane secrete digestive enzymes, provide an outlet port for the Brunner glands, and produce cells that replace surface membrane cells shed from the tips if the villi.

Small Intestine Components:     

The following is a list of the components of the small intestine in order of descent through the abdomen. The small intestine begins at the pyloric sphincter which is below the stomach.

• Superior Duodenum

• Descending Duodenum

• Horizontal Duodenum

• Ascending Duodenum (ends at the duodenojejunal flexure (DJF)

• Jejunum (longest part of the small intestine)

• Ileum (last part of small intestine before the large intestine begins)

Peristaltic waves move food through the small intestines and cause the enzymes to mix with the food as it passes through so digestion and absorption of nutrients takes place within the small intestine before emptying the food into the large intestines which are also part of the gastrointestinal tract.