The jejunum is the middle section of your small intestine and a particularly important part of your digestive system. Digestion of food -- especially starch -- starts in your mouth, although the jejunum is where most nutrients are absorbed into the bloodstream. Diseases and injuries that impact the jejunum negatively affect digestion and significantly increase the risk of nutritional deficiencies.
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The small intestine, also called the small bowel, accepts partially digested food from the stomach and is responsible for the vast majority -- about 90 percent -- of nutrient processing and absorption. The small intestine is about 22 or 23 feet long and is functionally divided into three sections: duodenum, jejunum and ileum. The duodenum is the initial and shortest section, and its main purpose is to further break down partially digested food with enzymes released from the pancreas and gall bladder. Most of the iron you need is absorbed in the duodenum. From the duodenum, nutrients pass through the jejunum and then the ileum before entering the large intestine. The main functions of the large intestine are to absorb any remaining water and allow friendly bacteria to ferment dietary fiber and produce vitamin B-12.
The jejunum portion of the small intestine is about 8 or 9 feet long and contains numerous finger-like projections called villi, which greatly increase its surface area and allow it to be very absorptive. The jejunum also contains lots of smooth muscle, which rhythmically contracts -- peristalsis -- and pushes food through the digestive tract. Enzymes released into the duodenum are still active in the jejunum, and eventually reduce carbohydrates, fats and protein into glucose, fatty acids and amino acids, respectively. Glucose, fatty acids and amino acids are absorbed through the wall of the jejunum and into the bloodstream, as are vitamins, minerals, electrolytes, water and bile salts. The spaces between the cells of the jejunum are relatively wide apart, making it the most porous or “leaky” section of the small intestine.
In contrast to the jejunum, the ileum is less porous and absorbent. However, a small proportion of nutrients are absorbed in the ileum, especially amino acids, vitamin B-12 and the majority of bile salts, which are released from the gall bladder. There is no clear division between the jejunum and ileum, although the tissues have structural and functional differences that become more apparent the farther along the small intestine you look. For example, the ileum is characterized by shorter villi and more lymphoid tissue compared to the jejunum.
A variety of conditions -- including infections, bleeding ulcers, celiac disease, Crohn's disease and cancer -- negatively affect the small intestine. When the jejunum is damaged or injured, the ileum can partially compensate over time by becoming more absorptive, but nutrient deficiency is still a concern. In general, significant destruction or loss of the jejunum most greatly impacts fatty acid, glucose, vitamin and mineral absorption. In contrast, significant loss of the ileum typically leads to dehydration issues and electrolyte imbalance.