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The small intestine is important for digestion and absorption of nutrients from food.
The small intestine is the portion of the digestive tract that connects the stomach and the large intestine. The small intestine consists of three different parts: the duodenum, jejunum and ileum. The small intestine contains small finger-like projections of tissue called villi which increase the surface area of the intestine and contain specialized cells that transport substances into the bloodstream. Although these villi do not aid in the digestion of nutrients, they do help with nutrient absorption.
Digestion in the small intestine occurs via two different processes. The first, mechanical digestion, helps break food from the stomach into smaller pieces. In the small intestine, muscles surrounding the intestinal walls contract to roll, mix and chop the partially digested food, also known as chyme. These muscle contractions occur several times per minute so that the chyme is moved back and forth. Other muscles gradually propel the chyme through the digestive tract via a process called peristalsis. Because the villi are not muscular, they do not contribute to this process.
In addition to mechanical digestion, food from the stomach is broken down chemically. Acids made by the stomach and enzymes secreted by the stomach and pancreas further digest the carbohydrates, lipids and proteins found in food. Additional enzymes that help break down proteins and carbohydrates are found in a portion of the villi known as the brush border. These brush border enzymes break down nutrients into components small enough to be absorbed.
Villi and Absorption
Although the villi play a role in digestion, they also are essential for the absorption of digested nutrients. The cells of the villi and another part of the small intestine, known as the crypts, transport food from the digestive tract into the bloodstream, where they can be used by the body. The villi aid in absorption by increasing the surface area of the intestine and contain specialized cells which transport different types of nutrients into the blood.
Anything that causes inflammation of the villi in the small intestine can affect digestion and absorption. One prominent cause of villi damage is celiac disease, a disorder caused by an immune reaction to gluten. When people with celiac disease eat gluten, the inflammation blunts the villi, making them unable to participate effectively in the breakdown and absorption of nutrients. Other conditions, such as Crohn's disease, can also affect the small intestine and lead to malabsorption of nutrients.