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What do the small intestines do?

The small intestines are an essential part of the human digestive system and are used to absorb the nutrients found in the already digested food - it is said that around 95% of absorption takes place along this six-metre tract. This article will discuss the structure and function of the small intestines in much greater detail.


Duodenum The small intestine consists of three main parts. The first section is called the duodenum and is a hollow tube measuring approximately 25cm in length. It is directly connected to the stomach and serves to further break down foods using enzymes. It also controls the rate that the stomach empties at with levels of hormone secretion (such as secretin). Jejunum The second part of the small intestines is called the jejunum and is approximately 2.5m in length in adult humans. It has an extremely large surface area (which will be discussed in the section on absorption) which aids in the speed at which nutrients are absorbed. Ileum The final section is known as the ileum. There is no definitive boundary where the jejunum ends and the ileum begins, but there are some structural differences; the ileum is somewhat smaller in diameter and contains many more lymphoid nodules ("Peyer's Patches") that secrete mucus. Its function is primarily to absorb the digestive products not dealt with in the previous section, such as bile salts.


There are three enzymes acting within the small intestines that correspond to the three primary nutrient groups, degrading them so that they can pass through the intestinal walls and into the bloodstream to be used by the body. They are: Proteolytic enzymes: These break down proteins into amino acids. Lipase: Lipids (fats) are broken down into glycerol and fatty acids. Amylase: Many carbohydrates are degraded into glucose (those that are not digested in the small intestines are dealt with by the bacteria in the large intestines).

Intestinal villi

The surfaces of the jejunum and ileum are covered in tiny structures called intestinal villi, that serve to significantly increase the surface area of the small intestines, allowing much faster rates of absorption of nutrients. They are finger-like in appearance and are approximately 1mm in length. The walls of the villi are also extremely thin and contain extensive networks of capillaries which aid in efficient absorption of nutrients.

Common problems associated with the small intestines

Crohn's disease is an autoimmune disease where the small intestines (or any other part of the digestive tract) are attacked by the immune system, resulting in severe inflammation. This can lead to abdominal pain and diarrhoea, and is much more prevalent in smokers. Similarly, Coeliac disease also results from attack by the immune system, with related symptoms like diarrhoea, fatigue and weight-loss.

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