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Small Intestine

The small intestine (also known as the small bowel) is the longest portion of the digestive tract - it is more than 6 meters long and is located within the middle of the abdomen. It has three sections, the duodenum, jejunum and ileum (more information provided below).

Much of the small intestine is coiled and suspended in a thin layer of fat - which gives the intestine a lot of flexibility and mobility.

What does it do?

Digestion of fats, proteins and carbohydrates contained in the foods you consume, is completed within the small intestine.

The resulting nutrients produced are absorbed through the lining of the small intestine and transferred to the bloodstream.

Duodenum

Located at the junction of the stomach and the small intestine, the duodenum is the first part of the small intestine. It is C-shaped and about 25 cm long.

What does the duodenum do?

  • The duodenum receives the thick liquid mixture of partly-digested food and acid from the stomach. This acid is quickly neutralised in the alkaline environment of the duodenum.
  • The duodenum also receives bile from the gallbladder, and other digestive enzymes from the pancreas. These enter the duodenum through small ducts or tubes.
  • Other glands produce mucus that coats the digestive mixture to help ease its passage.
  • The food mixes with bile, mucus, and pancreatic and other digestive enzymes.
  • The bulk of the digestion of proteins, fats and carbohydrates takes place in the duodenum before the material travels further into the small intestine.

Jejunum and Ileum

The jejunum is the 1-2 m long, coiled mid-section of the small intestine. The ileum is the final portion of the small intestine, which leads into the large intestine. The ileum measures 2-4 m in length.

The role of the jejunum and ileum

The inner linings of the jejunum and ileum contain very small finger-like bumps called 'villi'. The presence of these tiny bumps on the inside of the small intestine means that the surface area is much larger than if the lining were just a flat surface. This increased surface area improves the small intestine's ability to absorb nutrients.

The final stages of digestion are completed in this portion of the small intestine, where foods and liquids are fully broken down into their nutrient components such as:

  • smaller molecules (eg, glucose from carbohydrates; amino acids from proteins; fatty acids and cholesterol from fats)
  • vitamins
  • minerals
  • salts
  • water

As the digestive mixture travels through the small intestine, the villi absorb the nutrients and transfer them to the bloodstream and the liver. Any food that has not been digested in the small intestine, ie fibre
(along with some water and vitamins) then reaches the large intestine.

Problems/disorders of the small intestine

There are a number of things that can go wrong in the small intestine. Information leaflets on some of these conditions are available by clicking below.