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Small intestine

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Anatomy and physiology of the small intestine

The small intestine is part of the digestive system. It is a hollow, tube-like organ that connects to the stomach on one end and the large intestine on the other. The small intestine is the longest part of the gastrointestinalgastrointestinalReferring to or having to do with the digestive organs, particularly the stomach, small intestine and large intestine. (GI) tract, making up about three-quarters of the digestive system. It is also known as the small bowel.

The gastrointestinal tract is a tube that extends from the mouth to the anus. It includes the:

  • esophagus – a tube-like structure that carries food from the mouth and throat to the stomach
  • stomach – a sac-shaped organ that receives food from the esophagus, starts the digestive process and empties partly digested food into the small intestine
  • small intestine (small bowel) – a tube-like organ that continues digestion and absorbs nutrients
  • large intestine (large bowel) – a tube-like organ (larger in diameter than the small intestine) that absorbs water from partly digested food, forms it into stool and stores it

Several other organs of the digestive system help to digest food, including the liver, gallbladder and pancreas.


The small intestine is about 4.75–6 metres (15–20 feet) long and an average of 2.5 centimetres (1 inch) in diameter. It is looped and coiled and fills up much of the abdominal cavity (the space in the abdomen that contains the intestines and other organs). The small intestine has 3 parts:

  • duodenum
    • first part of the small intestine, which connects to the stomach
    • the shortest and widest part of the small intestine
    • about 25 centimetres (10 inches) long
    • considered the mixing bowl and is where most digestive enzymesenzymesA protein that speeds up certain chemical reactions in the body. enter the small intestine
  • jejunum
    • middle part of the small intestine, between the duodenum and ileum
    • about 2.4 metres (8 feet) long
    • most of the chemical digestion and nutrient absorption takes place here
    • layers of muscle within the jejunum contract (tighten) to help move food along the digestive tract
  • ileum
    • last part of the small intestine, which connects to the large intestine
    • about 3.6 metres (12 feet) long
    • absorbs nutrients from digested food and empties into the large intestine

The jejunum and ileum are supported by a sheet of mesentery. Mesentery is a membrane that suspends the small intestine from the back of the abdominal wall and supports blood vessels, nerves and lymph vessels that supply the small intestine. The small intestine is surrounded by the large intestine.

Layers of the small intestine wall

The inner surface of the small intestine is velvety in appearance. This is due to many finger-like projections of the mucous membrane called villi. The villi increase the surface area of the small intestine, which helps it absorb digested food.

The inside layers of the small intestine are made up of the same 4 layers that make up most of the gastrointestinal tract:

  • mucosa (mucous membrane)
    • innermost layer
    • made up of a layer of epitheliumepitheliumA thin layer of epithelial cells that makes up the outer surfaces of the body (the skin) and lines hollow organs, glands and all passages of the respiratory, digestive, reproductive and urinary systems., loose connective tissue called lamina propria and a very thin muscular layer called the muscularis mucosa
  • submucosa
    • lies next to the mucosa
    • contains connective tissue, blood vessels, lymph vessels and nerves
  • muscularis (muscularis propria)
    • the muscular layer that lies next to the submucosa
    • this band of smooth muscle helps move food through the small intestine
  • serosa (subserosa)
    • thin outermost layer that covers the small intestine
    • formed by the visceral layer of the peritoneum (layers of tissue that cover the outer surface of most organs in the abdomen)
    • attached to the mesentery


The main function of the small intestine is to break down (digest) food and absorb nutrients, such as electrolyteselectrolytesA substance in the blood and other body fluids that carries an electric charge. Electrolytes are responsible for the movement of nutrients and wastes into and out of cells to keep body fluids balanced and to allow muscles to function properly., vitamins and minerals. The small intestine is the most important absorbing organ in the gastrointestinal tract. About 90% of nutrient absorption takes place in the small intestine.


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