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How does the intestine work?

The intestine is a winding muscular tube extending from the stomach to the anus. Its main purpose is to digest food. But the intestine is not only there for digestion: it also produces various substances that carry messages to other parts of the body, and plays an important role in fighting germs and regulating the body’s water balance. For some people, the intestine reflects how they are feeling: for instance, they might get a stomach ache, diarrhoea or constipation when they are stressed or upset about something.

The small intestine, which is directly connected to the stomach, is 3 to 5 m long and is made up of the duodenum, jejunum and ileum. The inside wall of the small intestine is folded like the body of an accordion. This makes the surface area very big and the food can make contact with a large number of intestinal cells.

In the small intestine, substances produced by the body (enzymes) break down nutrients such as carbohydrates, proteins or fats into their building blocks. These enzymes are produced in the salivary glands in the mouth, in the pancreas and in the intestinal cells. The intestinal cells absorb the building blocks (for example sugar, amino acids or fatty acids) together with vitamins, salts and water. From there most of the nutrients enter the bloodstream to be carried to the parts of the body where they are needed.

Various intestinal hormones are produced in the small intestinal cells as well. These hormones influence the production of bile or pancreatic juice, for example. They also cause more water to be released into the intestine and make you feel full.

In the right lower abdomen the small intestine leads into the large intestine, which is roughly 1 to 1.5 m long. The large intestine is made up of the caecum together with the appendix, colon and rectum, which ends at the anus with the anal canal. In the large intestine, strong, wave-like movements help to push the contents of the intestine towards the anus. When stool enters the rectum, the urge to use the toilet and empty the bowels is triggered. If we suppress this urge, the rectum temporarily stores the stool. How often bowel movements occur varies greatly from individual to individual and mostly depends on how rich our diet is in fibre. Bowel evacuations ranging from three times a day to three times a week are completely normal.

Another important task performed by the large intestine is the absorption of water and salts. There are also millions of bacteria in the large intestine. These bacteria break down proteins in the food to produce protein building blocks (amino acids). They also make the important vitamins B and K.

Author: German Institute for Quality and Efficiency in Health Care (IQWiG)