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. There is a particularly high number of nerve cells in the wall of the intestine. For some people, the intestine reflects how they are feeling: for instance, they might get a stomach ache, diarrhea 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 end-to-end 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 gives it a very large surface area and food can make contact with a large number of intestinal cells.
The digestive system
The small intestine
In the small intestine, enzymes (substances produced by the body) 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.
The large intestine
In the large intestine, strong, wave-like movements help to push the contents of the intestine towards the anus. The urge to use the toilet and empty the bowels is triggered when stool enters the rectum. 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 fiber. 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.
Menche N. (ed.) Biologie Anatomie Physiologie. Munich: Urban & Fischer/ Elsevier; 2012.
Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.
Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer; 2011.
IQWiG health information is written with the aim of helping
people understand the advantages and disadvantages of the main treatment options and health
Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.
Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.