The small intestine

In the small intestine, enzymes break down the different parts of foods – such as carbohydrates, proteins or fats – into their building blocks (sugars, amino acids and fatty acids). These enzymes are produced in the salivary glands in the mouth, in the stomach and in the pancreas, and are released as digestive juices in the intestine. Bile fluid and intestinal secretions are also released. They help the enzymes to break down food. The stomach and the intestine also produce mucus, which prevents the enzymes from attacking the mucous linings. The partially digested food, the various digestive juices and the mucus are then mixed with a lot of water, so the contents of the small intestine become very liquidy.

The wall of the small intestine mostly absorbs nutrients, vitamins and salts. From there, the nutrients enter the bloodstream and are carried to all of the body's organs.

The wall of the small intestine also produces various hormones. These hormones influence things like the production of bile or pancreatic juice. This causes even more water to be released into the intestine. The hormones also send the message to your brain that you are full.

The large intestine

In the right lower belly, the small intestine leads into the large intestine, which is roughly 1 to 1.5 meters long. The large intestine is made up of the cecum, the appendix, the colon and the rectum, which ends at the anus in the anal canal.

Another important task performed by the large intestine is the absorption of water and salts. This helps to solidify the digested food to stool (poo). The muscles in the walls of the large intestine create strong, wave-like movements that help to push the contents of the intestine towards the anus. The urge to go to the toilet and empty your 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 person to person: Some people go three times a day, and others go three times a week. Both are completely normal. The frequency depends on various factors, including how much fiber you get in your diet.

The large intestine is also responsible for the absorption and production of vitamins. This job is done by gut microbes ("gut flora") – billions of bacteria that eat the undigested food matter. The bacteria produce the essential vitamins B and K, and prevent other (harmful) types of bacteria from settling in the intestine and multiplying there.

What happens when we have diarrhea or are constipated?

Two important jobs of the intestine are

• creating muscle movements that move the contents of the intestine towards the anus, and
• absorbing water and salts in the large intestine.

If one or both of these jobs isn't done properly, it can cause digestion problems: Constipation is usually caused by sluggish movements of the muscles in the large intestine. The contents of the intestine then move forward only very slowly. As a result, the stool gets more solid and harder as water is removed from it.

Diarrhea is caused by the opposite: The large intestine is no longer able to remove enough water from the stool. This can happen if too much liquid is passed on from the small intestine or if the contents of the intestine move too quickly through the large intestine. Intestinal infections are a common cause of diarrhea too.

Sources

• Lippert H. Lehrbuch Anatomie. München: Urban und Fischer; 2017.
• Menche N (Ed). Biologie Anatomie Physiologie. München: Urban und Fischer; 2016.
• Pschyrembel Online. 2021.
• Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2017.

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