How does the stomach work?

The stomach is a muscular hollow organ. It takes in food from the food pipe, mixes it and starts breaking it down. The pre-digested food is then passed on to the small intestine in small portions.

Shape and function

The digestive system is made up of a muscular tube that starts at the mouth and ends at the anus. The stomach is an enlarged pouch-like section of this tube. It is found on the left side of the upper abdomen. The stomach’s shape and size vary slightly from person to person, depending on things like their sex and build, but also on how much they usually eat. It is between 20 and 30 centimeters long on average, and can hold about 1.5 liters of food and drink.

At the point where the food pipe leads into the stomach, the opening is usually kept shut by muscles in the food pipe and diaphragm. This prevents the contents of the stomach from moving back up into the food pipe. When you swallow, these muscles relax and allow food to enter the stomach.

The upper part of the stomach is called the fundus. It is usually filled with air that enters the stomach when you swallow.

In the largest part of the stomach, called the body, food is churned and broken into smaller pieces, mixed with acidic gastric juice and enzymes, and pre-digested. Towards the lower end, the body of the stomach becomes narrower and joins the first part of the small intestine (the duodenum). The lower opening of the stomach is closed off by a circular muscle called the pyloric sphincter. When this muscle relaxes, the opening widens and allows the pre-digested food to pass into the small intestine bit by bit.

Illustration: Structure of the stomach

Churning machine made of muscle

The lining of the stomach consists of several layers: mucous membrane tissue, connective tissue with blood vessels, nerves, and muscle tissue. The muscle layer is made up of three sub-layers that each pull the stomach in different directions. They move the contents of the stomach around so vigorously that solid parts of the food are crushed, churned and mixed into a food pulp.

The muscles also move the pre-digested food towards the small intestine once it is ready. The food stays in the stomach for about three hours before it moves on. Depending on what you eat, it may stay there for a shorter or longer time.

The mucous membrane lining has folds in it that act as “pathways” for liquids to flow through. If the stomach is empty, liquids often enter and leave the stomach within fifteen minutes.

Gastric juice: Acid, salts and enzymes

There are lots of tiny glands in the lining of the stomach. These glands make digestive enzymes, hydrochloric acid, mucus and bicarbonate.

Gastric juice is made up of digestive enzymes, hydrochloric acid and other substances that are important for absorbing nutrients. About 3 to 4 liters of gastric juice are made each day. The hydrochloric acid breaks down the food, and the digestive enzymes split up the proteins in the food. The acid also kills bacteria.

Some of the glands produce a thick, protective layer of mucus that prevents the acid from attacking the wall of the stomach. This mucus also contains a lot of bicarbonate, which is alkaline (the opposite of acidic) and neutralizes the acid before it reaches the wall of the stomach.

Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.

Menche N. Biologie Anatomie Physiologie. München: Urban und Fischer; 2016.

Pschyrembel Online. 2022.

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Updated on January 4, 2022

Next planned update: 2025


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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