The stomach

An empty stomach is about the size of a fist, yet it can stretch to hold about 4 litres of food and drink.

A holding tank that mixes

You can eat a meal far faster than your small intestine can digest and absorb it. The stomach solves that problem. It holds the meal, mixes it with digestive juices and lets it out a little at a time. OpenStax notes that the empty stomach is only about the size of a fist but can stretch to hold up to 4 litres, more than 75 times its empty volume. When empty, its lining falls into large folds called rugae.

The stomach has four regions. The cardia is where food enters from the esophagus. The dome-shaped fundus rises above it, and the body is the main part. The funnel-shaped pylorus leads to the small intestine, with a ring of muscle, the pyloric sphincter, controlling the exit. The stomach wall has an extra, oblique layer of muscle besides the usual circular and long layers, which lets it churn food vigorously.

Acid, enzymes and a protective coat

The lining of the stomach is dotted with gastric pits that lead to glands. Different cells in these glands make different products. Parietal cells make hydrochloric acid, which gives the stomach contents a pH of about 1.5 to 3.5. The acid kills many of the bacteria that come in with food, unfolds proteins and turns the inactive enzyme pepsinogen, made by chief cells, into pepsin, which starts the digestion of protein. Parietal cells also make intrinsic factor, which the small intestine needs to absorb vitamin B12. OpenStax notes that this is the one stomach function essential to life.

Why does the stomach not digest itself? It has a mucosal barrier with three parts. A thick coat of mucus rich in bicarbonate covers the wall and neutralises acid. The lining cells are joined by tight junctions that stop gastric juice from leaking between them. And stem cells replace damaged cells quickly: the whole surface lining is renewed every 3 to 6 days.

Three phases and slow emptying

Gastric juice is controlled by nerves and hormones in three phases. The cephalic phase starts before food arrives: the sight, smell, taste or thought of food makes the stomach start secreting. OpenStax notes that this is a conditioned reflex, so it happens only if you like or want the food. The gastric phase begins when food stretches the stomach and lasts 3 to 4 hours. The hormone gastrin boosts acid production. In the intestinal phase the duodenum takes over: when it fills with chyme, a reflex closes the pyloric sphincter and slows the stomach down. The hormone ghrelin, made mainly in the fundus, rises before meals and acts on the brain to regulate hunger.

Inside the stomach, mixing waves pass about every 20 seconds and turn food into a soupy mixture called chyme. The pylorus holds about 30 mL and acts as a filter: each wave pushes only about 3 mL through the pyloric sphincter into the duodenum and sends the rest back to keep mixing. The stomach is usually empty 2 to 4 hours after a meal. Meals high in carbohydrate leave fastest, protein next, and fatty meals stay longest, sometimes 6 hours or more.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.