When animals eat, the chewed food passes through the mouth and down the throat through a tube called the esophagus. After the food moves down the esophagus it lands in the stomach. The stomach is a hollow organ that has many muscles. The stomach stores food and passes it along in small amounts to the intestines, where the food is digested. In some animals the stomach also partially digests the food.

The human stomach

The human stomach is more or less shaped like the letter J. It is located in the upper left side of the belly. The stomach of an adult is about 10 inches (25 centimeters) long and can easily expand to hold as much as 1 quart (0.9 liter) of food. The wall of the stomach is composed of four layers. The innermost layer, called the mucosa, is covered with millions of glands that produce mucus, acid, and substances called enzymes. The mucus forms a lining that protects the stomach from the gastric juices, which break down food. The second layer of the stomach, called the submucosa, is made up of tissue containing nerves and blood and lymph vessels. The third layer contains muscles that enable the stomach to churn, mix, and move food. The fourth layer, known as the serosa, connects the muscles to the outer covering of the stomach.

Circular muscles called sphincters close off the openings to the stomach at the esophagus and small intestine, except when food is passing through. Food is enclosed by the stomach until it is ready for final digestion.

Once food is in the stomach, the muscles of the stomach contract about three times a minute. This muscle action churns and kneads the food into a mixture of food and gastric juice called chyme. The amount of time that food remains in the stomach varies. Starches and sugars stay for an hour or two, proteins three to five hours, and fats even longer. After the stomach empties, the contractions of the stomach continue and may cause hunger pangs. (See also digestive system.)

The stomach is subject to a number of disorders, including hyperacidity (too much acid secretion), gastritis (swelling of the stomach lining), peptic ulcer (an injury of the mucous membrane), and cancer. Medications can be used to change the production of acid, but surgery may be needed to correct other problems. People who have had their stomachs removed live by eating small amounts of special foods many times a day.

Stomachs of other animals

The stomachs of some animals have more than one section, or chamber. These chambers have special purposes. The stomachs of cows and other animals that eat grassy food have four separate parts. These animals are called ruminants, or cud chewers. After ruminants eat, the food is stored in the rumen, the first chamber of their stomachs. The rumen adds mucus to the food and begins to digest it. Next the food goes back to the mouth to be thoroughly rechewed. This is called chewing cud. When swallowed again, the cud is passed to the second and third chambers, where water is removed from the food and absorbed back into the body. The food then goes to a final chamber, where the digestive enzymes are added.

Camels, llamas, and birds are among the animals with three-chambered stomachs. Camels and llamas can bring up the contents of their stomach to spit at approaching enemies. The chambers of a bird's stomach are called the crop, the true stomach, and the gizzard. The gizzard contains stones or toothlike structures that grind food.