Gastric juices are liquids found in the stomach. In their normal state, these liquids are usually primarily clear in color. The juices in the stomach begin the process of breaking down food so that nutrients can be extracted by the intestines, and they are produced by glands in the stomach as needed. The precise composition and pH balance of gastric juices varies, depending on the animal involved, but they are famously very acidic.

The major components of gastric juices are mucus, pepsin, and hydrochloric acid. Young mammals also produce rennin, an enzyme which breaks down milk so that it can be digested. In humans, the pH balance hovers between one and three, making this stomach secretion very acidic. The acidity is important, because it breaks down foods to make them accessible to the digestive tract. The high acidity of the stomach also kills many bacteria and microorganisms which cannot survive in that environment, protecting the body from infection with many common pathogens.

Production of gastric juices is triggered when the hormone gastrin is released in the blood. Gastrin is released by the body in response to the presence of food in the stomach, indicating that the stomach needs to kick into gear and start the digestion process. Various glands in the stomach are responsible for producing different components of gastric juices, and for achieving the right balance of components.

Because of the intense acidity of the gastric juices, the stomach lining is specially designed to withstand harsh conditions. This fluid would eat through ordinary tissue, just like it breaks down meat when people and animals consume it. The stomach is made from very tough material, and lined with mucus to prevent the gastric juices from irritating the stomach wall. People with ulcers can experience extreme stomach pain because the gastric juices irritate the ulcerated area of the stomach.

Gastric juices can cause problems when they enter the esophagus. In most people, a flap keeps the juices of the stomach contained while the stomach is working, so that the juices cannot creep their way up out of the stomach. However, people can experience heart burn and acid reflux syndrome, conditions in which the esophagus becomes irritated and inflamed by chronic exposure to gastric juices. People who are prone to vomiting can also develop esophageal damage, along with damage to the enamel of the teeth caused by the high acidity of the stomach contents.