Final answer:
Gastric juice is a mixture of water, hydrochloric acid, enzymes, and other proteins secreted by gastric glands in the stomach, initiating the digestion of proteins. Its production is regulated by the brain, stomach, and small intestine through different phases.
Step-by-step explanation:
Gastric juice is a complex digestive fluid that is secreted by gastric glands located in the mucosal epithelium of the stomach. It is composed primarily of water, hydrochloric acid (HCl), various enzymes, inorganic ions, and other proteins. Gastric juice begins its production during the gastric phase, which is when food enters the stomach after a period of fasting, initiating gastric secretion. The purpose of hydrochloric acid in gastric juice is to denature food proteins, which means it helps to unfold protein molecules to allow more efficient action by digestive enzymes. Pepsinogen, an inactive enzyme produced by cells in the stomach wall, is converted into its active form, pepsin, upon contact with the acidic environment of the stomach. Pepsin then catalyzes the hydrolysis of peptide linkages within protein molecules, commencing the digestion of proteins, which is completed later in the small intestine. Stimuli from the brain, stomach, and small intestine can activate or inhibit the production of gastric juice, regulating the secretion through the cephalic, gastric, and intestinal phases. All these phases of gastric secretion can happen simultaneously once initiated. The secretion of gastric juice plays an integral role in the early stages of digestion by providing the necessary acidic environment and enzymes needed to break down ingested foods.