The stomach plays a critical role in the early stages of food digestion. Asides from squeezing and churning the food bolus, it also secretes a mixture of compounds, collectively known as "gastric juice."
Gastric juice comprises water, mucus, hydrochloric acid, pepsin, and intrinsic factor. Of these five components, pepsin is the principal enzyme involved in protein digestion.
It breaks down proteins into smaller peptides and amino acids that can be easily absorbed in the small intestine. Specific cells within the gastric lining, known as chief cells, release pepsin in an inactive form, or zymogen form, called pepsinogen.
By doing so, the stomach prevents the autodigestion of protective proteins in the lining of the digestive tract. Since chief cells release pepsin as a zymogen, activation by an acidic environment is necessary.