(i) Inactive Enzymes: Protease enzymes secreted from the stomach are inactive. The inactive enzyme never participates in the reaction. So stomach enzymes can do no harm.
(ii) Mucous membrane: Stomach is covered by mucous membrane. Because of this, the enzyme cannot come in contact with the stomach wall. So the stomach is not damaged.
(iii) Anti-enzymes: Anti-enzymes are secreted from cells. Anti-enzymes block the action of various enzymes. So there is no damage to the stomach wall.
(iv) HCl : HCl secreted from the stomach converts the food into semi-solid liquid. Kills stomach bacteria. Absorbs harmful toxins or poisons. Acts as the main lining of the stomach. Converts gastric juice to pepsin and renin.
(v) Peristalsis Movement: The three-layered wall of the stomach increases circulation during digestion. Enzymes mix with food. Food moves inside it by the process of peristalsis. So the stomach wall is not damaged.
(vi) Tight junctions: Epithelial cells of the stomach are tightly packed. Hence HCl and digestive enzymes can do no harm to the stomach.
(vii) Cell replacement: Epithelial cells of the stomach die after 3-6 days. It is then replaced by new cells. It ensures the protection of the stomach.
(viii) Bicarbonate secretion: Bicarbonate is secreted from gastric mucosa. Bicarbonate solution neutralizes HCl.
From the above discussion, it can be said that the stomach digests various types of food, but does not digest itself.