Why your Stomach acid do not effect on stomach though it is strong enough to dissolve thin metals?

Image Credit: Britanica.com

Parietal cells in the mucosa, the inner cell layer of our digestive tract, secrete hydrochloric acid (HCl) into the stomach's lumen, or cavity. The solution in the lumen may have a pH of one or less 10 times as acidic as pure lemon juice. This HCL is essential to digest food. We have to wish a special thanks to HCL as our digestive enhanced partner.But amazing fact is that this hydrochloric acid with a pH of 2 to 3 and it is highly corrosive .  So this acid is able to dissolve thin metals, like blade, copper strip etc. If we can use it as in electroplating reaction it gives good result. 

  But more interesting things is that our stomach  do not effected or burned with Stomach Acid (HCL). Mucous-producing cells at the neck of the gastric pits create a layer of protective mucous covering the stomach lining. This mucous layer includes bicarbonate ions which act as a chemical barrier against the protons in gastric juice. actually this acid attacks your stomach lining, which protects itself by secreting an alkali bicarbonate solution. The lining still needs to be replaced continually, and it entirely renews itself every four days.

    The following chemical reaction is involved in the stomach all time.

First, the mucosa doesn't get digested as it secretes the HCl because the pH within the parietal cells remains near neutrality by the following mechanism (note that all parts of the mechanism take place simultaneously): (1) Potassium ions, K⁺, diffuse passively from the parietal cell into the lumen. (2) An active transport pump brings K⁺ back into the parietal cell, simultaneously secreting H⁺ from the cell to the lumen. As much K⁺ returns by this route as leaks in (1). (3) Chloride ions, Cl^-, diffuse passively from the cell to the lumen, and their negative charges balance the positive charges of the secreted H⁺.(4) An exchanger on the opposite face of the parietal cell balances this loss of Cl^- by importing Cl ^-from the blood in exchange for bicarbonate ions (HCO3^-). (5) Within the cell, water reacts with carbon dioxide (CO₂) to form carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃^-. The figure (right) summarizes these steps. In summary, the secreted HCl arises from H⁺(from water) and Cl^- from the blood. Both accumulate only in the lumen and not in the parietal cell.

Image Credit: Britanica.com