The stomach cells present along the inner wall of the stomach secrete around 2 liters (0.5 gallons) of hydrochloric acid each day.
Th HCl is necessary for the digestion of protein and also kills the microbes. The HCl present in the stomach is highly acidic and has a pH of 1.5 – 3.5.
To protect itself from the corrosive action of the stomach acid, the stomach lining has a thick coating of mucus.
The epithelial lining however, cannot provide protection indefinitely, so the stomach has to produce a new coat of mucus for protecting itself from the acid.
It takes a minimum of 5 days for the epithelial-cell lining of the stomach to completely rebuild itself. So we have a new stomach lining every one or two weeks.