"the stomach is a J-shaped, expandable sack, normally on the left side of the upper abdomen. Several muscle layers surround the stomach, serving to churn food. The stomach can expand to hold about 2 L of food (= 1/2 gal). The stomach contains hydrochloric acid (HCl) strong enough to dissolve metal (pH about 1.5 to 3, usually around 2), which kills bacteria and helps denature the proteins in our food, making them more vulnerable to attack by pepsin. The stomach secretes mucus to protect itself from being digested by its own acid and enzymes. The stomach also manufactures pepsin, an enzyme to digest protein. The average person secretes about 400 mL of gastric juice per meal, containing 50 to 300 µg pepsin/mL. For an average of around 200 µg/mL × 400 mL of gastric juice, this would be 80 mg (or 0.080 g) pepsin/meal. For HCl with a concentration of around 6.08 g/L × 400 mL, this would be 2.4 g/meal. Consumption of antacids does just what their name suggests: they drastically change the pH of the stomach contents, interfering with pepsin’s ability to digest protein. Here is more background on pepsin and the effects of antacids. Here is a photograph of results of an experiment we perform to study pepsin’s ability to digest the protein in egg white, and a summary of the contents of each of these test tubes."
