Dude, you gotta start reading about the small intestine! All the info is out there on this wonderful information highway. :) Digestion is an acidic process. How do you suppose the intestines aren't eaten up along with the food? Because they have a mucosa lining that is base and counteracts the acids. Sometimes they can't form enough and so you get acid pain.
Antacids slowly kill people because they turn off digestion. If nothing you eat digests, it turns your entire body into a compost pile. Next time you take Nexium or Maalox or whatever, check out your tongue the next day. It will be coated with undigested food particles, the same ones that line your arteries and cause impotence and heart failure.