You really are talking about two different things here. Antacid tablets are really not buffers. They are edible bases that neutralize excess acid upon reaching the stomach. Most antacid tablets are carbonate salts like calcium carbonate (CaCO3) which are actually the chemical component of chalk. Hence, flavor is added to mask the chalky taste. The chemistry at play here is simple acid base chemistry.
CaCO3 + 2HCl --> CaCl2 + H2CO3
Buffers in the bloodstream are more complex. A buffer is a chemical system that is designed to try to maintain a relatively constant pH. In other words, a system where the addition of excess acid or base can be neutralized without a major change in overall pH. Carbon dioxide in water is a perfect buffer system for the bloodstream, since the bicarbonate ion (HCO3-) can reaction with both acids and bases. All of this chemistry is demonstrated below.
NaHCO3 + HCl --> NaCl + H2CO3
NaHCO3 + NaOH --> Na2CO3 + H2O