Explanation

A buffer is a chemical or combination of chemicals that can both take up and release hydrogen ions. Carbonic acid $(H_{2}CO_{3})$ and sodium bicarbonate $(NaHCO_{3})$ help buffering human blood because $H_{2}CO_{3}$ is a weak acid that does not totally dissociate, when excess hydrogen ions are present in blood, the reaction goes to the left and carbonic acid forms to maintain the pH. $H_{2}{CO}_{3}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \rightleftharpoons \ \ \ \ \ \ \ \ \ \ \ \ \ \ H^{+}\ \ \ \ \ \ \ + \ \ \ \ \ \ \ \ \ \ \ \ \ {HCO}_{3}^{-}$ Carbonic acid Hydrogen ion Bicarbonate ion