What is Buffer?

A buffer is a solution containing a weak acid and its conjugate base in comparable amounts; it resists a change of pH, because added $\mathrm{H^+}$ is taken up by $\mathrm{A^-}$ and added $\mathrm{OH^-}$ by $\mathrm{HA}$. It works best within one unit of the p$K_a$, and its capacity grows with the total concentration of the pair. The cell’s buffers: phosphate ($\mathrm{H_2PO_4^-}/\mathrm{HPO_4^{2-}}$, p$K_a$ 6.8) and the side chains of proteins inside cells; bicarbonate ($\mathrm{CO_2}/\mathrm{HCO_3^-}$, effective p$K_a$ 6.1) in the blood, whose acid partner is a gas that the lungs can remove.