text: All of them stabilize pH (concentration of H+).\n\n1. Carbonic acid (weak acid) dissociates partly bothsides:\nH2O + CO2 <=> H2CO3 <=> HCO3-, H+. Whatever you add, the equilibrium of this reaction shifts to one or another direction and H+ concentration of the solution (acidity) remains the same.\n\na) If you add acid ( more H+ then), a disbalance occurs. Excessive H+ react with HCO3- and a larger quantity of H2CO3 is formed. And it dissociates to water and CO2, neutralising addition of some acid.\n\nb) When a base is added, a part of H+ is removed (H+ and OH- form water, which is neutral). Then also a disbalance occurs, which causes CO2 combination with water, formation of H2CO3 and appearance of new H+. \n \n2. Na2HPO4 <=> 2Na+ + HPO4/2- + H+ <=>  H2PO4-  \nAnalogically, a shift occurs when adding acid or base, neutralising their effect:\na) HCl + Na2HPO4 => NaH2PO4 + NaCl \nb) NaOH + NaH2PO4 --> Na2HPO4 + H2O \n\n3. Different proteins act as buffers. Hemoglobin (located in red blood cells) buffer is one of strongest. Imidazole groups of histidine residues of this protein are most significant for this effect.\n\nHemoglobin without O2 <=> reduced hemoglobin \nReduced hemoglobin takes on H+, decreasing amount of free H+ in solution and the solution becomes not so acid.