The carbonic acid bicarbonate buffer system in the blood helps maintain normal pH. If you have too many H+ ions in the blood the renal system will remove them using bicarbonate as the buffer. This of course assumes that renal function is not impaired. An excess of H+ ions will shift the pH to the left and change the pH to an acidotic state. Bicarbonate causes a shift in H+ ions from extracellular to intracellular thus moving the pH of the blood back to the right more towards a normal pH. If the renal system fails to accomplish this task, metabolic acidosis ensues. The respiratory system also plays a large role in maintaining the pH by altering the respiratory rate.