When O2 is used by cells to produce energy, CO2 is created as a byproduct. This is handed off to the bloodstream to be carried back to the lungs and released during expiration. But CO2 is acidic, which means that it lowers the pH of the blood. That is why the body has the bicarbonate buffer system. Within this system, CO2 reacts with water (H2O) to create carbonic acid (H2CO3). This, in turn, dissociates quite rapidly and forms a bicarbonate ion (HCO3-) and a hydrogen ion (H+). This system makes sure the pH is balanced by having both a weak acid and its conjugate base present so that any excess acid or base that is introduced into the system is neutralized.