How does the buffer system prevent carbonic acid from changing the pH of the blood?

Answer 1

Changes in pH is harmful to the cell and any living organism. In order to minimize this harm cells contain buffering systems. In order to change the pH of a solution H+ ions must be added or taken from it. Buffers do just that. In blood the carbon dioxide is mainly transported by hemoglobin (23%). 70% of it is transported in the plasma in the form of bicarbonate ions. When CO2 enters the blood it enters the blood cell forming carbonic acid which then dissociates into a H+ ion and a bicarbonate ion. The H+ ions attach to the hemoglobin and the bicarbonates leave and enter the plasma. So the pH of the blood does not change.

Answer 2

Any buffer system consists of a weak acid and its conjugate base in similar concentrations. Since you have a weak base in the solution, it will react with protons formed by carbonic acid, absorbing them from the solution. So, protons produced by carbonic acid will change the ratio of weak acid to weak base in the solution, and will not be available to change the pH of the solution.