The blood keeps it's pH levels fairly constant (7.4 I believe) by use of a buffer system. The buffer system is formed from dissolved carbon dioxide in the blood. The carbon dioxide in your blood is in the form of carbonic acid which is in dynamic equilibrium with hydrogen carbonate. Low pH blood level is referred to as acidosis, high levels as alkalosis. If the pH levels of the blood were to fluctuate it would have an effect on the functioning of proteins etc.