Blood could be considered the "ultimate buffer solution" because its role within organisms is to carry oxygen to every cell in the human body, and also to regulate many systemic functions involving the metabolism and production of hormones and enzymes.
A buffer solution is defined as a watery substance that contains either a weak or a strong acid. These acids are, in turn, balanced by a counterpart base which is either strong when the acid is weak, or weak when the acid is strong. With this balance, the buffer solution is capable of balancing the PH acid in chemical reactions.
Concisely, blood possesses the balanced combination of acid and base that makes it a buffer solution. Moreover, blood is ever-present in every single chemical reaction that takes place in the human body. This being said, the consistency and composition of blood, as well as the vast amount of it, makes it capable of performing the function of regulating the PH of the everyday metabolic reactions that occur within the human body.