Statement I: False. A buffer solution is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) in specific ratios. The statement's claim of "any particular quantities" is incorrect.
Statement II: True. Blood is a naturally occurring buffer system, primarily maintained by the H$_2$CO$_3$/HCO$_3^-$ system, which helps regulate pH within a narrow range.
Thus, only Statement II is correct.