Blood plasma is a weak buffer. Remember that it is designed to buffer around pH 7.4 and therefore its capabilities are limited to weak acids or alkalis. For example, carbon dioxide dissolved in it forms a weak acid, which plasma can buffer. Plasma would not be able to buffer strong mineral acids.....it was never designed for that purpose! The effect of acid on a standard buffer solution depends on the strength of the acid. A standard buffer will more easily cope with a 1 mM solution of HCl than a 1 M solution of the acid. Mineral acids (HCl H2SO4 and HNO3) will require higher buffering than weaker acids eg. acetic acid.