buffer, solution that can keep its relative acidity or alkalinity constant, i.e., keep its pH constant, despite the addition of strong acids or strong bases. Buffer solutions are frequently solutions that contain either a weak acid and one of its salts or a weak base and one of its salts. Many acid-base reactions take place in living organisms. However, for organisms to perform certain vital functions, the body fluids associated with these functions must maintain a constant pH. For example, blood must maintain a pH of close to 7.4 in order to carry oxygen from the lungs to cells; blood is therefore a powerful buffer.