Acid–base biochemistry encompasses the physical chemistry of the constituents
of biological solutions that influence the dissociation of, and therefore
the concentration of, hydrogen ions (Hþ) in those solutions. In the biological
solutions that comprise the body fluids, these constituents include electrolytes
that are essentially completely dissociated at the solute strength that exists in the body fluids, termed ‘‘strong ions’’ (1,2), a wide variety of weak acids and, most importantly, the volatile weak acid H2CO3 (carbonic acid). Central to an understanding of acid–base homeostasis is knowledge of the chemistry of weak acids and, in particular, carbonic acid. In this chapter, we review the physical chemistry that underlies acid–base homeostasis, incorporating the concepts of Brønsted and Lowry, who defined acids as Hþ donors and bases as Hþ acceptors (3,4). The additional role of strong ions, which are regulated independently of the dictates of acid–base homeostasis but influence [Hþ] is discussed at the end of this chapter.