Although pure water is considered a neutral molecule with a pH of 7,
water plays a large role in acid-base homeostasis. The very important bicarbonate
buffer system in the mammalian body involves an equilibrium
in which carbon dioxide combines with water to produce carbonic acid,
which in turn dissociates into a free proton and a bicarbonate ion. This
equilibrium finds importance in many parts of the body, but is particularly
significant in the blood because it contributes to the removal via the lungs
of relatively toxic carbon dioxide, produced in the peripheral tissues. The
linkage of the bicarbonate buffer system with hemoglobin means that relatively
large quantities of carbon dioxide can be removed from the body
with a very small change in pH.