Internal reference standards in ionic regulation and the predictability of ionic concentrations in animals

1. The regulation of ions at similar concentrations in most individuals of a species suggests the existence of internal reference standards. 2. Few have been identified, but many probably relate to cell membrane properties, including potentials, surface charge densities and equilibrium constants of receptor molecules. 3. Solubility may sometimes determine the product [Ca2+][CO32−]. 4. Reference standards must generally each relate to more than one ionic species. 5. For some concentrations, including osmolality, there may be no direct reference standard