Organization within the body: from molecules to body compartments

The volumes of different compartments of body water are first considered in this article, with a description of how these can be measured by the ‘dye-dilution’ method. Then the physicochemical properties of water, inorganic ions and organic molecules are discussed in terms of the biological roles that they perform. Particular emphasis is placed on amphipathic molecules and their three-dimensional structure in aqueous and non-aqueous environments. The movement of water and dissolved substances across a membrane separating two compartments is described in some detail. To illustrate the different possibilities, several examples are described where the properties of the separating membrane and the solutions in the two compartments differ. This section of the article covers osmotic pressure, oncotic pressure, the development of transmembrane electrical potentials and the Gibbs–Donnan equilibrium. Differences between intracellular and extracellular fluids are described. The article ends by comparing the movement of substances by simple diffusion, facilitated diffusion and active transport. In all cases, the effect of the movement, the source of energy giving rise to the movement and factors limiting the rate of movement are considered.