Evolving ideas about osmosis and capillary fluid exchange.

When a solute is dissolved in water at (T, pel), the temperature and external pressure applied to the solution, the water in the solution is altered as is pure liquid water at (T, pel - piH2Ol). The liquid water and the water in the solution are in equilibrium when piH2Ol is the osmotic pressure of the water in the solution. Every partial molar property of the water in the solution at (T, pel), including its vapor pressure, chemical potential, volume, internal energy, enthalpy and entropy, is identical with the same molar property of pure liquid water at (T, pel - piH2Ol). This elementary fact was deduced by Hulett in 1903 from a thought experiment; he concluded that the internal tension in the force bonding the water is the same in both solution and pure liquid water, in equilibrium, at these differing applied pressures. Hulett's understanding of osmosis and the means by which the water was altered by the solute were neglected and abandoned. Competing ideas included the notions that the solute attracts the water into the solution and that the solute lowers the activity (or concentration) of the water in the solution. These ideas imply that the solute acts on the solvent at the semipermeable membrane separating the solution and water. Hulett's theory of osmosis requires that the solute alter the water at the free surface of the solution where the solute exerts an internal pressure on the boundary of the solution retaining the solute. Fluid exchange across the capillary endothelium is influenced, in part, by colloidal proteins in the plasma. The role of the proteins in capillary fluid exchange must be reinterpreted based on Hulett's view, the only valid view of osmosis.