Intravenous literature: Campbell, I. (2009) Osmolarity and partitioning of fluids. Anaesthesia and intensive care medicine. 10(11), p.562-565.
Substances move down concentration gradients. When fluids are mixed together, they diffuse down their own concentration gradients and come to a dynamic equilibrium such that the concentrations of the various substances in all parts of the medium are the same. When two solutions are separated by a semipermeable membrane (i.e. permeable to the solvent (water) but not the solutes), water moves down its concentration gradient such that the osmotic pressure on both sides of the membrane is the same. Cell membranes are effectively semipermeable membranes. Water passes freely but the movement of solutes across the membrane, particularly charged molecules, is usually via a specific carrier protein. This may sometimes involve the expenditure of energy.