Basic Physiology, Body Fluids, Membrane Transport, Nerve and Muscle Function

Diffusion

Molecules of gas and solutes in solution are in constant random movement as a result of their thermal energy. This random movement scatters the molecules evenly within a given volume. This is called diffusion.
In a situation where a concentration difference exist between two areas, diffusion would tend to eliminate this difference. Thus, though diffusion of molecules tends to occur in all directions, in cases where a concentration gradient is present, net diffusion occurs from an area of high concentration to an area of lower concentration.