Molecular diffusion
The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) of the particles.
The result of diffusion is a gradual mixing of material such that the distribution of molecules is uniform.
In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing.
Metabolism and respiration rely in part upon diffusion in addition to bulk or active processes.
Lungs contain a large surface area to facilitate this gas exchange process.
However, there sometimes occur so-called quasi-steady states, where the diffusion process does not change in time, where classical results may locally apply.
Before this point in time, a gradual variation in the concentration of A occurs along an axis, designated x, which joins the original compartments.
The rate of diffusion of A, NA, depend on concentration gradient and the average velocity with which the molecules of A moves in the x direction.
This relationship is expressed by Fick's law where D is the diffusivity of A through B, proportional to the average molecular velocity and, therefore dependent on the temperature and pressure of gases.
Restricting discussion exclusively to steady state conditions, in which neither dCA/dx or dCB/dx change with time, equimolecular counterdiffusion is considered first.
If no bulk flow occurs in an element of length dx, the rates of diffusion of two ideal gases (of similar molar volume) A and B must be equal and opposite, that is
