An equation that describes the rate at which a solute diffuses between two locations in a solution. It has many applications and can be written in various forms, but in biology it is frequently applied to the movement of molecules of respiratory gases, i.e. oxygen or carbon dioxide, during gaseous exchange between an organism’s internal body fluids and the external environment. It can be expressed thus:
where Q is the rate of diffusion, D is the diffusion coefficient of the solute, A is the area over which diffusion is occurring, P1 and P2 are the concentrations of solute at the two locations, and L is the distance between the two locations. The diffusion coefficient depends on the nature of the solute, the medium through which it is diffusing, and the temperature. The equation was derived by the German-born physiologist Adolf Fick (1829–1901).