Diffusion is, transport of a molecule due to its own moveability. The driving force is its 'activity': the (electro)chemical potential. Mostly, the concentration is taken for that but that is not fully correct: it is only valid in a homogeneous medium. For gases, the partial pressure is better since that reflects the chemical potential also in non-homogeneous media. The related diffusion parameters are Diffusion coefficient D for concentration and Permeability ℘ for gases - but note, that in literature there are other forms of permeability too! In physiology, the 'Krogh's Diffusion Coefficient' is the same as 'Oxygen Permeability'.
Diffusion is the only way of transport in regions that cannot be reached by other methods, such as inside tissue cells, and thus critically important for the body.
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Basic Principles
Steady State
Nonsteady State
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