Gas diffusion, as a basis for complex biological processes such as respiration, is a core principle for understanding fundamental physiology. Students, however, often find these concepts challenging, in particular when expressed formally as in Fick’s law of gas diffusion:

In this version, m/t describes the oxygen flux (i.e., the amount [m] of oxygen, moving in a given time [t] from one compartment of a system to another, e.g., from surrounding water to the blood of a fish). D, the diffusion constant, expresses how “easily” the gas passes a material layer. S is the surface area over which diffusion takes place, and which separates compartments (e.g., gill or body surface). And ΔC/x describes the concentration gradient between the two compartments, such as “blood” and “water,” as the difference between concentrations in these compartments divided by their distance. The distance between water and blood (source and sink) is x.

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