Geometry Influences Diffusion

Transit time depends heavily upon dimensionality. Flat, thin animals can live by absorption and diffusion without convection. Tapeworms do not require a circulatory system and can elongate without altering diffusion distances. In one dimension, transport by diffusion is proportional to distance. At three dimensions, however, the multiplicative factor is distance divided by the radius of the sink consuming the particles (Ref: Hardt, 1980). Therefore, to speed transit in larger forms, we must add convection, and this means adding a circulation with a heart or two or three.

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