concept

Fick's law of diffusion

The physical rule behind every respiratory organ that ever existed: how fast a gas crosses a living membrane depends on three things - the surface area for exchange, the steepness of the partial-pressure difference across it, and the thinness of the membrane. More surface, steeper difference, thinner wall, faster the gas moves. It is not a biological law but a physical one, as true on Pandora as in a laboratory. Life cannot escape it, only obey it cleverly - which is why every lung and gill is a vastly folded structure with walls shaved thin enough that blood and air nearly touch.