Partial pressure gradients [(P1 – P2)/L] drive diffusion across gas exchange surfaces; the larger the gradient, the greater the rate of gas exchange. These gradients can be maximized in several ways:
Minimizing path length: Very thin tissues in gills and lungs reduce the diffusion path length (L).
Ventilating: Actively moving the external medium over the gas exchange surfaces (i.e., breathing) regularly exposes those surfaces to fresh respiratory medium containing maximum O2 and minimum CO2 concentrations. This maximizes the partial pressure gradients.
Perfusing: Actively moving the internal medium (e.g., blood) over the internal side of the exchange surfaces transports CO2 to those surfaces and O2 away from them, thus maximizing the partial pressure gradients driving diffusion.