L Q is the rate at which a substance diffuses between two locations. A is the cross-sectional area over which the substance is diffusing. L is the distance between those locations. L Animals can maximize D for respiratory gases by using air rather than water for the gas exchange medium if it is at all possible. All other adaptations must influence surface area or the concentration gradient across that surface area where the gas exchange occurs.
Gills are highly branched and folded to provide a large surface area for gas exchange.
Lungs are highly divided by elastic air sacs to increase surface area. External gills can increase the concentration gradient across them by being waved around in the environment. This increases the concentration gradient because the gills are exposed to fresh respiratory medium this way. Internal gills and lungs are exposed to fresh respiratory medium by ventilation - moving the medium across the gas exchange surface. Movement of gases between the gas exchange surface and the tissues is called perfusion .