A small organism, like an amoeba, has a large surface area : volume ratio, and so it can accomplish all the exchange it needs by simple diffusion across its body surface. However, a large organism, like a mammal, has a much smaller surface area: volume ratio, so it can not accomplish all the exchange it needs in this way. (A large surface area : volume ratio is preferable for carrying out exchange of substances). Such large organisms need special respiratory organs such as lungs for taking in oxygen and removing carbon dioxide.
Fick's Law shows how the factors which affect the rate of exchange interact
Example
Calculating the ratio of surface area : volume for organisms
 Look at surface area and volume values
 Check they are in the same units
 Divide the surface area value by the volume value. This gives you a figure which = answer
 The answer : 1 is the ratio of surface area to volume.
e.g.
 Surface area = 54 cm^{2}
 Volume = 27 cm^{3}
 Surface area to volume = 54/27 = 2
 Therefore 2 : 1 is the ratio of surface area to volume.
