 Another potential problem that arises at the interface between wheel and axle (or axle and body) is the ability of an organism to transfer materials across this interface. If the tissues that make up a wheel are living, they will need to be supplied with oxygen and nutrients and have wastes removed in order to sustain metabolism. A typical animal circulatory system, composed of blood vessels, would not be able to provide transportation across the interface. In the absence of circulation, oxygen and nutrients would need to diffuse across the interface, a process that would be greatly limited by the available partial pressure and surface area, in accordance with Fick's law of diffusion. For large multicellular animals, diffusion would be insufficient. Alternately, a wheel could be composed of excreted, nonliving material such as keratin, of which hair and nails are composed.
