Another problem is the flow of materials to and from a living wheel. If the tissues that form the wheel are living, then they must receive oxygen and nutrients and have wastes taken away in order to continue metabolism (the process of using energy to maintain functioning). A normal animal circulatory system (the network of tubes that allow blood to flow), would not be able to provide for the necessary flow of materials. Without blood flowing, materials would have to diffuse to and from the wheel. Diffusion is limited by partial pressure and surface area, by a rule called Fick's law of diffusion. For large animals, diffusion would not be enough to keep tissues alive. Instead of a living wheel, a wheel could be made of dead material like keratin, which makes up hair and nails.