Final answer:
Diffusion of oxygen through tissues is effective at a distance of 1mm or less, as diffusion is a slow process and effective over only small distances, such as the tear layer of the cornea.
Step-by-step explanation:
According to a common rule of thumb, diffusion of O2 through tissues can occur at a distance of 1mm or less. This is due to the fact that diffusion is a very slow process and over greater distances it becomes impractical for the efficient transport of oxygen. The cornea of the eye serves as a good example of this concept. It obtains most of its oxygen by diffusion through the thin tear layer covering it, which is typically around 0.500-mm thick. For larger organisms or those with thicker tissues, specialized respiratory tissues such as gills and lungs, along with circulatory systems, are required to adequately transport oxygen throughout the body.