Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms like humans?

Multicellular organisms like humans require a more efficient circulatory system for transportation of oxygen as diffusion cannot transport oxygen to longer distances in the body. In multi-cellular organisms, all the cells may not be in direct contact with the surrounding environment. Thus, simple diffusion will not meet the requirements of all the cells. So, in multicellular organisms like humans, transport of oxygen through blood stream ensures its efficient supply to each cell of body.