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

Diffusion is a passive mode of transport of substances like ions, atoms etc. from higher concentration to lower concentration region.This process takes place slowly and can only transfer substances up to a very small distance.The size of the multicellular organisms is very large compared to unicellular organisms and also the body cells are not in direct contact with the surrounding environment so, every cell of the body will not get oxygen as per need by the process of diffusion from the environment. Therefore diffusion is insufficient to meet the oxygen requirements of multicellular organisms.