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

Diffusion is the passive movement of ions, atoms, and molecules from a region of higher concentration to a region of low concentration, but it occurs slowly and only up-to small distances. The body size of multicellular organism is very large so diffusion is not sufficient to meet the oxygen requirements of multicellular organisms since oxygen will not diffuse quickly to meet the demands of respiratory pigment in the body.