why is diffusion in suficient to meet the oxygen requirements of multicellular organisms like humans?

In multicellular organisms especailly human beings ... the many layers of cells cause the ratio of surface area to volume ratio to be very low .... Hence rate of diffusion is very slow and hence most of the multicellular organisms won't survive .... In addition to it diffusion can transfer gases to small distances only .... Hence, diffusion of O2 is insufficient to meet the requirement of multiucellular oragnisms like human beings .....

multicellular organisms with a few cells can only survive by this process of oxygen diffusion ..... not the ones like human beings ...
Multicellular organisms such as humans possess complex body designs. They have specialised cells and tissues for performing various necessary functions of the body such as intake of food and oxygen. Unlike unicellular organisms, multicellular cells are not in direct contact with the outside environment. Therefore, diffusion cannot meet their oxygen requirements