Why is diffusion insufficient to meet the oxygen reqirements of multicllular organisms like human?

In single-celled organisms the entire surface of the organisms is in contact with the environments for the diffusion of substances. In multi-cellular organisms, the body size is large and all the cells are not in direct contact with the surrounding environment. Through diffusion, oxygen will not reach the cells lying deep within the body.So simple diffusion will not meet the oxygen requirements of all the cells.

Since diffusion pressure alone cannot take care of oxygen delivery to all parts of the body, respiratory pigments are usually present to carry oxygen from lungs to the tissues.