Question

why is diffusion insufficient to meet the oxygen requirement in the body

Answer 1

Our body is made up of numerous cells all of which require energy. Moreover the worm blooded animals like us need huge amount of energy to maintain body temperature. As diffusion is a very slow process it will take a long time to reach oxygen to each cell. Also not all the cells of the body are exposed to environmental oxygen that can be directly diffused in the cell. Thus, diffusion only is not sufficient to meet the oxygen requirement of such complex multicellular body.