Question

diffusion pressure will not be able to deliver oxygen all over the body in bigger animals comment

Answer 1

Large animals cannot maintain gas exchange by diffusion across their outer surface. They developed a variety of respiratory surfaces that all increase the surface area for exchange, thus allowing for larger bodies. A respiratory surface is covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange. Those gases can only cross cell membranes when they are dissolved in water or an aqueous solution, thus respiratory surfaces must be moist.

Answer 2

Its easy. All the cells of our body aren't in contact with the environment and also the diffusion process is slow and we need more o2 molecules for our complex reactions.