Diffusion is a process of movement of substances from its higher concentration to its lower concentration. The pressure which is responsible for creating the diffusion ability of a molecule is called as diffusion pressure.
This diffusion pressure plays a major role in exchange of gases in plants and in smaller animals but fails in bigger animals. It is so because in bigger animals not every body part faces the external environment. That is why they cannot take part in the diffusion process. Due to this reason, diffusion pressure is unable to deliver O2 to all over the body of bigger animals.
