In both external respiration as well as internal respiration, exchange of respiratory gases occurs. In external respiration there is exchange of carbon dioxide of blood with oxygen of air or water while in internal respiration, there is exchange of oxygen of blood with carbon dioxide of the body cells. 2. These gas exchanges are physical process and depend partly upon the principle of simple diffusion, and partly on facilitated diffusion.
3. Diffusion is simply random motion of molecules which exert kinetic energy. 4. Diffusion is defined as the flow of the substances from a region of higher concentration(or higher partial pressure or higher kinetic ene to a region of lower concentration(or lower partial pressure or lower kinetic energy