Oxygen is an important fuel in living animals. In the unicellular animal like the amoeba, the oxygen needed for metabolism diffuses from the water in which the organism lives into its body cell. Metabolism of food substances in the body leads to production of energy, water and carbon dioxide. The concentration of carbon dioxide inside the body cells is therefore higher than outside the cells. The excess CO2 must be excreted from the body. In the case of amoeba, CO2 diffuses along its concentration gradient from the inside of the organism to the surrounding water. The taking in of oxygen and given off of carbon dioxide is called external respiration. The utilization of the oxygen for cellular metabolism and the resultant production of energy, water and carbon dioxide is internal respiration.
As one ascends the evolutionary scale and the organism become multicellular and more, it is no longer possible to meet the requirements of oxygen intake and carbon dioxide excretion by simple diffusion. The higher animals like mammals have a respiratory system which is devoted to the function of gaseous exchange. The respiratory system functions in close collaboration with the circulatory system which acts as the transport system that conveys oxygen from the respiratory apparatus to the tissues and carbon dioxide from the tissues to the respiratory apparatus for expulsion from the body. The respiratory system is thus an “air pump” while the cardiovascular systems is a “blood pump”
Source: http://www.physiologynotes.com/category/respiration/