We inhale all the gases present in the atmosphere. When this mixture of gases reaches our lungs .... alveoli (air sac in the lungs), where the impure blood (with CO2) exchanges its CO2 with O2 from the mixture of gases, what we inhaled. The Red Blood Cells (RBC) has the capacity to absorb only oxygen only, which is verymuch required to convert the stored carbohydrate and fat into energy in the muscles. The RBC has the haemoglobin, which is responsible for selectively absorbing O2 for this mixture of gases. A person suffering from anaemia (lack of haemoglobin in the RBC) has less capacity to trap O2 from the mixture of gas, he/she inhaled. Thus, this will lead to fatigue. In the human physiology, oxidation is the main process where the stored carbohydrate or fat is burnt and converted into energy. Thus respiration is a catabolic action. Now-a-days, oxygen parlours are very famous in the thickly populated and polluted cities. When we inhale air,all the gases in it also enter our lungs. Pure oxygen is actually not good to breathe for long periods of time . We have externally no selective mechanism to inhale only oxygen. In the terminal region of lungs, the alveoli ,exchange of gases takes place. The haemoglobin in red blood cells has an affinity for this oxygen present in the inhaled air and it is immediately absorbed. The rest are inert gases which are sent out during exhalation. Oxygen is vital to our body functions. so the body takes what it needs and rejects the rest.