Question

why we need only oxygen for breathing

Answer 1

At the External interface No… we breath in air not Oxygen.

At the Internal interface Yes… Blood selectively picks up Oxygen from the alceoli that are full of AIR not just Oxygen.

The reason No: 1 is low partial pressure of Oxygen in the deoxygenated blood coming to the lungs. This facilitates movement of Oxygen towards blood, (the reverse is the case of Carbon dioxide)

The reason No: 2 is enormous affinity (love) of Hemoglobin for Oxygen. The Hemoglobin devoid of Oxygen in the deoxygenated blood simply embraces the Oxygen in alveoli and carries it in its affectionate bond to the tissues where it is released (due to less partial pressure of Oxygen in the tissues).

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Answer 2

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.