Question time!!How is the oxygen differentiated from other gases in our respiratory system?

The atmosphere contains several gases. A person inhales that combination but how is the oxygen differentiated from that in our respiratory system?

We inhale oxygen, carbon dioxide and nitrogen and also exhale the same gases, though in different proportions to inhalation; that is, oxygen is exchanged for carbon dioxide during breathing.

When inhaling, we take in approximately 21 per cent oxygen, 0.04 per cent carbon dioxide and 79 per cent nitrogen. On exhalation, we give off approximately 16 per cent oxygen, 4 per cent carbon dioxide and 79 per cent nitrogen; only the amount of nitrogen remains constant in the exchange. Exhaled air also contains water vapour. The inhaled air reaches lungs and enters alveoli where oxygen diffuses out from alveoli into blood, which enters into lungs via pulmonary capillaries, and carbon dioxide diffuses into alveoli from blood.

This diffusion happens because of partial pressure difference between oxygen and carbon dioxide in blood and alveoli.

In alveoli, partial pressure of oxygen is relatively higher than that of carbon dioxide whereas, in the blood which enters back in to the lungs, partial pressure of carbon dioxide is higher than that of oxygen.

Carbon dioxide entered into the alveoli by diffusion is exhaled by lungs while we breathe out.

For our body cells to perform various functions, they need energy, and this energy is generated by producing ATP molecules via burning fuel molecules such as carbohydrates using oxygen.

The byproduct of this reaction is carbon dioxide. So in the body cells, partial pressure of carbon dioxide is higher than oxygen. When oxygen rich blood reaches body cells by systemic circulation, because of partial pressure gradient, oxygen will diffuse into the body cells and carbon dioxide which is at higher pressure will diffuse into blood.

The carbon dioxide rich blood returns to the heart (right atrium) and then pumped into lungs. In the lungs, carbon dioxide is exhaled. This process is a cycle and oxygen is taken up and carbon dioxide is released out continuously in our body.

It is general concept that carbon dioxide is bad for the body. It is not so. Carbon dioxide plays one of the most significant roles in body health and well-being. Carbon dioxide regulates the distribution of oxygen in the body. It is also the body’s relaxer. CO is the body’s way of dilating the arteries, especially those in the brain, the heart and the periphery (hands and feet).

S.P.S. JAIN

Former Member, Engineering, Indian Railways

Greater Noida, Uttar Pradesh