Exchange of Gases MCQ - Practice Questions & Answers

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Exchange of Gases

Alveoli are the primary sites of exchange of gases. Exchange of gases also occurs between blood and tissues.
There are approx. 30 crore alveoli in the human lungs.
O2 and CO2 are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient.
The solubility of the gases, as well as the thickness of the membranes involved in diffusion, are also some important factors that can affect the rate of diffusion.
Dalton’s Law: Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO2 for oxygen and pCO2 for carbon dioxide. It is equal to the total pressure of the mixture divided by the percentage of that gas in the mixture. For instance, the pressure of atmospheric gas at sea level is 760 mmHg and oxygen forms 21% of the air, so pO2 will be 21% of 760 or 159 mmHg.

As the solubility of CO2 is 20-25 times higher than that of O2, the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O2.
The diffusion membrane is made up of three major layers:
- thin squamous epithelium of alveoli
- endothelium of alveolar capillaries
- Basement substance (composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries) in between them

However, its total thickness is much less than a millimetre.
Therefore, all the factors in our body are favourable for the diffusion of O2 from alveoli to tissues and CO2 from tissues to alveoli.