Exchange of gases
- Alveoli are the primary sites of exchange of gases.
- O2 and CO2 are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient.
- 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.
- A gradient is present for CO2 in the opposite direction, i.e., from tissues to blood and blood to alveoli.
- 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
- the thin squamous epithelium of alveoli,
- the endothelium of alveolar capillaries
- the basement substance in between them.
- All the factors in our body are favourable for diffusion of O2 from alveoli to tissues and that of CO2 from tissues to alveoli.
Fig. Diffuson membrane of alveoli