17. BREATHING AND EXCHANGE OF GASES
Gas exchange occurs between- Alveoli and blood
- Blood and tissues
Alveoli are the primary sites of gas
exchange.
O2 & CO2 are exchanged by simple diffusion. It depends upon the following factors:
O2 & CO2 are exchanged by simple diffusion. It depends upon the following factors:
- Pressure/ concentration gradient: The Partial pressures (individual pressure of a gas in a gas mixture) of O2 and CO2 (pO2 and pCO2) are given below.
|
Respiratory gas |
pO2 (in mm Hg) |
pCO2 (in mm Hg) |
|
Atmospheric air |
159 |
0.3 |
|
Alveoli |
104 |
40 |
|
Deoxygenated blood |
40 |
45 |
|
Oxygenated blood |
95 |
40 |
|
Tissues |
40 |
45 |
pO2 in alveoli is more (104 mm Hg) than that in blood capillaries (40 mm Hg). So O2 diffuses into capillary blood. pCO2 in deoxygenated blood is more (45 mm Hg) than that in alveoli (40 mm Hg). So, CO2 diffuses to alveoli.
- Solubility of gases: Solubility of CO2 is 20-25 times higher than that of O2. So, the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is higher than that of O2.
- Thickness of membranes: The diffusion membrane is made up of 3 layers:
a) Squamous epithelium of alveoli.b) Endothelium of alveolar capillaries.c) Basement substance between them.Its total thickness is only 0.5 mm. It enables easy gas exchange.
- Surface area: Presence of alveoli increases the surface area of lungs. It increases the gas exchange.
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