52. Gaseous exchange in the lungs and tissues
The gas exchange occurs in the gas exchange zone of the lung, including the respiratory bronchioli, alveolar ducts and alveoli. The total surface area of the gas exchange zone is on the order of 100 square meters.
The respiratory membrane:
All the membranes O2 and CO2 must diffuse through during gas exchange are collectively called the respiratory membrane. It is extremely thin, at around 0,5 µm. The membrane has six layers, from the alveoli to the capillaries:
- The surfactant fluid
- The alveolar epithelial layer
- The basement membrane of the alveolar epithelium
- A thin interstitial space
- The capillary basement membrane
- The capillary endothelial layer
Rate of diffusion:
What drives the diffusion of O2 in the alveolar air to the blood, and the CO2 in the blood to the alveoli? Diffusion is a passive process which is driven by the difference in concentration of the gas in the blood and in the alveolar air. According to Fick’s law, the rate of diffusion depends on four factors:
- The concentration gradient of the gas
- Meaning the difference in concentration of the gas in the blood and in the alveolar air
- The surface area of the exchange surface
- The thickness of the diffusion membrane
- The diffusion constant for the gas
Let’s look at the concentration gradients for the two gases:
| Atmospheric air | Alveolar air | Arterial blood | Venous blood | Concentration gradient between alveolar air and venous blood | |
| pO2 (mmHg) | 159 | 100 | 95 – 100 | 40 | 60 |
| pCO2 (mmHg) | 0,3 | 40 | 40 | 46 | 6 |
Note how concentration gradient of O2 is ten times larger than of CO2. However, the diffusion constant for CO2 is 20 times higher than the diffusion constant for O2. The result is that CO2 diffuses much faster than O2 across the respiratory membrane.