Surface Tension Magnitude Inside The Lung
Even though the surface tension can be greatly reduced by pulmonary surfactant, this effect will depend on the surfactant's concentration on the interface. The interface concentration has a saturation limit, which depends on temperature and mixture composition. Because during ventilation there is a variation of the lung surface area, the surfactant's interface concentration is not usually at the level of saturation. The surface increases during inspiration, which consequently opens space for new surfactant molecules to be recruited to the interface. Meanwhile at the expiration the surface area decreases, the layer of surfactant is squeezed, bringing the surfactant molecules closer to each other and further decreasing the surface tension.
SP molecules contribute to increase the surfactant interface adsorption kinetics, when the concentration is below the saturation level. They also make weak bonds with the surfactant molecules at the interface and hold them longer there when the interface is compressed. Therefore, during ventilation, surface tension is usually lower than at equilibrium. Therefore the surface tension varies according to the volume of air in the lungs, which protects them from atelectasis at low volumes and tissue damage at high volume levels.
| Condition | Tension (mN/m) |
|---|---|
| Water at 25°C | 70 |
| Pulmonary surfactant in equilibrium at 36°C | 25 |
| Healthy lung at 100% of TLC | 30 |
| Healthy lung between 40 and 60% of TLC | 1~6 |
| Healthy lung below 40% of TLC | <1 |
Read more about this topic: Pulmonary Surfactant
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