 Dear students, in this topic we shall discuss the pulmonary surfactants and alveolar collapse. Dear students, the lungs have the ability to stretch and expand due to tension in their walls. This tension in the lung wall depends on the properties of alveolar wall that allow stretching. And the surface tension at the liquid-air interface that causes resistance to stretch. One factor is that it allows stretching and the other factor is that it resists stretching. Overall, what is the significance of these factors? The lungs produce tension in the wall. Dear students, the surface tension of the alveolar is kept low due to the presence of surfactants. Surfactants as a lipoprotein complexes which reduce the surface tension on the liquid-air interface of the alveolar. These surfactants are produced by the type 2 cells of alveolar lining. These compounds or lipoproteins have a half-life of about 12 hours in humans and other mammals. The pulmonary surfactants reduce the effort associated with breathing. They help to prevent alveolar collapse. These surfactants are found in the lungs of all terrestrial vertebrates that is in amphibians, reptiles, birds and mammals. Dear students, now we shall discuss alveolar collapse. You know that alveoli are very small bubble-like structures with very thin walls. Their walls are only 1 to 10 micrometre thick or thin. During breathing, they are inflated and deflated alternately. This continuous inflation or deflation in bubble-like structures that creates mechanical problems that might cause their collapse. Imagine that a bubble that is formed expands and then contracts again and again. But for alveoli, this is possible. However, there is still a danger that it can burst like a bubble. We call this condition alveolar collapse. The air pressure inside the thin walled alveoli may cause them to burst. When they burst, they may join with other larger alveoli. The larger alveoli have lower pressures while the smaller alveoli have larger pressures. As a result, larger sacs are formed. When larger sacs are formed from alveoli, it overall reduces the surface area available for gas exchange. Dear students, alveolar collapse does not normally occur. It is prevented because of two factors. 1. The surrounding tissue prevents over-expansion of alveoli. 2. The presence of pulmonary surfactants on the alveolar surface increases wall tension when surface film is expended. When alveoli is expended, the surfactants increase the wall tension which resists further stretching. This wall tension is decreased because of surfactants when the alveoli are compressed. When alveolar volume is decreased, it is fully compressed. In that case, alveoli are folded. These folds are covered with surfactants. This thick layer of surfactants reduces the surface tension. This prevents them to inflate easily because the folded alveoli has surfactant layer. This effect of surfactants also minimizes the pressure difference between the larger and smaller alveoli. When the pressure difference is reduced, the effect is that the chances of collapse are reduced.