Elastic behavior of lungs

The second effect of surface tension is that it causes the alveolus to become as small as possible. As the water molecules pull toward each other, the alveolus forms a sphere, which is the smallest surface area for a given volume. This generates a pressure directed inward on the alveolus, or a collapsing pressure. The magnitude of this pressure is determined by the Law of LaPlace  

The collapsing pressure (P) is proportional to the alveolar surface tension (ST) and inversely proportional to the radius (r) of the alveolus. In other words, the greater the surface tension and the smaller the radius, the greater the collapsing pressure. 

Due to this collapsing pressure, alveoli are inherently unstable. For example, if two alveoli of different sizes have the same surface tension, the 

Normal lungs, however, produce a chemical substance referred to as pulmonary surfactant. Made by alveolar type II cells within the alveoli, surfactant is a complex mixture of proteins (10 to 15%) and phospholipids (85 to 90%), including dipalmitoyl phosphatidyl choline, the predominant constituent. By interspersing throughout the fluid lining the alveoli, surfactant disrupts the cohesive forces between the water molecules. As a result, pulmonary surfactant has three major functions  

Pharmacy application infant respiratory distress syndrome 

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