FIGURE 4 The law of Laplace predicts instability between alveoli of different radii (r) because pressure (P) = 2T/r, where T = tension. Therefore, small alveoli will empty into larger alveoli unless surface tension is lower in small alveoli.
Therefore, smaller alveoli would collapse and empty into larger alveoli if the surface tension were equal in both alveoli. This is an oversimplification because alveoli are not simple bubbles, but are an interdependent network of air cells resembling a sponge. The tendency of one alveolus to collapse is opposed by elastic forces in adjacent alveoli resisting further expansion. Still, the law of LaPlace predicts a mechanical advantage if surface tension could be decreased in smaller alveoli, so they would not tend to empty into larger alveoli.
Pulmonary surfactant is a biologic secretion lining the alveoli that reduces surface tension in the lung. Figure 5 shows two effects of surfactant on surface tension. First, surfactant reduces the surface tension below that of water (which would otherwise line the alveoli), and second, surface tension of a surfactant layer depends on surface area, in contrast with either water or detergent. The low surface tension of surfactant reduces the pressure necessary to inflate the lungs and reduces the work of breathing. Lung compliance is reduced and the work of breathing is increased if disease or injury depletes surfactant from the lungs (see Clinical Note later in this chapter on respiratory distress in newborns).
Detergent also has a lower surface tension than water; this explains why bubbles in soap solutions last longer than bubbles in a pure water. However, the surface tension of detergent does not change with surface area as it does for surfactant (Fig. 5), and this dependence on area is physiologically important. Surfactant promotes alveolar stability by decreasing surface tension in small alveoli, and the law of LaPlace predicts that this will reduce the difference in pressures between alveoli of different radii (see Fig. 4).
Surfactant is a lipoprotein synthesized in lamellar bodies in alveolar type II cells and turns over rapidly in the lungs. It is 90-95% phospholipids with a glycerol backbone on a polar head and two nonpolar fatty acid chains. Dipalmitoylphosphatidylcholine (DPPC) is the main phospholipid, and it spontaneously forms a
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