49.4). In clean water with high surface tension, the pressure required to keep open a bubble as the radius decreased would be greater than the pressure keeping open a larger bubble, so the smaller bubbles would eventually collapse into larger ones (or small alveoli would collapse into larger ones), as shown in Figure 46-3, B. This effect is demonstrated in Figure 4-24, which represents two drops placed on wax, one of which is water and the other water with detergent. Surface tension is exploited by alveoli by means of a surfactant that is produced by one of the cells and released to lower the surface tension of the fluid coating the inside of the alveoli to prevent these sacks from collapsing. Surface tension effects are also important, resulting in about 60% of the necessary force required to expand the lung. The force of these covalent bonds effectively creates an inward force on surfaces, such as lung tissue, with the effect of lowering the surface area of that surface as the tissue is pulled together. The force acting on one half is the product of pressure and cross-sectional area, which is balanced by the force acting on the other side, which is the product of surface tension and length of circumference. The surface tension of alveolar fluid is regulated by pulmonary surfactant, allowing efficient respiration. Surface tension is generated from molecular attractive forces within a liquid that oppose spreading; it is the reason that water ‘beads up” on a clean surface. A molecule on the surface, however, is only attracted to its neighbors. During expiration surfactant reduces surface tension, splinting open the alveolus and making it generally more stable. From: Nunn's Applied Respiratory Physiology (Eighth Edition), 2017, B.S. For example, the surface tension of water (in dynes/cm) is 76 at 0° C, 72 at 25° C, 68 at 50° C, and 59 at 100° C. The surface tension of liquids is also reduced by the presence of impurities, some of which are exceedingly effective. When SO (1000 cSt) comes in contact with pure water, interfacial tension was found to be 40 mN/m. If the surface tension remained high, the pressure required to keep smaller alveoli open as their radius decreased would be greater than the pressure keeping open larger alveoli, so the small alveoli would collapse into larger ones, as shown in Fig. 46.3B. Collapse of the lungs is called alectasis. Without normal surfactant, the tissue surrounding the air sacs in the lungs (the alveoli) sticks together (because of a force called surface tension) after exhalation, causing the alveoli to collapse. Surface tension causes the pressure within the alveolar lining fluid to be less than the alveolar pressure. This reduces the surface tension to increase wetting. Surface tension is measured in terms of force (dynes) per centimeter of the surface of liquid. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. 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Dekerlegand, ... Christiane Perme, in, Respiratory Disorders in the Preterm Infant, Avery's Diseases of the Newborn (Tenth Edition), Respiratory Distress in the Preterm Infant, Avery's Diseases of the Newborn (Ninth Edition), Craig's Restorative Dental Materials (Thirteenth Edition), Foundations of Anesthesia (Second Edition), Surfactant consists of phospholipids, which contain both hydrophobic and hydrophilic domains. Newborns may also attempt to prevent alveolar collapse by grunting. During inflation, as the radius of each alveolus increases, surface tension increases even faster. SP-B and SP-C facilitate the initial adsorption and the reincorporation of surfactant lipids into surface films and help prevent inactivation of surfactant, presumably by their ability to both order and disorder lipid membranes (Johansson and Curstedt, 1997; Kruger et al, 2002; Perez-Gil, 2002; Wang et al, 2002). Onset of axisymmetric instability causing bead formation in the fibers; pictures taken at different distances from the needle: (a) 1 cm, (b) 3 cm, (c) 5 cm, (d) 7 cm, (e) 9 cm, (f) 12 cm, (g) 15 cm, (h) 30 cm.25 Reprinted with permission from reference 25. J. Craig Jackson, in Avery's Diseases of the Newborn (Ninth Edition), 2012. Surface tension is the force exerted by water molecules on the surface of the lung tissue as those water molecules pull together. For an alveolus the equation reads: This shows that as the radius of an alveolus becomes smaller the pressure required to balance the tension in the wall and so prevent collapse must rise. • In normal lungs, most stiffness is due to surface tension (although it is greatly reduced as area decreases, due to surfactants). However, if the bubbles are lined with high-quality surfactant, the surface tension falls quickly as the radius gets smaller because the surfactant molecules become crowded during deflation (Fig. During inflation, more pressure is required to achieve similar tidal volume (Figure 46-5, B), because of poor compliance (蜐V/蜐P) from having to reopen collapsed alveoli. Surfactants can be added to the surface to artificially decrease the contact angle. However, if the bubbles are lined with good-quality surfactant, the surface tension falls quickly as the radius gets smaller because the surfactant molecules become crowded during deflation (Figure 46-4). As a result, the surface energy always acts to try and reduce the surface for a given volume. In short, surface tension created by water lining the respiratory system prevents the lungs from expanding. • Change in tissue compliance or in surfactants causes diseases. 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