The term elasticity refers to the tendency of a structure to return to its initial size after being distended. Because of their high content of elastin proteins, the lungs are very elastic and resist distension. Since the lungs are normally stuck to the chest wall, they are always in a state of elastic tension. This tension increases during inspiration when the lungs are stretched and is reduced by elastic recoil during expiration.
The elastic nature of lung tissue is revealed when air enters the intrapleural space (as a result of an open chest wound, for example). This condition, called a pneumothorax, is shown in figure 16.10. As air enters the intrapleural space, the intrapleural pressure rises until it is equal to the atmospheric pressure. When the intrapleural pressure is the same as the intrapulmonary pressure, the lung can no longer expand. Not only does the lung not expand during inspiration, it actually collapses away from the chest wall as a result of elastic recoil, a condition called atelectasis. Fortunately, a pneumothorax usually causes only one lung to collapse, since each lung is contained in a separate pleural compartment.
The elastic nature of lung tissue is revealed when air enters the intrapleural space (as a result of an open chest wound, for example). This condition, called a
Law of Laplace
■ Figure 16.10 A pneumothorax of the right lung. The right side of the thorax appears uniformly dark because it is filled with air. The spaces between the ribs are also greater on the right side due to release from the elastic tension of the lungs. The left lung appears denser (less dark) because of shunting of blood from the right to the left lung.
Clinical Investigation Clues
Remember that Harry's stab wound caused the collapse of the lung.
What condition does Harry have?
How can he still be alive when the paramedics found him?
What physical factors caused the lung to collapse?
■ Figure 16.11 The law of Laplace. According to the law of Laplace, the pressure created by surface tension should be greater in the smaller alveolus than in the larger alveolus. This implies that (without surfactant) smaller alveoli would collapse and empty their air into larger alveoli.
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