The lungs are very distensible (stretchable)—they are, in fact, about a hundred times more distensible than a toy balloon. Another term for distensibility is compliance, which here refers to the ease with which the lungs can expand under pressure. Lung compliance can be defined as the change in lung volume per change in transpulmonary pressure, expressed symbolically as AV/AP.A given transpulmonary pressure, in other words, will cause greater or lesser expansion, depending on the compliance of the lungs.
The compliance of the lungs is reduced by factors that produce a resistance to distension. If the lungs were filled with concrete (as an extreme example), a given transpulmonary pressure would produce no increase in lung volume and no air would enter; the compliance would be zero. The infiltration of lung tissue with connective tissue proteins, a condition called pulmonary fibrosis, similarly decreases lung compliance.
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