An increase in lung volume leads to greater lung recoil pressure and hence to greater distending pressures on intrapulmonary airways; conversely diminishing lung volume leads to a smaller distending force applied to the airway circumference. If the external airway surface area increases in proportion to lung volume (hence Fel/A = constant), the afterload to the contracting muscles is independent of lung volume. In man and dog maximal airway narrowing has been shown to increase as the lung is deflated.