596 SECTION VIIRespiratory Physiology
the lung. The curve is shifted upward and to the left (compli-
ance is increased) in emphysema. It should be noted that com-
pliance is a static measure of lung and chest recoil. The
resistance of the lung and chest is the pressure difference re-
quired for a unit of air flow; this measurement, which is dy-
namic rather than static, also takes into account the resistance
to air flow in the airways.
ALVEOLAR SURFACE TENSION
An important factor affecting the compliance of the lungs is the
surface tension of the film of fluid that lines the alveoli. The mag-
nitude of this component at various lung volumes can be mea-
sured by removing the lungs from the body of an experimentalFIGURE 35–9 X-ray of chest in full expiration (left) and full inspiration (right). The dashed white line on the right is an outline of the
lungs in full expiration. Note the difference in intrathoracic volume. (Reproduced with permission from Comroe JH Jr.: Physiology of Respiration, 2nd ed., Year Book, 1974.)
FIGURE 35–10 Intrapulmonary pressure and volume relationship, the relaxation pressure curve. The middle curve is the relaxation
pressure curve of the total respiratory system; that is, the static pressure curve of values obtained when the lungs are inflated or deflated by various
amounts and the intrapulmonary pressure (elastic recoil pressure) is measured with the airway closed. The relaxation volume is the point where
the recoil of the chest and the recoil of the lungs balance. The slope of the curve is the compliance of the lungs and chest wall. The maximal in-
spiratory and expiratory curves are the airway pressures that can be developed during maximal inspiratory and expiratory efforts.
Total
lung capacityFunctional
residual capacity
Residual volumeLiters Liters
6 5 4 3 2 1Maximal
inspiratory
curveMaximal
expiratory
curveRelaxation
pressure
curveRelaxation
volume
Vital
capacity+ 3+ 2+ 10− 1− 120 − 80 − 40 0 40 80 120 160 200− 2Intrapulmonary pressure (mm Hg)Change from resting volume