Lung Compliance
Compliance is the change in volume divided by the change in pressure, and is the inverse of stiffness. A compliant lung has a large change in volume over a small change in pressure, signified by a steep slope. Compliance decreases as volume increases.
Compliance decreases with restrictive diseases or pulmonary edema, due to restriction of lung volume. Compliance increases with emphysema due to loss of elastic recoil. Work of breathing increases in situations with both increased and decreased compliance.
Pleural Pressure
Pleural pressure (PP) is normally - 5 as the elasticity of the lungs pulls inward while the chest wall pulls outward.
FRC, the volume of lungs after passive expiration, is the equilibrium between these two forces.
Surface Tension
The Law of LaPlace states that P = 2 T / r, with P: inside pressure, T: surface tension, and r: radius
This means that smaller alveoli contain higher inside pressure and are therefore unstable.
Surfactant, produced by Type II cells, decreases surface tension (T) which stabilizes smaller alveoli. This increases compliance and reduces opening pressure.