ELASTICITY, COMPLIANCE,

SURFACE TENSION and RESISTANCE

 

COMPLIANCE is the ability of the lung to stretch; its distensibility.   It represents the change in volume that occurs for a given change in pressure.  It is inversely related to ELASTASTICITY, the ability of the lung to recoil to its resting volume after the stretching force is released.  Elastance depends on the elastic tissue of the lung and chest wall.

SURFACE TENSION is the collapsing pressure exerted upon the alveoli.  It results from the attractive forces between molecules of liquid lining the alveoli and follows LaPlace’s Law, P=2T/r, where P is the collapsing pressure, T is the surface tension and r is the radius of the alveolus.  SURFACTANT lines the alveoli and reduces surface tension by disrupting intermolecular forces between molecules of liquid.  This reduction in surface tension prevents alveoli from collapsing and increases COMPLIANCE.

RESISTANCE of the airway opposes the flow of gases.  Air flow is characterized as Laminar when it is stream-lined, low velocity and follows Poiseuille's Law (see below); it is usually confined to the small peripheral airways. Air flow is characterized as TURBULENT when the movement of molecules of gas is disorganized; it occurs when velocity of flow exceeds a limiting value or when irregularities in the configuration of the airway preclude laminar flow.

It follows POISEUILLE’s LAW, an equation which describes laminar flow in a straight tube, ie laminar flow.

V=Pr4/8nl

where V= flow P= driving pressure r= radius of tube n= fluid viscosity l= length of tube (4 represents power of 4) 

Since length and viscosity of the airway are usually constant, the radius (or diameter) of the airway is the most powerful determinant of airway resistance (resistance and radius are inversely related).  The smaller the airway radius, the greater the resistance to flow.  Nearly 90% of airway resistance can be attributed to the trachea and bronchi, both characterized by rigid structures and together accounting for the smallest total cross-sectional area of the airway.

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