As shown in the previous text and animation flow during a forced expiration is determined by:
The elastic properties of the lung:
the stiffer the lung, i.e. the larger PL,el,
the wider the diameter of small, intrapulmonary airways.
the stiffer the lung, the greater (for the same
flow) the distance between alveoli and that part of
the airway where the pressure loss equals = PL,el:
the ‘equal pressure point’ is then established
in larger and usually stiffer airways.
the further the lung is stretched, the larger PL,el.
Resistance to flow of smaller intrathoracic airways:
for the same flow viscous pressure losses increase
in proportion to resistance. Hence increased resistance
leads to greater pressure losses, and a pressure loss
equal to PL,el occurs closer to the alveoli. An ‘equal pressure
point’ in smaller and usually more compliant
airways will lead to greater dynamic airway compression
and hence to greater flow limitation.
Airways resistance increases at smaller lung volumes
(see 1), and PL,el becomes smaller; therefore maximum expiratory flow
decreases during a forced expiration.
Airway stiffness at the flow limiting segment.
the stiffer the airway down from the ‘equal
pressure point’, the larger the cross-sectional
area of the flow limiting segment and the larger the
flow through it.
Particularly in elderly subjects airway closure is likely
to occur; it occurs in dependent lung regions towards
the end of expiration.
The animation illustrates how these factors
interact during the recording of a maximum expiratory flow-volume