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 small