Growth of the small airways and alveoli from childhood to the adult lung measured by aerosol-derived airway morphometry

Understanding the human development of pulmonary air spaces is important for calculating the dose from exposure to inhaled materials as a function of age. We have measured, in vivo, the air space caliber of the small airways and alveoli at their natural full distension [ total lung capacity ( TLC)] by aerosol- derived airway morphometry in 53 children of age 6 - 22 yr and 59 adults of age 23 - 80 yr. Aerosol- derived airway morphometry utilizes the gravitational settling time of inhaled inert particles to infer the vertical distance necessary to produce the observed loss of particles to the airway surfaces at sequential depths into the lung. Previously, we identified anatomical features of the lung: the caliber of the transitional bronchioles [ transitional effective air space dimension ( EAD(trans))]; the mean linear dimension of the alveoli ( EAD(min)); and a measure of conducting airway volume [ volumetric lung depth ( VLDtrans)]. In the present study, we found that EADmin increased with age, from 184 mu m at age 6 to 231 mu m at age 22, generally accounting for the increase in TLC observed over this age range. EADtrans did not increase with TLC, averaging 572 mu m, but increased with subject age and height when the entire age range of 6 - 80 yr is included {EAD(trans) (mu m) = 0.012[height (cm)] x [age (yr)] + 508; P = 0.007}. VLDtrans scaled linearly with lung volume, but VLDtrans relative to TLC did not change with age, averaging 7.04 +/- 1.55% of TLC. The data indicate that from childhood ( age of 6 yr) to adulthood a constant number of respiratory units is maintained while both the smallest bronchioles and alveoli expand in size to produce the increased lung volume with increased age and height.