Growth of lung volume
It is common use in medicine to predict an individual's lung volume and other lung indices from body length. Because it is so commonly done it provides one with the reassuring feeling that standing height is indeed an important scaling factor that can be used to study or describe growth processes. But is that feeling justified?
The graph on the right shows the relationship between the FRC (functional residual capacity - the volume of the lung at the end of a normal expiration) and body length of a considerable number of neonates, infants and toddlers. Let us start with the neonate and first develop some feeling for the order of magnitude of body dimensions and volumes. The FRC in a healthy neonate who measures 50 cm from crown to heel is on average 80 mL. That is the contents of about ½ a coffee cup. The graph discloses that the FRC increases non-linearly with body height. In a 6-year old with a standing height of 120 cm the FRC has increased to about 900 mL. Let us assume that all body dimensions (length, width, height) increase proportionally (one then speaks of isometric or isotropic growth), then volume increases as the third power of this scale factor. From 50 cm to 120 cm the scale factor is 2.4; this factor cubed gives 13.8. The expected FRC therfore comes to 80·13,8 or about 1100 mL. The actual FRC of about 900 mL is less than expected in the case of isometric growth.
- The growth pattern between ages 0-6 year does not appear to be governed by simple isometry.
- Empirically body length may be quite useful in predicting average FRC for a given length, but the empirical relationship does not prove or elucidate a fundamental relationship between body length and lung volume.