Lung elasticity, thorax and age
When one compares the PV-curves of the 20-yr old subject that you saw earlier, with those of a newborn, differences are pronounced. The newborn has a very flaccid thorax: small changes in pressure applied to the thorax result in considerable changes in its volume. The thorax being so pliable is a very useful feature, as it allows easy passage through the birth canal. The lung is similarly quite flaccid. These features are responsible for the fact that at elastic equilibrium the volume of the lung is not very different from the minimum volume of the thorax, which in turn is the main determinant of residual volume (RV).
In the 20-yr old subject both the lung and the thorax are stiffer, and residual volume is now a larger proportion of the TLC. Loss of lung elastic recoil, and the changes in the elastic properties of the chest wall after adolescence, lead to an increase in minimal lung volume (RV), as well as to and increased FRC.
During the process of aging the thorax therefore gradually moves towards a more inspiratory position: the lateral and postero-anterior diameters increase.
In pulmonary emphysema (lower figure) loss of elastic lung recoil is excessive. Not only can the lung be inflated to a larger volume than normal (TLC), the residual volume and the functional residual capacity will also increase appreciably. Such patients can be recognized by the pronounced inspiratory position of their thorax (hyperinflation).
It follows that the elastic properties of lungs and thorax change during growth and aging, and that this is associated with a change in the relative contribution of RV and FRC to the total lung capacity, as well as to a change in the shape of the thorax; particularly in the case of pulmonary emphysema the changes in RV and FRC, and in the shape of the thorax, are much more pronounced.
- The lung and thorax become stiffer (more elastic) from birth to adolescence.
- These changes are accompanied by an absolute and relative increase in FRC.
- From adolescence to old age the lung becomes more flaccid.
- As the lung loses elasticity its relaxed volume (the volume when transpulmonary pressure is nil) increases; this also holds for the FRC.