Lung growth and aging

The figures on the left show schematically the dimensions of the airways and lung volume at rest and during a forced expiration in a neonate. The figures on the right show the same in an adult, where all dimensions have been isometrically enlarged, i.e. all lengths and diameters were enlarged by the same amount.

The number of alveoli increases, and peripheral airways still differentiate, until a few years after birth (ref. 1). Thereafter growth occurs by increasing the dimensions of alveolar spaces and airways. The growth of these structures cannot be gauged from the relationship between body dimensions (such as standing height) and lung volume. This is because body proportions change from birth to adulthood, and this in turn alters the relationship between length and contents of the thoracic cage. A biologically more relevant question is whether growth of lungs and airways occurs in a fashion that they stay in tune. Dimensionally this is the case if growth is isometric, i.e. the relationship between diameter and length of airways and air spaces remains the same. This appears to be the case in the majority of subjects, even if they had respiratory problems in early childhood (ref. 2). In the latter case the airway is on average somewhat narrower than in healthy subjects; it is unclear whether this caused the respiratory problems or is a sequel of them. Asthma in childhood is associated with on average about a 10% larger total lung capacity (ref. 3).