Hopefully you have been able to follow the thread of this exploration into lung growth and aging:
- Body height can be used to empirically scale lung volumes and ventilatory flows. However, this does not contribute to a fundamental understanding of the laws that govern a successful growth pattern as it does not take into account changing body dimensions during growth.
- Throughout the process of lung growth the normal functioning of the organism as a whole must be warranted. In the first years there is growth of the number of alveoli and bronchi, and subsequently they simply grow larger.
- As from age 6 year lungs and airways appear to grow proportionally: isometric growth.
- The lung growth pattern in patients with asthma is similarly isometric.
- Asthma patients, as well as individuals who experienced recurrent respiratory symptoms during infancy, exhibit a normal isometric growth pattern and yet appear to have narrower airways.
- Due to the elastic properties of lungs and chest, the changes that occur with growth and aging, and the changes that occur in the shape and properties of the thorax, neonates and elderly subjects have dependent lung areas that are poorly or nonventilated (airway closure) during normal tidal breathing. On account of the shape of the oxyhemoglobin dissociation curve the poorer oxygenation of capillary blood in such lung compartments cannot be compensated by increased ventilation elsewhere. This may explain the lower arterial oxygen pressures observed in these subjects. Neonates, infants, toddlers and elderly subjects are therefore at greater risk of more extensive airway closure in the case of e.g. viral infections, and thus to a compromised oxygen supply, particularly when supine.
- Soon after birth the arterial carbon
dioxide tension attains adults values, which are maintained
to a very high age. This indicates that the lung is up to
its task of exchanging gas across the alveolar-capillary