FRC and airway closure
We have previously noted the fact that the thorax and lung parenchyma become stiffer during growth from neonate to adult. During this process the functional residual capacity (FRC) increases in both absolute and relative terms. Due to these changes there is no airway closure during normal tidal breathing in healthy young adults. Particularly in infants and toddlers this phenomenon, and the change in body posture (larger lung volume when erect), helps to explain the rise to adult values of the arterial oxygen pressure.
The aging process modifies the elastic properties of lungs and thorax. The lung volume at which airway closure (closing capacity, CC) occurs during normal tidal breathing in basal parts of the lung encroaches upon the FRC. On average at age 65 yr airway closure occurs at the level of the FRC in healthy lifelong nonsmokers. While supine the FRC is smaller as the diaphragm is pushed up by the abdominal contents; in this posture airway closure occurs at normal end-expiratory level on average at age 45 yr. In smokers the downsloping curve in figure is displaced considerably to the left.
The changes in the mechanical properties of lungs and thorax are accompanied by changes in alveolar ventilation, of which airway closure is obviously a potentially important contributing factor. These changes are held responsible for the gradual decline in the partial pressure of oxygen in arterial blood.
- The gradual decline in arterial oxygen tension in adults can be atrributed to greater unevenness of alveolar ventilation distribution and to airway closure in dependent parts of the lung, associated with a gradually more flaccid lung.
- This is exacerbated by breathing in the supine posture, as the abdominal contents now pushes the diaphragm into the thorax so that brathing occurs at a lower lung volume.