Effects of age and posture on closing volume.

Closing volume and closing capacity increased linearly with age. In the seated position, the closing volume exceeded the expiratory reserve volume in subjects older than about 65 years, while in the supine position, as the expiratory reserve volume is markedly reduced, this occurred at about 44 years. Above these two "critical ages", ventilation to the dependent parts of the lungs is reduced during the normal tidal volume breathing. Thus, in seated subjects older than 65 years and in supine individuals older than 44 years there is significant impairment of ventilation distribution to the dependent lung zones, which necessiarly causes impaired gas exchange within the lungs.