MODULE SUPPLEMENT: PULMONARY SYSTEM
The Impact of Age Changes on Air Exchange

So far we've noted that the chest wall gets stiffer, the lungs get more compliant, and the muscles get less forceful with age. This means that air is exchanged less efficiently. Older adults can not get air out as quickly or as fully as a young adult, and some airways close before the alveoli empty of air.

Does this matter?

Consider that, normally, the airways within the lungs are held open by tissue forces; there's a vertical gradient of lung distending pressures so that the alveoli and airways at the apex (top) of the lungs are more fully expanded than at the base. At some point in active expiration, pressures high enough to close the small airways develop and, because of the gradient, the airways at the bases will close while air is still expelled from the apex. The volume of air left in the lung at this time, that is, at the start of small airway closure, is called the 'CLOSING VOLUME'.