MODULE SUPPLEMENT: PULMONARY SYSTEM Changes that Occur with Age in the
Structure and Compliance of the Lung and Chest Wall
Three structural changes in the respiratory system associated with aging that influence pulmonary ventilation include changes in the chest wall, respiratory muscles, and lung parenchyma (Janssens, et al., 1999).
The thorax or chest wall becomes less compliant with age, meaning it gets stiffer, because of calcification of the chondral cartilage and kyphoscoliosis. This results in the chest wall becoming "fixed" in a slightly expanded position from which there is restriction in its ability to expand outward further or to contract inward. During normal breathing, rib cage expansion accounts for about 40% of the change in lung volume in young adults but only 30% in the elderly (Rossi, et al, 1996).
The muscles of the thoracic cage are the only skeletal muscles that must contract on a regular basis throughout the life span (Rossi et al, 1996). These muscles loose strength and function with aging, as do peripheral muscles. The strength of an older adult's diaphragm is noted to about 25% less than that of a young adult (Enright, 1999). These changes are probably related to loss of lean body mass (muscle mass) as well as denervation and degeneration of the muscle motor fibers (see Muscle Fibers -- Note: this link will open in a new browser window). At the same time, there seems to be wide variability in strength and resilience between individuals and improvement may occur with training. However, diminished muscle strength influences respiratory function, especially under stress, and decreases the effectiveness of coughing to clear secretions.
In contrast to the chest wall, the lungs become more compliant, or less elastic, with age. Thus they don't recoil or force air out as easily as we get older. Changes in the lung are related to alterations in the its connective tissues, specifically, changes in the characteristics of the elastin and collagen components (Culver & Butler, 1985; Rossi, et al, 1996). The actual processes that occur with aging, whether cross linkage or changes in location and orientation of the individual elastic fibers within the lung (De Martinis & Timiras, 2003; Culver & Butler, 1985; Sparrow & Weiss, 1988), aren't too clear. However, these changes cause the alveolar ducts to become enlarged and the alveoli to flatten. This results in more air staying within the alveolar duct rather than within the alveoli where oxygen exchange is most efficient (De Martinis & Timiras, 2003).
In addition, the elastic fibers in the alveoli are normally tethered to the respiratory and terminal bronchioles which helps keep these passages open, especially at low lung volumes. With age, these connections may be lost, leading to the increased compliance of the alveoli themselves and subsequent collapse of the passages. As a result, the alveoli often do not empty fully before the airway passages collapse, leading to uneven ventilation and air trapping.
