Structural and Mechanical Changes in the Aging Lung
As the body ages, the respiratory system undergoes a series of natural and gradual changes. The lungs reach full maturity around age 25, and a slow, steady decline in function follows, becoming more apparent after age 35. These changes are not typically the result of disease, but they can diminish the respiratory system's reserve and increase vulnerability during illness.
Changes to Lung Tissue and Airways
- Loss of Elastic Recoil: The elastic fibers in the lung parenchyma (the functional tissue) gradually degenerate. This loss of elasticity causes the small air sacs, or alveoli, to become larger and more floppy, an effect sometimes termed "senile emphysema". This reduces the lungs' ability to recoil and expel air effectively during exhalation.
- Thickening of Alveolar Membrane: The membrane separating the alveoli and capillaries, where gas exchange occurs, thickens with age. This slightly increases the distance for oxygen to diffuse into the bloodstream.
- Diminished Airway Support: Supportive tissues and muscles around the smaller airways lose their tone, causing them to close more easily, especially during forced exhalation. This can trap air within the lungs.
Changes in the Chest Wall
- Stiffening Ribcage: The cartilage connecting the ribs to the breastbone becomes stiffer and more calcified with age. This, combined with thinning and changing shape of the ribcage bones due to osteoporosis and a possible curvature of the spine (kyphosis), reduces the flexibility of the chest wall.
- Weakened Respiratory Muscles: The diaphragm and intercostal muscles, which aid in breathing, become weaker. This decreases the force of inspiration and makes breathing more strenuous, particularly during physical exertion. The strength of inspiratory and expiratory muscles declines, often more significantly in men than in women.
Impacts on Pulmonary Function and Gas Exchange
The structural changes lead to measurable effects on how the lungs function, as seen in pulmonary function tests.
Decreased Ventilatory Capacity
- Reduced Vital Capacity: The maximum amount of air that can be exhaled after a full inhalation decreases with age. This is because the increasing residual volume (air left in the lungs after exhalation) reduces the functional volume available.
- Lower Expiratory Flow Rates: The loss of elastic recoil and weakened muscles mean that older adults cannot exhale air as quickly or forcefully. This is reflected in a reduced forced expiratory volume in one second (FEV1).
- Increased Work of Breathing: With a stiffer chest wall and weakened muscles, breathing requires more energy. For a 60-year-old, the energy spent on respiration can be 20% higher than for a 20-year-old.
Less Efficient Gas Exchange
- Reduced Diffusing Capacity: The age-related decline in alveolar surface area and thickening of the alveolar-capillary membrane impairs the transfer of oxygen to the bloodstream. The diffusing capacity of the lung for carbon monoxide (DLCO) decreases.
- Lower Arterial Oxygen Tension (PaO2): A reduced PaO2 is a normal consequence of aging, resulting from less efficient gas exchange and increased ventilation-perfusion mismatch.
Impairment of Protective Mechanisms and Immunity
Beyond mechanics, aging affects the body's protective systems, leaving older adults more susceptible to infection.
Weakened Airway Clearance
- Less Sensitive Cough Reflex: The nerves that trigger coughing become less sensitive over time. This makes it harder for older adults to clear foreign particles, mucus, or irritants from their lungs.
- Decreased Mucociliary Clearance: The cilia, tiny hair-like projections in the airways that sweep mucus and trapped particles upward, function less efficiently with age. This further impedes the clearance of pathogens and debris.
Compromised Immune Response
- Weakened Immune System: Aging leads to a less robust immune system (immunosenescence), which makes older adults more vulnerable to lung infections like pneumonia and influenza.
- Lower Antioxidant Capacity: The lungs' antioxidant defenses decline with age, leaving tissues more susceptible to oxidative stress and damage from environmental toxins.
Comparison of Respiratory Function in Younger vs. Older Adults
| Feature | Younger Adult (Approx. 20–35 Years) | Older Adult (Approx. 65+ Years) |
|---|---|---|
| Peak Function | Reaches maximum lung function around age 25. | Represents the declining phase of lung function. |
| Lung Elasticity | High elastic recoil; lungs snap back easily after inflation. | Decreased elastic recoil; lungs become more compliant and "baggy". |
| Chest Wall Compliance | Flexible and compliant, allowing for easy expansion. | Stiffer and less compliant due to cartilage calcification. |
| Diaphragm & Muscle Strength | Strong and efficient respiratory muscles. | Weakened diaphragm and intercostal muscles, requiring more effort to breathe. |
| Vital Capacity | Higher vital capacity, with lower residual volume. | Lower vital capacity due to increased residual volume. |
| Gas Exchange | Efficient oxygen transfer across a thin alveolar membrane. | Less efficient due to thickening membrane and reduced surface area. |
| Cough Reflex | Sensitive and effective for clearing airways. | Less sensitive, increasing the risk of aspiration and infection. |
| Immune Defense | Stronger, more effective immune response against respiratory pathogens. | Weaker immune system, increasing susceptibility to infections. |
Conclusion
In summary, an expected age-related change in respiratory function in older adults involves a gradual decline in the mechanical efficiency and immune defenses of the lungs. Structural changes include a loss of lung elasticity and an increase in chest wall stiffness, which reduce lung capacity and expiratory flow rates. On a cellular level, gas exchange becomes less efficient, and protective mechanisms like the cough reflex and mucociliary clearance weaken. While these are normal and slow changes, they reduce the respiratory reserve, making older individuals more susceptible to complications during acute illnesses. However, it is important to remember that significant or sudden changes should always be evaluated by a healthcare professional, as they may indicate an underlying pathological condition rather than normal aging. Regular exercise, avoiding smoke, and proper vaccination can help manage and mitigate some of these effects throughout a person's life. For more information on maintaining lung health, consult the American Lung Association.
