Vital capacity decreases with age due to physiological, anatomical and immunological changes that weaken the respiratory system over time. The lungs mature at age 20 to 25, and then progressively decline, according to a study published on the National Center for Biotechnology Information.
Lung function is dependent upon lung volumes, according to NCBI. The lung volumes are vital capacity, total lung capacity and residual volume. Vital capacity is the maximum amount of air that an individual can forcibly exhale after breathing in as deeply as possible, says The Free Dictionary by Farlex.
The vital capacity depends on the maximum inspiratory pressure, says NCBI. The MIP indicates diaphragm muscle strength, and the diaphragm is the most important breathing muscle. A decline in MIP correlates with a decline in vital capacity.
Men have an MIP that is 30 percent higher than women, but the decline in MIP with age is steeper for men than women, reports NCIB. They also found that the MIP declines by 0.8 centimeters to 2.7 centimeters of H2O per year. This means the diaphragm strength weakens with age, thus decreasing the vital capacity. Other age-related changes in the respiratory system, including structural changes in the chest wall, lead to an overall decline in the function of the respiratory system.