How does aging reduce the reserve capacity of organs?

Aging reduces the reserve capacity of organs through a combination of structural and functional declines that occur throughout the life span. This reduction is driven by two primary factors:1

• Loss of Cellular Mass: Many organs, including the heart, kidneys, brain, and lungs, experience a gradual decline in the total number of functional cells. This loss of tissue directly diminishes the organ's overall capacity to perform at peak levels.1
• Diminished Cellular Efficiency: The remaining cells in an elderly individual often do not perform as effectively as those in younger people. Changes in cellular constitution and reduced activity of certain enzymes mean that metabolic processes and chemical reactions take significantly longer to complete.12

Consequently, while organs may maintain normal function under resting conditions, they struggle to adapt to physiological stress. This manifests as a slower recovery time following physical exertion or metabolic changes, as well as a reduced maximum work capacity, such as a lower maximum heart rate during strenuous activity.21